CHAPTER VI._A Text-book of Tanning A treatise on the conversion of skins into leather, both practical and theoretical.

METHODS OF CHEMICAL ANALYSIS FOR THE TANNERY.

It is assumed that the reader has an elementary knowledge of chemistry,

and of the common manipulations of the laboratory; but at the risk of

giving information which to many is already familiar, the principles

that underlie those methods of testing which are most applicable to

technical purposes must be briefly explained.

_Standard Solutions._--If 40 _grm._ of pure caustic soda (NaHO) be

dissolved in water, and a little tincture of litmus added, it will be

coloured a bright blue. If hydrochloric acid be now added, drop by

drop, the litmus will at last become purple, and a single drop more

would turn it a bright red. At this point the liquid is neither acid

nor alkaline, and if it be evaporated to dryness, nothing will be left

but 58·5 _grm._ of common salt (NaCl), while 18 _grm._ of water will

be formed and have escaped. We have therefore used exactly 36·5 _grm._

of pure HCl, and if we dissolve 40 _grm._ of caustic soda in 1 _litre_

of water, and 36·5 _grm._ of pure HCl in another, equal parts of these

liquids will always exactly neutralise each other, forming nothing

but common salt and water. It will be obvious that if we have a soda

solution of the strength named, we can find the amount of hydrochloric

acid in any solution of unknown strength, by seeing how much of it

is required to neutralise, say, 10 _c.c._ (= 0·4 _grm._ soda) of the

known solution. Instead of 40 _grm._ of caustic soda, we may take 56

_grm._ of potash to the _litre_, and it will exactly neutralise an

equal volume of the hydrochloric solution containing 36·5 _grm._ If,

again, we make a solution containing 49 _grm._ of pure sulphuric

acid (SO_{4}H_{2}) per _litre_, it will neutralise an exactly equal

volume of either the soda or the potash solution, thus being precisely

equivalent to the HCl solution. Such solutions are called normal, and

any normal acid solution will neutralise an equal volume of any normal

alkali, and _vice versâ_. For many purposes normal solutions are too

strong, and solutions containing 1/10 of the quantities required for

normal solution are preferable; such solutions are called decinormal.

All solutions containing known quantities of chemicals, and intended

for use in volumetric analysis, are called Standard solutions.

_Indicators._--The tincture of litmus used to show when the solution

is exactly neutral is called an indicator, and many materials are

used in a similar way in different analytical processes. Thus the

indigo solution in Löwenthal's process is an indicator. A more useful

indicator than litmus for tannery purposes is Dr. Lunge's "methyl

orange," which is indifferent to carbonic acid, and may therefore be

used in the cold with solutions of alkaline carbonates; which are much

more easily made and preserved than those of the caustic alkalies

necessary with litmus. It is very sensitive to mineral acids, but not

equally so to organic. It may be obtained of Messrs. Mawson and Swan,

of Newcastle; and as a minute quantity only must be used for each

test, it is really cheaper than litmus, and a few _grm._ will last a

lifetime. It must be dissolved in water, and not more than 2 or 3 drops

taken for each titration. (Titration signifies an estimation by means

of a standard solution.) Other indicators will be named in connection

with the analytical methods in which they are used.

[Illustration: Fig. 9.]

_Instruments._--To practically carry out analysis by standard

solutions, measuring glasses are required. One or more flasks marked

in the neck to hold exact quantities (Fig. 9), one at least, holding

1 _litre_, are indispensable. One or two graduated cylinders (Fig.

10), holding 100 _c.c._, and divided into tenths of _c.c._, are very

useful, and it is well also to have one holding a _litre_, and provided

with a stopper (Fig. 11). This is called a "test mixer," but is not

absolutely essential.

[Illustration: Fig. 10.]

[Illustration: Fig. 11.]

[Illustration: Fig. 12.]

[Illustration: Fig. 13.]

[Illustration: Fig. 14.]

Pipettes (Fig. 12) are tubes with a mark on the stem by which exact

quantities of liquid can be taken. Several holding 5, 10, 20, and 25

_c.c._ are necessary, and one holding 10 _c.c._ and divided into tenths

is advisable. Most important of all is the burette (Fig. 13). If only

one is to be had, it must be a Mohr's burette with a glass tap, but as

alkaline solutions are apt to set glass taps fast, it is well to have

one with a tap, and another with a pinchcock (Fig. 14). They should

hold 50 or 25 _c.c._ and be divided into tenths. The burette in use is

fixed in a stand (Fig. 15) and filled up to the top of the graduation,

and the quantity of solution delivered is then shown by the scale. It

is usual to read by the under side of the hollow of the liquid, keeping

the eye carefully level with it.

[Illustration: Fig. 15.]

A chemical balance suitable for the preparation of standard solutions

and general analytical use, is shown in Fig. 16. The beam is provided

with steel or rock-crystal knife-edges at the centre, which are

supported on agate planes, and similar edges _a_ support the pans.

Except at the moment of weighing, the beam, and in good balances the

pans also (at _b_), are steadied by supports raised by turning the

milled head _c_. The long pointer _d_ moving over a scale, shows when

the beam is horizontal; but the weighing is performed, not by waiting

till the balance comes to rest, but by noting when the oscillations

are equal on each side of the zero point. The _weights_, which should

run from 50 _grm._ downwards, are usually of brass (preferably gilded)

down to 1 _grm._, while the fractions to 0·01 _grm._ are of platinum

foil. Milligrammes and fractions are weighed by a "rider" of wire

weighing 0·01 _grm._, and moved along the beam (which is graduated for

the purpose like a steelyard) by the arms _e_. A fair balance should

turn distinctly with 0·001 _grm._, and a good one with 0·0001 _grm._

If equal weights are placed on each pan, they should of course balance,

and if changed side for side the balance should be maintained. If not,

the arms of the beam are unequal. Weights always have trifling errors,

but if by a really good maker, these are generally so small that they

may be disregarded except in very delicate researches. The weights

should always be placed on the scale in regular order, beginning with

the heaviest, and it is well to accustom oneself to reading the weight

by the vacant places in the box as well as by the weights on the scale.

[Illustration: Fig. 16.]

While of course it is most important, and for accurate work essential,

to have as good a balance as possible, much may be done in technical

work, even with a good pair of druggists' scales; and most standard

solutions may be bought ready made; while from two or three accurately

adjusted solutions many others may be made volumetrically.

_Preparation of Standard Acid and Alkaline Solutions._--In practice

it is very difficult to obtain perfectly pure caustic soda, free from

water and carbonic acid, both of which are greedily absorbed by it from

the air, so that a standard solution cannot practically be made by

directly weighing out the substance as suggested in the introductory

paragraph. In sodic carbonate, however, we have a substance which is

easily obtained pure and dry, and which may be used for almost all the

purposes to which a caustic solution could be applied. A decinormal

solution is strong enough for most of the work in a tannery, though it

is a convenience to have both normal and decinormal, and a stock of the

stronger solution will last a longer time and is readily diluted to

decinormal strength by adding 1 part to 9 parts of distilled water. To

make a normal solution, about 60 _grm._ of the purest sodic carbonate

are placed in a porcelain basin or platinum crucible and heated over

a Bunsen gas-burner or spirit-lamp, nearly to redness, and allowed

to cool closely covered up. Of the salt thus dried 53 _grm._ are

accurately weighed into a beaker and dissolved in distilled water. The

solution is then poured into a gauged _litre_ flask, and carefully

filled up with water at a temperature of 59° F. (15° C.) to the mark on

the neck. The whole is then poured into a good-sized stoppered bottle

(40 oz.) and vigorously shaken for 5-10 minutes. This thorough shaking

is important with all standard solutions, and without experience no

one would believe how much shaking is required uniformly to mix a

solution. Probably more difficulty to beginners in analysis arises from

neglect of this matter than from any other cause. To make a decinormal

solution, proceed in precisely the same way, using 5·3 _grm._ instead

of 53; or dilute as above.

_Standard Acid Solution._--For this purpose any one of several acids

may be used, each of which has its special advantages.

Oxalic acid is the easiest to make of any. A sufficient quantity of

pure crystallised oxalic acid is powdered and pressed between filter

paper, so as to absorb the moisture which occasionally is retained

in cavities of the crystals. 6·3 _grm._ is then weighed out and

dissolved in water, exactly as was done with sodic carbonate, forming

a decinormal solution. It is used in Löwenthal's tannin estimation

process and may also be employed to determine alkalies, but forms

insoluble calcium oxalate with lime salts, and does not give a sharp

reaction with methyl orange indicator. Hence litmus must be used, or a

few drops of a neutral solution of calcium chloride added to the methyl

orange, when hydrochloric acid will be liberated as soon as there

is excess of the acid, and the indicator will be promptly reddened.

Sulphuric acid is the most permanent of any acid solution, and may be

generally employed. It forms insoluble sulphates with lime, baryta, and

strontia. To make a normal solution, 35 _c.c._ of the pure concentrated

acid are poured into at least 3 or 4 times as much distilled water,

and allowed to cool, and are then made up to about 1 _litre_ and well

shaken. The burette is filled with the mixture, 10 _c.c._ of the

standard sodic carbonate are measured into a beaker, 2 or 3 drops of

methyl orange solution are added, and the acid is run in with constant

stirring till the indicator is just beginning to redden. This must be

repeated, and the two titrations should exactly agree. Suppose that

9.5 _c.c._ are required, then 950 _c.c._ of the trial acid are equal

to 1 _litre_ of the soda. If therefore 950 _c.c._ be measured into a

test mixer, and made up to 1 _litre_, the solution should be accurately

decinormal. Of course great care must be used in the whole process.

If a gauged flask only is at hand it will be easier to measure into

it the water required to make up the _litre_, and then fill to the

mark with the trial acid. Normal hydrochloric acid may be made exactly

as described for sulphuric acid, but using about 100 _c.c._ of the

strongest acid. Decinormal solutions of both these acids may be made by

the same methods; using 1 tenth the quantities, or by dilution of the

normal solution.

Beside comparison with sodic carbonate solution, hydrochloric acid

may also be checked by determining the amount of chlorine present,

with silver nitrate (see p. 98) 10 _c.c._ of decinormal acid should of

course be equal to 10 _c.c._ of decinormal silver nitrate.

Table giving the Quantity of the Following Substances contained in or

equivalent to 1 _litre_ of Normal or 10 _litres_ of Decinormal Standard

Solution.

Sulphuric acid 49 _grm._ SO_{4}H_{2} = 40 _grm._ SO_{3}

Hydrochloric acid 36·5 " ClH = 35·5 _grm._ Cl.

[G]Oxalic acid 63·0 " C_{2}O_{4}H_{2} + 2 Aq.

Acetic " 60·0 " C_{2}H_{3}O_{2}H.

Soda 40·0 " NaHO.

Sodic carbonate 53·0 " Na_{2}CO_{3}.

[G]Lime 28·0 " CaO = 37·0 _grm._ CaH_{2}O_{2}.

[G]Calcic carbonate 50·0 " CaCO_{3}.

Ammonia 17·0 " NH_{3}.

[G]Barium hydrate 76·5 " BaO = 85·5 _grm._ BaH_{2}O_{2}.

Barium chloride 104·0 " BaCl_{2}.

Zinc chloride or sulphate 32·6 " Zn = 16·0 _grm._ S. as sulphide.

Silver nitrate 170·0 " AgNO_{3} = 35·5 _grm._ Cl.

Potassic permanganate 31·6 " K_{2}MnO_{4}.

[Footnote G: Insufficiently soluble in water to form a normal solution.]

EXAMINATION OF WATER.

_Hardness_ (Hehner's process). (_a_) Temporary Hardness.--As has

been stated (p. 84), this consists of lime and magnesia carbonates.

As methyl orange is not affected by carbonic acid, bicarbonates of

alkaline earths have an alkaline reaction, and may be estimated in

solution by standard acid like the alkalies themselves. 100 _c.c._,

or in soft waters 200 _c.c._, of the water is measured into a beaker,

a drop or two of solution of methyl orange added, and decinormal

hydrochloric or sulphuric acid run in from the burette with constant

stirring till the colour just changes to pink. This is repeated, and

the average taken. The two determinations should not at the most differ

more than 1/10 _c.c._ Each _c.c._ represents 5 parts per 100,000 of

CaCO_{3} or 2·8 parts of CaO; or corresponding quantities of magnesia

(4·2 parts of MgCO_{3} or 2 parts MgO), when 100 _c.c._ of water are

used.

(_b_) Permanent Hardness.--200 _c.c._ are measured into a beaker and

boiled for 15 minutes with 40 _c.c._ decinormal sodic carbonate. The

mixture is then allowed to cool and made up to 250 _c.c._; or the flask

and its contents may be weighed before boiling and made up again to the

same weight. It is then filtered, and 60 _c.c._ representing 50 _c.c._

of the original water, is twice titrated with decinormal acid and the

result added. If the water were pure, exactly 10 _c.c._ should be

required to neutralise the 10 _c.c._ of sodic carbonate, but if there

be permanent hardness a part of the sodic carbonate will be already

neutralised with the acids of the lime and magnesia salts, which have

been precipitated as carbonates together with the carbonates of these

bases originally present in the water. The hardness will therefore be

represented by the loss, i. e. the number of _c.c._ of acid used for

100 _c.c._ of the original water must be subtracted from 20 and the

remainder calculated as before, or if calculated as sulphates, each

_c.c._ represents 6·8 parts of CaSO_{4} or 6 parts of MgSO_{4} per

100,000. If, as is sometimes the case, more acid is required than is

needed for the sodic carbonate used, the excess corresponds to sodic

carbonate originally present in the water. In this case there can be no

permanent hardness.

_Chlorine in Water._--If silver nitrate be added to a solution of any

chloride, the silver is precipitated as white curdy insoluble silver

chloride. As indicator, a few drops of neutral potassic chromate are

used. So long as any chloride is present the red silver chromate

which forms is at once decomposed, and the silver converted into

white chloride. But as soon as all the chloride is exhausted, the

red chromate becomes permanent. To prepare a standard decinormal

solution of silver, 17 _grm._ of pure recrystallised silver nitrate are

dissolved in 1 _litre_ of distilled water. To perform the estimation

50 _c.c._ of water are measured into a beaker, 2 or 3 drops of strong

solution of pure yellow potassic chromate are added, and then silver

nitrate from the burette till a permanent red is formed. This is

repeated, and the results are added together, representing 100 _c.c._

of water. Each _c.c._ of silver nitrate used represents 3·55 parts of

chlorine, or 5·85 parts of sodic chloride per 100,000. If more than 10

_c.c._ of silver solution are required to 50 _c.c._, it is advisable

to use a smaller quantity of water. If the process be applied to

other liquids than natural water, it must be borne in mind that the

solution must not contain free acids or alkalies except carbonic acid.

If this is not the case the liquid may be rendered faintly alkaline,

with lime-water free from chlorides, and the excess of lime removed by

passing carbonic acid through it; or it may be slightly acidified with

sulphuric acid, and shaken with a little pure precipitated calcic or

baric carbonate.

_Detection of other Impurities._--Sulphuric acid (as sulphates) is

seldom wholly absent, but its presence may be proved, by adding excess

of barium chloride to the water slightly acidified with hydrochloric

acid (2-3 _c.c._ of saturated solution of BaCl_{2} are sufficient for

any ordinary water); if the mixture be allowed to stand overnight in a

100 _c.c._ cylinder beside a solution containing a known, and not very

different quantity of decinormal sulphuric acid, the quantity present

may be roughly compared by measuring the bulk of the precipitates.

Lime may be similarly detected and roughly measured by precipitation

with excess of ammonic oxalate in presence of ammonium chloride, to

hinder precipitation of magnesia. Lime-water, which may be used as a

standard, contains about 128 parts of lime per 100,000.

Magnesia is detected by adding ammonium phosphate to the filtrate from

the precipitated oxalate of lime. If the mixture be allowed to stand

in a warm place for 24 hours all the magnesia will be precipitated as

ammonio-magnesic phosphate.

Silica, &c.--100 _c.c._ of the water is acidified with a little HCl

evaporated to dryness, moistened with HCl, and treated with a little

hot water. The silica or silicic acid is left undissolved. The solution

from which the silicic acid has been filtered off is evaporated to

small bulk and ammonia added, when iron will be precipitated as brown

ferric oxide, which is coloured black by tannin or tanning liquor.

If copper be present it will give a blue solution with the ammonia.

Iron may also be recognised by evaporating the water to small bulk

with a trace of HCl, and adding a little sodium acetate, when if

iron be present it will be coloured black by tannin, red by ammonium

sulphocyanide, and blue by potassium ferrocyanide (prussiate of

potash). Its quantity may be estimated (Thomson, Chem. Soc. Abstracts,

May 1885) by measuring 100 _c.c._ of the water to be tested and 100

_c.c._ distilled water into two similar cylinders, adding to each 5

_c.c._ of dilute hydrochloric acid (1:5) and 15 _c.c._ of a solution

of potassium sulphocyanide (40 _grm._ per _litre_), and then adding to

the distilled water cylinder a very dilute standard solution of ferric

salt, till its colour matches the other. If the iron contained in the

water is in the ferrous condition, it must be oxidised with potassic

permanganate before testing.

A suitable ferric standard solution may be made by dissolving 0·1

_grm._ of clean, bright, soft iron wire in a little hydrochloric acid

in a long-necked flask, adding nitric acid so long as red fumes are

produced, evaporating nearly to dryness, and making up to 1 _litre_

(more accurately 996 _c.c._). Each _c.c._ will then equal 0·0001 _grm._

Fe.

Lead (and copper) may be detected by passing sulphuretted hydrogen

through the water acidified with HCl, or by adding a drop of fresh

ammonium or sodium sulphide to the slightly acidified water, when a

brownish coloration clearly visible in a deep beaker set on a sheet of

white paper will be produced. Iron also gives a black with sulphides in

alkaline solution. Copper may be distinguished from lead by the blue

given with ammonia, and by a reddish-brown precipitate with potassium

ferrocyanide.

For accurate quantitative estimation of these impurities, the regular

works on the subject, such as Thorpe's 'Quantitative Analysis,'

Sutton's 'Volumetric Analysis,' or Fresenius' 'Quantitative Analysis,'

must be consulted.

EXAMINATION OF COMMERCIAL ACIDS.

_Sulphuric acid_ 10 _grm._ may be made up to 100 _c.c._ and well

mixed, and of this 10 _c.c._ may be tested with normal sodic carbonate

in presence of methyl orange. Each 1 _c.c._ of soda solution used

corresponds to 0·049 _grm._ or 4·9 per cent. of H_{2}SO_{4}. For most

purposes, the strength may be ascertained from the specific gravity, as

measured by a hydrometer or weighed in a specific gravity bottle. The

following table gives the strength at 59° F. (15° C.):--

─────────┬───────────────┬────────────

Specific │ Degrees │ Per cent.

Gravity. │ Twaddell.[H] │ H_{2}SO_{4}

─────────┼───────────────┼────────────

│ ° │

1·8426 │ 168·5 │ 100

1·8376 │ 167·5 │ 95

1·822 │ 164 │ 90

1·786 │ 157 │ 85

1·734 │ 147 │ 80

1·675 │ 135 │ 75

1·615 │ 123 │ 70

1·557 │ 111 │ 65

1·501 │ 100 │ 60

1·448 │ 90 │ 55

1·398 │ 80 │ 50

1·351 │ 70 │ 45

1·306 │ 61 │ 40

1·264 │ 53 │ 35

1·223 │ 45 │ 30

1·182 │ 36 │ 25

1·144 │ 29 │ 20

1·106 │ 21 │ 15

1·068 │ 14 │ 10

1·032 │ 6 │ 5

─────────┴───────────────┴────────────

[Footnote H: Degrees of Twaddell's hydrometer may be reduced to

specific gravity by multiplying by ·005 and adding 1·, thus 10° Tw. =

1·050 sp. gr.]

The impurities of sulphuric acid most common and injurious for tanning

purposes are iron and nitrous acid. Iron is detected on neutralising

with soda or ammonia, when it falls as a yellowish precipitate, which

may be recognised by the ordinary tests (p. 100). Nitric and nitrous

acids are detected by pouring a strong solution of ferrous sulphate

cautiously on to the top of the strong cold acid, when a dark ring is

formed at the junction of the two liquids.

_Hydrochloric acid_ may be tested with soda solution like sulphuric. 1

_c.c._ of normal soda = 0·0365 _grm._ or 3·65 per cent. HCl. It may

also be calculated from specific gravity.

──────────────────┬───────────────

Specific Gravity, │ Per cent. HCl.

15° C. │

──────────────────┼───────────────

1·200 │ 40

1·177 │ 35

1·151 │ 30

1·126 │ 25

1·100 │ 20

1·075 │ 15

1·050 │ 10

1·025 │ 5

──────────────────┴───────────────

The presence of iron is indicated by a yellow colour, and may be

confirmed by the usual tests as in sulphuric acid.

_Oxalic acid_ should be pure white and soluble in distilled or

rain-water. 6·3 _grm._ may be weighed out, and made up to 200 _c.c._

If 20 _c.c._ of the solution for a test be used, each _c.c._ of

normal soda solution equals 10 per cent. of pure crystallised acid,

C_{2}O_{4}H_{2} + 2 Aq. The end-reaction with methyl orange is rendered

sharper by the addition of a few drops of neutral calcic chloride

towards the end of the titration.

_Acetic acid_ may be similarly determined, each _c.c._ of normal alkali

being equivalent to 0·06 _grm._ of C_{2}H_{4}O_{2}. Caustic soda, or

lime-water and litmus, give sharper results than sodic carbonate and

methyl orange. Brown pyroligneous acid is difficult to test from the

dark compounds formed with soda, but may be indirectly determined

by the quantity of marble, baric carbonate, or magnesia which it

will dissolve (compare p. 100), or very possibly by lime-water like

tan-liquors with a little tannin as indicator.

EXAMINATION OF LIME AND LIME-LIQUORS.

The quantity of caustic lime in either quicklime or lime-bottoms may

be determined by weighing a quantity of the finely powdered material

containing not more than 1 _grm._ of caustic lime, and shaking it

thoroughly with 1 _litre_ of distilled water and filtering. 100 _c.c._

should be taken, and decinormal acid, sulphuric or hydrochloric (or

if oxalic, with addition of neutral calcic chloride, or with litmus

instead of methyl orange as indicator). Each _c.c._ of decinormal

acid corresponds to 0·0028 _grm._ of CaO. If the filter and residue

be treated with sufficient normal acid to dissolve the whole of the

carbonates, and then titrated back with normal sodic carbonate and

methyl orange, the loss (less soda solution required than acid was

originally employed) is equal to the carbonate of lime and carbonate

and hydrate of magnesia present. 1 _c.c._ of normal acid = 0·05 _grm._

of CaCO_{3}.

[Illustration: Fig. 17.]

[Illustration: Fig. 18.]

Lime-water and lime-liquors may be titrated as above, with sulphuric

or hydrochloric acid and methyl orange; but in the latter case ammonia

(and if soda ash or "Inoffensive" is used, soda and potash also),

and the lime salts of weak organic acids will be estimated with it.

It is difficult to get a sharp end-reaction in old liquors from the

organic acids (caproic, amidocaproic, &c.) present. To determine the

ammonia, 50-100 _c.c._ of the liquor may be distilled in a small

retort or flask, and the escaping NH_{3} collected in a =U=-tube or

"nitrogen bulb" (Fig. 17), containing 20-50 _c.c._ of normal acid,

which is afterwards titrated back with sodic carbonate and methyl

orange. Kathreiner employs the arrangement shown in Fig. 18. 30 _c.c._

of the liquor to be examined is placed in a shallow vessel on a piece

of ground-glass, and 10 _c.c._ of normal acid in a second cup, which

is supported over the other by a glass or wire triangle. The whole

is covered with a small bell-glass, of which the edges are smeared

with, vaseline. At the end of 24 hours, all the ammonia will have been

absorbed by the acid, which is titrated back. The lime-liquor sample

should be drawn after well plunging the lime, and rapidly filtered into

a flask from a funnel covered with a clock-glass.

_Determination of Gelatin and Coriin in Lime-liquors._--This cannot

be done directly, though considerable quantities of dissolved

hide-substance are precipitated on acidification of the liquor with

hydrochloric acid and saturation with common salt. If the liquor be

neutralised with hydrochloric acid, and evaporated to dryness on the

water-bath, nitrogen may be determined in the residue by combustion,

and the hide-substance calculated from it (compare p. 108). This method

is serviceable in determining the amount of hide dissolved by different

solutions, or under different conditions.

The total solids of lime-liquors are estimated by evaporating 20-30

_c.c._ in a porcelain crucible at 212° F. (100° C.). The organic matter

is then found by igniting and determining loss (using ammonia nitrate

if necessary to complete the combustion of the carbon). The ash is

mostly lime carbonate. Soda, potash, and other bases may be determined

in it by the usual methods, if required.

ESTIMATION OF SULPHUR AS SULPHIDE IN SODIUM SULPHIDE, &c.

32·6 _grm._ of chemically pure zinc is dissolved in dilute sulphuric or

hydrochloric acid. This is readily accomplished in a flask, if a piece

of platinum foil, or a few drops of platinic chloride are added to form

a galvanic couple with the zinc. After solution, sufficient ammonia is

added to redissolve the precipitate at first formed,[I] and the whole

is made up to 1 _litre_. Each _c.c._ = 0·016 _grm._ sulphur or 0·242

_grm._ of sodic sulphide. This solution is added drop by drop from a

burette to the solution of sulphide, and forms a white precipitate of

zincic sulphide. The end of the reaction is known by placing a drop

(with a glass rod) side by side on a piece of white filter paper,

with a drop of solution of lead acetate. So long as sulphide remains

in solution, it will form a black margin of lead sulphide where the

drops touch. The drops must not be placed too close, as the solid zinc

sulphide is _always_ darkened if it comes in contact with lead acetate.

It must be noted that tank-waste liquors, and many other sulphur

solutions, contain polysulphides which are estimated by zinc, but which

do not unhair, at any rate in an unaltered state.

[Footnote I: If any brown residue remains, the zinc is contaminated

with iron.]

CHEMICAL EXAMINATION OF LEATHER.

[Illustration: Fig. 19.]

_Estimation of Grease._--To determine oil and grease, a weighed

quantity (5-10 _grm._) of the leather in fine shavings or raspings

is exhausted with petroleum-ether (gasoline) in a fat-extraction

apparatus, of which a convenient form is represented in Fig. 19. The

leather is placed in the upper vessel, of which the lower opening is

loosely plugged with cotton-wool, and the petroleum-ether in the flask,

which is gently heated in a water-bath. The petroleum-ether boils and

condenses in the inclined condenser through the casing of which a

stream of cold water is passed, whence it drops back into the flask

through the material to be exhausted. When the exhaustion is complete

(when a drop of petroleum-ether from the leather leaves no grease when

allowed to evaporate on a clean glass), the upper part of the apparatus

is removed, and the ether is distilled off. If the flask has been

previously weighed, it is maintained in an air-bath at 212°-248° F.

(100°-120° C.) for some hours, allowed to cool, and weighed, when the

gain of weight is the grease and oil. Paraffin would also be extracted

and reckoned, and probably traces of resin if present. Ordinary ethylic

ether cannot be used, since tannins and many of their products are

soluble in it. Probably carbon disulphide might be substituted. Care

must be taken to avoid explosion, as the vapours of petroleum are very

combustible. The residue left in the percolator may be examined for

matters soluble in water, by extracting again with hot distilled water,

or for resins (and phlobaphenes) by extraction with alcohol.

_Estimation of matters soluble in water._--This is important both to

detect weighting, and to draw conclusions as to the materials used in

tanning. Fine raspings or shavings may be exhausted with warm water

in a percolator, or roughly a weighed piece (20 _grm._) of leather,

air-dry, may be well kneaded and worked in 100 _c.c._ of warm water

in a basin. 50 _c.c._ of this may be evaporated to dryness in a light

basin over the water-bath (or under a paper hood on a steam boiler),

and the gain of weight will give the amount dissolved from 10 _grm._

This is more accurate and quicker than redrying the leather and

weighing loss. The residue will contain tannins and their products,

often in considerable quantities, and may be examined by the table of

reactions, p. 112, though these are as yet very imperfect. It will

also contain glucose, dextrin, and soluble salts, if these have been

used to give weight and firmness. The absolute proof of weighting with

glucose or dextrin is difficult, since tanning materials naturally

contain these and analogous principles. The residue may be powdered

and exhausted with cold water, and the tannins and colouring matter

removed by shaking with magnesia (p. 108) or lead carbonate. Fehling's

solution[J] is then added and the mixture is heated nearly to boiling.

A rapidly formed and considerable precipitate of red cuprous oxide

indicates weighting with glucose or dextrin. Leather extracts, however,

invariably reduce Fehling's solution more or less, and a conclusion can

only be drawn after some experience and comparative tests. Gallotannic

acid and pyrogallol reduce it when heated, but not cane sugar or gum

arabic. If a solution of cane sugar be heated to 68° C. for 1/4 hour

with 10 per cent. of fuming hydrochloric acid, it is "inverted," and

then after neutralising the acid with potash or soda, will reduce

Fehling's solution when heated.

[Footnote J: 4 _grm._ cryst. cupric sulphate are dissolved in 20 _c.c._

of water; and 16 _grm._ of neutral potassic tartrate and 13 _grm._ of

fused sodic hydrate are dissolved in 60 _c.c._ The two are mixed, made

up to 100 _c.c._, and boiled for some minutes. It should always be

tested before use by boiling a portion, which should remain perfectly

clear.]

The soluble mineral salts are detected by igniting the residue left

after evaporation of a separate portion in a porcelain crucible.[K]

From unweighted leather, the quantity is very small. The ash is

exhausted with a few _c.c._ of distilled water, which will dissolve

most sulphates and chlorides, which may be detected in small portions

of the solution by baric chloride and silver nitrate respectively.

Baric chloride and lead acetate are precipitated by a drop of sulphuric

acid, and the latter is blackened with ammonic or sodic sulphide.

Lime is precipitated by addition of ammonic chloride, ammonia, and

ammonic oxalate; magnesia by the subsequent addition of sodic phosphate

(see p. 109). The carbonates in the insoluble part (mostly derived

from salts of organic acids) may be taken up by dilute hydrochloric

acid and tested separately, or the acid may be used at first. Any

residue undissolved by the acid is probably lead chloride, and will be

dissolved by hot water.

[Footnote K: A platinum crucible must not be used for fear of its

destruction by lead, unless this metal has been proved absent.]

_Estimation of ash._--The leather in small pieces (either after or

before extraction with water) is incinerated in a porcelain crucible.

The ash is extracted with hydrochloric acid. The insoluble portion may

contain barium sulphate (barytes), lead sulphate, sand, clay, &c. For

further examination, ordinary chemical text-books must be consulted.

Any large amount of ash indicates weighting. Müntz found only about 0·5

per cent. of ash from bark-tanned leather.

_Determination of hide substance._--It is sometimes of interest to

determine the proportion of dry hide-substance in a sample of leather,

but there is no known means of doing this directly. If, however,

the leather be dried, finely powdered by rasping, and the nitrogen

determined by combustion, either with soda-lime (Will and Varrentrapp's

method), or with copper oxide (Dumas), the hide-substance may be

calculated, since tannin contains no nitrogen. Müntz found unhaired

skin dried at 230° F. (110° C.) to contain 51·43 per cent. of nitrogen

(compare also p. 20).

DETERMINATION OF FREE ACIDS IN TAN-LIQUOR.

The lime-water method mentioned on p. 172 is, from its simplicity, well

suited for daily use in the tannery as a control method for ordinary

working; but where it is necessary to make very exact estimations, or

to determine the various acids separately, it is not so satisfactory as

one recently published by Kohnstein and Simand (Dingl. Polyt. Jour.,

1885, cclvi. 38).

The acids usually present in liquor consist of several members of the

fatty or acetic group, which distil over with boiling water, of other

non-volatile organic acids, and sometimes sulphuric acid, which is

added to assist the swelling of the leather.

To determine the acids of the acetic group, Kohnstein and Simand

proceed as follows:--100 _c.c._ of the liquor are distilled, in a flask

or retort with a good condenser, to about 30 _c.c._, allowed to cool

a little, made up again to 100 _c.c._, and again distilled; and this

is repeated till about 300 _c.c._ have passed over. The distillate

is then made up to 300 _c.c._, well mixed by shaking, and the acid

is determined with standard soda. Methyl orange and sodic carbonate

is not so suitable for this titration, as caustic soda and litmus,

since methyl orange is not very sensitive to vegetable acids. If it be

desired to ascertain what quantity of acids of the acetic group exist

in combination with lime and other bases in the liquor, small excess

of sulphuric acid may be added to the residue in the retort, and the

distillation repeated, when the organic salts will be decomposed and

the volatile acids come over.

To determine the total free organic acids, Kohnstein and Simand shake

about 80 _c.c._ of the liquor with 3-4 _grm._ of freshly ignited

magnesia, quite free from carbonate and from lime, and allow to stand

for some hours with frequent vigorous shaking, till the liquor, which

at first is brown or dirty green, becomes almost colourless and gives

no reaction of either acid or tannin. The mixture is then filtered,

and the tannin and colouring matter are retained on the filter in

combination with magnesia, while the organic salts of magnesia, which

are mostly soluble, pass through with the filtrate. 10-30 _c.c._ of

the filtrate, according to the amount of acid present, is evaporated

to dryness, and gently ignited so as not to decompose any magnesic

sulphate present. The residue is moistened with water saturated with

carbonic acid, to convert any magnesic oxide into carbonate, and then

dried, in order to make the mass powdery, and easier to wash, It is

next taken up with hot distilled water, filtered and well washed.

Any sulphate which is present passes into the filtrate, while the

carbonate, which corresponds to the organic salts present before

ignition, remains on the filter, and after solution in hydrochloric

acid, is estimated as magnesic pyrophosphate. To the hydrochloric

solution is added excess of ammonia and sufficient ammonic chloride

to redissolve the precipitate formed, and prevent the precipitation

of the magnesia; the solution is heated and then ammonic oxalate

solution, first dilute, and then concentrated, is added to precipitate

any lime which may be derived from lime salts present in the liquor.

After filtering out and washing the precipitate, 10-15 _c.c._ of 10

per cent. sodic phosphate solution is added, and the liquid is stirred

with a glass rod without touching the sides of the beaker, and allowed

to stand 12 hours. The crystalline precipitate is then rinsed on to

a filter, and washed with a mixture of 1 of ammonia and 3 of water,

till the washings no longer give any milkiness with silver nitrate.

The filter is then dried and the precipitate is placed in a platinum

crucible and first gently, and then strongly ignited with the cover on;

the filter paper, freed as much as possible from the precipitate, is

burnt in the usual way on the crucible lid, the ashes are added to the

precipitate in the crucible, and the whole is again ignited and allowed

to cool in the desiccator, and finally weighed. 111 parts of magnesia

pyrophosphate correspond to 120 parts of acetic, or 180 parts of lactic

acid. Kohnstein and Simand calculate the pyrophosphate corresponding to

the acetic acid already found by distillation, and after deducting it

reckon out the remainder as lactic acid. Of course the volatile acids

are really a mixture consisting of acetic, propionic, butyric and other

members of the fatty group; but it would be difficult if not impossible

to separate them. Similarly other fixed acids exist in mixture beside

the lactic acid, but as their action is similar and lactic acid is

always the most abundant, these acids are to be reckoned as lactic.

It has been mentioned that when sulphuric acid is present in the liquor

it is found in the filtrate from the magnesia carbonate as sulphate.

After removal of the lime as oxalate, as previously described, the

magnesia may be similarly determined as pyrophosphate, and reckoned

out as sulphuric acid (111 parts of pyrophosphate being equal to 98

parts sulphuric acid, H_{2}SO_{4}). It may also be estimated with

barium chloride, but in this case regard must be had to the sulphates

originally present in the liquor.

Since waters invariably contain both lime and magnesia salts, a

portion (50 or 100 _c.c._) must be evaporated, ignited, and after

precipitation of the lime, the magnesia must be estimated as already

described, and deducted from the amount found in a similar amount of

liquor after saturating with magnesia. If, together with the organic

acids, the liquor contains sulphuric acid, the correction may be

divided equally between the two.

The method is not applicable in presence of phosphoric, tartaric, or

oxalic acids. To overcome this difficulty, Messrs. Kohnstein and Simand

are at present investigating a method dependent on decolorisation of

the liquor with bone charcoal, completely free from mineral salts, and

subsequent titration with soda.

It may be interesting to add the determinations of a complete set of

handlers in a Continental upper-leather tannery, in which larch bark is

used. 100 _c.c._ of liquor contained as follows, in _grm._:--

─────────┬────────────┬───────────────┬──────────────────────

No. of │Total Acids │ Volatile Acids│ Fixed Organic Acids

Handler. │ reckoned │ reckoned │ reckoned as Lactic.

│ as Acetic. │ as Acetic. │

─────────┼────────────┼───────────────┼──────────────────────

1 │ 0·205 │ 0·050 │ 0·232

2 │ 0·628 │ 0·237 │ 0·586

3 │ .. │ 0·372 │ ..

4 │ 0·688 │ 0·426 │ 0·393

5 │ 0·569 │ 0·432 │ 0·206

6 │ 0·509 │ 0·453 │ 0·084

7 │ 0·487 │ 0·456 │ 0·047

─────────┴────────────┴───────────────┴──────────────────────

QUALITATIVE DETECTION OF TANNINS.

It is often desirable to determine from what tanning materials an

extract or liquor is made, or with what a sample of leather is tanned.

The following table gives reactions of the principal tanning materials,

which will enable any one of them to be recognised with certainty,

and in many cases will determine the constituents in a mixture of

several, though this is naturally far more difficult. In such cases,

colour reactions are apt to mislead, that of one tannin being modified

by another, and it is safest to rely on the categorical test of

precipitate or no precipitate, coloration or no coloration, without

regard to the tint. The infusions must be very weak, not exceeding

1-2° Bktr., or precipitates will be formed where mere coloration or

clouding is noted. In some cases only negative peculiarities can be

given, and the material cannot be positively determined in mixture

with materials where these peculiarities are present. Thus myrobalans

could not be distinguished from divi with certainty, where any other

material, such as gambier, was present, which gave a deep coloration

with concentrated sulphuric acid. The writer will feel greatly obliged

by the communication of more distinctive reactions.

CHEMICAL ANALYSIS FOR THE TANNERY

────────────┬────────────┬────────────┬─────────┬───────────┬─────────────┐

Reagent. │Myrabolanes.│ Divi-divi. │ Valonia.│ Oak Bark. │Chestnut wood│

────────────┼────────────┼────────────┼─────────┼───────────┼─────────────┤

Boiled with │Pale deposit│Pale deposit│ Slight │Slight pale│ Slight red │

equal volume│ (eliagic │ (eliagic │ pale │ deposit or│ deposit on │

of sulphuric│ acid) on │ acid) on │ deposit.│ turbidity │ cooling. │

acid (1 vol.│ cooling. │ cooling. │ │on cooling.│ │

to 9 vol. │ │ │ │ │ │

water). │ │ │ │ │ │

Bromine │ No pp. │ No pp. │ No pp. │ Pale pp. │ No pp. │

water. │ │ │ │ │ │

Dilute │Blue-black │Dark blue │Blue- │Bluish │Blue- │

ferric │pp. │ pp. │black pp.│ black pp. │ black pp. │

chloride. │ │ │ │ │ │

Add │Brown pp. │Dark red pp.│Red brown│ Red brown │Dull red pp. │

ammonia. │ │ │ pp. │ pp. │ │

Sol. │ No pp. │Faint │ No pp. │ No pp. │Slight │

tartar │ │ clouding. │ │ │ clouding. │

emetic. │ │ │ │ │ │

Add ammonic │Light pp. │Dense pp. │Pale pp. │Whitish │ Pale pp. │

chloride. │ │ │ │ pp. │ │

Copper │Faint │Slight │ No pp. │Slight pp. │ No pp. │

sulphate. │ clouding. │ green pp. │ │ │ │

Add │Dense dark │Dense dark │Dark │Brown pp. │Dark │

ammonia. │ pp. │ pp. │ reddish│ │ brown pp. │

│ │ │ pp. │ │ │

Lime-water. │Yellow pp. │Yellow pp. │Yellow │Brown pp. │Purplish │

│ turning │ turning │ pp. │ │ brown pp. │

│ greenish. │ purple. │ turning│ │ │

│ │ │ red- │ │ │

│ │ │ purple.│ │ │

Ammon. │Dirty │Dark │Dark │Greenish │Dirty │

molybdate │ yellow │ greenish │ greenish│ pp. │ green pp. │

in nitric │ pp. │ pp. │ pp. │ │ │

acid. │ │ │ │ │ │

With sodic │Yellow │ Yellow │Turns │Turns │Reddish pp. │

sulphide │ colour. │ colour. │purpulish│ red. │ │

exposed to│ │ │ red. │ │ │

air on a │ │ │ │ │ │

tile. │ │ │ │ │ │

Add │Yellow │Intense │Deep │Deep red │Dark brown. │

concentrated│ colour. │ crimson. │ yellow.│ pp. on │ │

sulphuric │ │ │ │ dilution │ │

acid to │ │ │ │ │ │

1 drop │ │ │ │ │ │

infusion. │ │ │ │ │ │

Lead │Light │Dark │Pale pp. │Brown pp. │ Brown pp. │

Nitrate │ yellow pp.│ yellow pp.│ │ │ │

Cobalt │Buff pp. │Buff │Dirty │ Ditto. │Dirty │

Acetate │ │ pink pp. │ pink pp.│ │ yellow pp. │

Manganese │ Yellow pp. │ Yellow pp. │ Dirty │ Ditto. │ Grey pp. │

acetate. │ │ │ yellow │ │ │

│ │ │ pp. │ │ │

Uranium │Dark red │Dark red │Dark red │Dark brown │ Dark red │

acetate. │ colour. │ colour. │ colour.│ pp. │ colour. │

Ammoniacal │ No pp. │ No pp. │Brown pp.│ No pp. │ No pp. │

picric │ │ │ │ │ │

acid sol. │ │ │ │ │ │

Potassic │ Brown pp. │ Brown pp. │Brown pp.│ Brown pp. │ Brown pp. │

dichromate.│ │ │ │ │ │

────────────┴────────────┴────────────┴─────────┴───────────┴─────────────┘

CHEMICAL ANALYSIS FOR THE TANNERY -- Continued.

┬───────────┬───────────┬───────────┬────────────┬────────────┬───────────┐

│Hungarian │ Hemlock │Mimosa │Cutch │ Gambier │Gallotannic│

│Larch │ (Extract).│ bark. │ (Pegu). │ (Cuba). │ Acid, 1 │

│(Extract). │ │ │ │ │ per cent. │

┼───────────┼───────────┼───────────┼────────────┼────────────┼──────────-│

│Yellow │Abundant │Heavy red │Light red │ Reddish │ Usually │

│Flocculent │ red │ deposit │ deposit │ deposit │ some pale │

│deposit │ flocculent│ on │ on cooling.│ on cooling.│ deposit. │

│separates │ deposit. │ cooling. │ │ │ │

│quickly. │ │ │ │ │ │

┼───────────┼───────────┼───────────┼────────────┼────────────┼───────────┤

│Yellow pp. │Yellow pp. │Yellow pp. │ Yellow pp. │ Yellow pp. │ No pp. │

│Dull brown │Dirty green│Full brown │Green-black │Intense │ Blue │

│ pp. │ pp. │ pp. │ pp. │ green │ -black │

│ │ │ │ │ colour. │ pp. │

│Dull red │Reddened │Purple │ Dark red │ Reddened │ Reddened │

│ pp. │ pp. │ colour. │ pp. │ pp. │ pp. │

│No pp. │No pp. │White pp. │ No pp. │ No pp. │ No pp. │

│Pale pp. │Slight pale│Dense white│ Pale pp. │ Faint │ White pp. │

│ │ pp. │ pp. │ │ clouding. │ │

│Slight │Pale pp. │Slight pp. │ Dense pp. │ No pp. │ No pp. │

│ cloud. │ │ │ │ │ │

│Deep blue │Dark green │Deep red │Deep violet │Dark green │ Brown pp. │

│coloration.│coloration.│ pp. │ coloration.│ coloration.│ │

│Dirty │ Brown pp. │ Slight │Slight cloud│ No pp. │ Pale pp. │

│brown pp. │ │reddish pp.│ soluble in │ │turns blue.│

│ │ │ │ excess. │ │ │

│Slight │ Slight pp.│ Brown pp. │ Ditto. │ Ditto. │ Yellow │

│ clouding. │ │ │ │ │ colour. │

│No change. │ No change.│ Turns red.│ Slight │ No change. │ No change.│

│ │ │ │ reddening. │ │ │

│Dark brown │ Intense │ Intense │ Deep red no│ Dark brown │ Yellow. │

│or crimson.│ crimson. │purple-red.│ pp. on │ or crimson.│ │

│ │ │ │dilution. │ │ │

│Pale pp. │ Pale pp. │ Clouding. │ No pp. │ Faint │ White pp. │

│ │ │ │ │ clouding. │ │

│ Purplish │ Purple pp.│ Brown pp. │ Brown pp. │ No pp. │ Purple pp.│

│ pp. │ │ │ │ │ │

│Slight │ Slight pp.│ No pp. │ No pp. │ Ditto. │ White pp. │

│ clouding. │ │ │ │ │ │

│Slight │Light brown│ Dark red │ Dark red │ Dark red │ Crimson │

│ darkening.│ pp. │ colour. │ colour. │ colour. │ colour. │

│ │ │ │ │ │ Brown pp. │

│ No pp. │ Clouding. │ No pp. │ No pp. │ No pp. │ No pp. │

│ │ │ │ │ │ │

│ Ditto. │ Brown pp. │ Brown pp. │Brown │ Brown pp. │ Brown pp. │

│ │ slowly │ │ colour. │ slowly │ │

│ │ formed. │ │ │ formed. │ │

┴───────────┴───────────┴───────────┴────────────┴────────────┴───────────┘

QUANTITATIVE DETERMINATION.

Many processes have been proposed for the quantitative estimation of

tannins, but it cannot be said that any method yet known is wholly

satisfactory. The oldest, that of Sir H. Davy, recently improved by

Stoddart and others, consists in precipitating with gelatin, and drying

and weighing the precipitate. This is almost impossible to filter off

as directed by Davy; but by the use of a little alum, and by pouring

hot water on the precipitate, it becomes curdled into a mass which

may be washed by decantation. As the precipitate contains varying

quantities of tannin, according to the strength of solution employed;

as it is soluble in excess of gelatin solution, and as it is almost

if not quite impossible to wash it free from gelatin and alum, the

method can hardly lay claim to much accuracy. A somewhat better one

consists in the employment of a standard solution of gelatin with a

little alum, determining the end of the reaction by filtering off a

portion and ascertaining if another drop of the reagent produces a

further precipitate. This method is very tedious, the end reaction

is difficult to hit, the standard solution is very unstable, it is

inapplicable to gambier and cutch because the mixture will not

filter clear, and its results are irregular, probably from the power

of tannin to combine with various proportions of gelatin. A plan,

which has a seductive appearance of simplicity, is that of Hammer;

he takes the sp. gr. of the infusion, then absorbs the tannin with

slightly moistened hide-raspings, again takes the sp. gr., and from

the difference calculates the percentage of tannin, a difference of

5 per cent. of tannin corresponding to one of 1·020 sp. gr. (20°

barkometer). Unfortunately the hide is more or less soluble in the

liquor, and absorbs acids other than tannic with considerable energy;

the moistening of the raspings introduces an error, and the smallness

of the quantity to be measured makes a slight error completely vitiate

the results. With extreme care, due corrections for temperature, for

the water introduced with the raspings, and for their solubility,

and by substituting evaporation of the infusions to dryness for mere

calculation from their sp. gr., the method is useful as giving almost

the only information obtainable as to the actual weight of tannin in

any material capable of being absorbed by hide. It is, however, only

suitable for use as a check on easier and more rapid methods, such as

Löwenthal's, which give accurate relative results, but no information

as to absolute weight of unknown tannins. A modification of Hammer's

method has been introduced by Müntz and Ramspacher, in which the liquor

whence the tannin is to be removed is forced through a piece of raw

hide by pressure. This method, except that it is more rapid, has all

the evils of Hammer's in an intensified form, and gives such variable

results as to be quite useless in practice. A set of very careful

determinations of one sample of sumach gave results ranging from 18 to

28 per cent., and similar variations occurred when the experiment was

repeated with valonia. Wagner's method by precipitation with a standard

solution of cinchonine and magenta has proved wholly unreliable.

Gerland's method with a volumetric solution of tartar emetic, used in

presence of ammonic chloride, gives constant results with sumachs,

2/3 of those given by permanganate and Neubauer's equivalent.

Tartar emetic does not precipitate the tannins of cutch and gambier.

Fleck's, by precipitation with copper acetate, and subsequent washing

with ammonic carbonate and gravimetric estimation, either of the

tannate dried at 212° F. (100° C.), or of the copper oxide left

on ignition; and Carpene's, by precipitation with ammoniacal zinc

acetate, and subsequent estimation with permanganate and indigo,

though giving fairly accurate results on some tannins, are only of

limited application. They may therefore be passed over, as well as

Jean's method with a volumetric solution of iodine in presence of sodic

carbonate, and Allen's method with lead acetate, which are tedious and

difficult, and present no advantage over Löwenthal's improved process.

This last is easy of execution, constant in results, and universally

applicable. Before proceeding to describe it in detail, it may be well

to give some hints as to the best modes of sampling and preparing

tanning materials for analysis, since this is often more difficult and

tedious than the actual analysis.

Sampling.--Samples should always be drawn from at least 10 sacks or

separate parts of the bulk, and, in the case of valonia, special

care should be taken to have a fair average quantity of "beard."

No attention is usually paid to this point by merchants, and the

proportion varies greatly in different parts of the same cargo. If

several sacks are spread in layers on a level floor, and then portions

going quite to the ground are taken from several parts of the floor,

this will be accomplished. Where samples must be dealt with which have

not been specially drawn, it might be safest to weigh out from each the

same proportion of beard and whole cups, bearing in mind that the beard

is always the richest part of the valonia. In sampling myrobalans, it

should be remembered that the poor and light nuts will rise to the

top, and hence the hand should be plunged well into the sack. Grinding

of valonia and myrobalans when practicable is probably best done in a

small disintegrator, fitted with gratings. The material, of which some

pounds must be used, is screened over a sieve of say 15 wires per in.,

and all coarser parts are returned to the mill till they will pass. The

mill must grind into a close box, that no dust may be lost. Bark may

be reduced to fine saw-dust by cutting a portion of each piece in the

sample with a circular saw or rasp driven by a lathe. The advantage of

these methods is that samples can be ground without previous drying,

and thus in many cases time may be saved and separate determination

of moisture avoided. When this is not practicable, the sample of some

lbs. at least is ground in an ordinary bark-mill, well mixed, spread

out flat on a floor or table, and several portions are taken as already

described, say 50-100 _grm._ in all, and dried in a water- or air-oven

at 212° F. (100° C.). The moisture is best determined, to save time,

in a small separate portion of 10 _grm._, which must be dried till

it ceases to lose weight, and the loss taken as moisture. It must be

weighed in a covered capsule, as it is very hygroscopic. When the

larger portion of the sample has been dried some hours, it is passed

twice through a good coffee-mill, and then returned to the oven till

thoroughly dried, for which, 12-24 hours is generally sufficient.

Another method sometimes convenient is to take each acorn, or each

piece of bark of the sample to be tested, and snip a piece from it with

a pair of tinners' shears, taking care that in the case of valonia the

section runs right to the centre of the cup; and in bark, that fair

shares of the outer and inner layers are taken. The reason for drying

before grinding is, that unless hard dried, tanning materials cannot be

passed through a small mill. Bark and valonia usually contain 12-16 per

cent. of moisture.

Exhaustion.--10 _grm._ of valonia, 20-30 _grm._ of bark, or

corresponding quantities of other material, are boiled briskly for

half an hour with 1 _litre_ of distilled water, a funnel being placed

in the neck of the flask, and great care being taken at first to avoid

frothing and boiling over. The flasks used should have a capacity of

at least 1-1/2 _litre_. The whole contents are finally rinsed into

a gauged flask, allowed to cool to 59° F. (15° C.), and made up to

1 _litre_. In the case of sumach, a little more boiling even than

this is desirable. This method has been found by the writer to give

better results than boiling with successive portions of water. Another

method is to boil for 1/2 hour with 250 _c.c._ of water, then pour the

whole on a filter, wash with boiling water so long as a drop of the

filtrate blackens paper moistened with a dilute solution of ferric

acetate, and finally make up to 1 _litre_. Many materials, however,

clog the filter to such an extent that washing is almost impossible.

Kathreiner has used 15 _litres_ of water, and corresponding quantities

of material, in a large steam-jacketed copper pan, for each exhaustion,

making the weight up finally to 15 _kilos._, with very uniform and

excellent results. (See also p. 130.) With all materials which deposit

ellagic acid or other insoluble derivatives, on cooling and standing,

considerably higher results will be obtained if the titration be made

as soon as the liquor is cold, than if it be allowed to stand 24 hours;

in this respect, a uniform practice should be adhered to. Addition of

1/2 _c.c._ of glacial acetic acid renders the infusions less liable to

change.

Analysis.--Of all the methods which have been proposed for the

estimation of tannins, the only one which has met with any general

acceptance is that of Löwenthal, and indeed it is the only one which in

rapidity of execution and constancy of results is fitted for general

use. The method, as originally proposed, depends on the oxidation of

the astringent solution by permanganate in presence of indigo, which

not only serves as an indicator, but controls the oxidation, limiting

it to those bodies which are more oxidisable than indigo. As, however,

these include gallic acid and other substances which are useless to

either tanner or dyer, it is necessary to remove the tannin, and by

a second titration to obtain its value by difference. This Löwenthal

(Zeitschrift f. Anal. Chemie, 1877, p. 33) accomplished by a solution

of gelatin and common salt, to which, after mixture with the tannin

infusion, a small quantity of sulphuric or hydrochloric acid was added.

It was necessary to let this stand at least some hours before a clear

filtrate could be obtained, and the gelatin remaining in solution had

a slight though generally negligible effect on the permanganate. In

some cases, even after long standing, perfect filtration was extremely

difficult and tedious, and it was also clearly proved by Simand

(Ding. Polyt. Jour., ccxliv. 400) that a certain proportion of the

tanno-gelatin precipitate, varying with the acid present, and with

the species of tannin, remained in solution, and thus gave too low a

result. He therefore proposed to revert to the old method of separating

tannin with hide raspings, or, as an improved substitute, with the

gelatinous tissue of bones, and this is probably the most accurate

method, but has the disadvantage of requiring considerable time for

its execution. (See also p. 130.) The writer has therefore tried, and

he thinks successfully, so to modify Löwenthal's original method as to

increase its accuracy, and at the same time to make it more rapid and

easy of execution. It was found that by saturating the clear filtrate

with salt, a further precipitate containing tannin was formed, but

unfortunately, it was so finely divided that no amount of standing,

or even of warming, and repeated passing through the paper, would

obtain a clear filtrate. Finally, he hit on the device of mixing with

the liquid, before filtration, a portion of the pure kaolin used by

photographers. The effect was instantaneous and complete. A perfectly

clear filtrate was obtained without any of the tedious waiting which

before was necessary, and it was not only free from tannin, but also

nearly so from gelatin, so that it only gave the faintest cloudiness

with tannin solution. Gelatin gives a more considerable precipitate,

but this is simply due to its insolubility in the saturated salt

solution, and it is redissolved on dilution with water.[L]

[Footnote L: Hunt (Jour. Soc. Chem. Industry, April 1885) states, that

saturation with salt causes partial precipitation of gallic acid when

present, and that results agreeing more closely with those obtained by

absorption with hide are obtained by employing a mixture of 50 _c.c._

liquor, 25 _c.c._ 2 per cent. gelatin solution, and 25 _c.c._ saturated

solution of salt containing 50 _c.c._ of concentrated sulphuric acid

per _litre_ and a teaspoonful of kaolin. This approaches very nearly to

Löwenthal's original method, but with the addition of the kaolin, and

as in it, it is to be feared that a portion of the tannate of gelatin

will remain in solution. For accurate work, therefore, absorption by

hide-raspings is preferable, though even that has been shown by the

writer to remove gallic acid and other matters beside tannin. Hunt

states that raw hide also absorbs catechin.]

A slight error is introduced by the presence of a trace of oxidisable

matter in the gelatin, and when very great accuracy is required, it

is well to make a blank estimation of "not-tannin" without tannin

infusion, and deduct 1/2 of the permanganate consumed as a correction

from the not-tannin; but this may usually be disregarded. Each

titration should be made twice, and successive tests should not differ

by more than 0·1 _c.c._ of permanganate.

Reagents.--Solutions are required of (1) Pure potash permanganate, 1

_grm._ per _litre_. (2) Pure soda or potash sulphindigotate, 5 _grm._,

and concentrated sulphuric acid, 50 _grm._ per _litre_. (3) Pure

oxalic acid, 6·3 _grm._ per _litre_ (decinormal). The sulphindigotate

(indigo carmine), must be filtered, and when oxidised by permanganate,

should give a pure clear yellow, free from any trace of brown or

orange. Any contamination with indigo-purple, which gives brown

oxidation-products, is quite fatal to the accuracy of the analysis.

The permanganate solution is standardised by measuring 10 _c.c._ of

the (decinormal) oxalic acid solution, adding a little pure sulphuric

acid and distilled water, warming to 136° F. (58° C.), and running in

the permanganate till a faint permanent pink is produced, for which

about 32-33 _c.c._ should be required. The indigo-carmine solution

should be of such strength that 14-16 _c.c._ of permanganate are

required to bleach the quantity employed, which may be 20-25 _c.c._, as

convenient. (4) Gelatin solution: 2 _grm._ of Nelson's or other good

gelatin are allowed to swell in distilled water for two hours, melted

by setting the glass in a pan of boiling water, and made up to 100

_c.c._ This will not keep. (5) Dilute sulphuric acid: 10 _c.c._ of pure

concentrated acid are added to 90 _c.c._ of distilled water. (6) Good

table salt. (7) Purified kaolin.

The analysis is performed in the following manner:--20 _c.c._ of indigo

solution, and 5 _c.c._ of the infusion of tanning material is added,

in a white basin as recommended by Kathreiner, to about 3/4 _litre_

good water, which it is best to measure approximately, so that if it

contains any impurity which affects the permanganate it should be

constant, and thus be eliminated with the indigo. Permanganate solution

is then allowed to drop in, with constant stirring till the pure yellow

liquid shows a faint pinkish rim, most clearly seen on the shaded

side. This end-reaction, which is of extraordinary delicacy, is due

to Kathreiner, and is quite different to the pink caused by excess of

permanganate, being an effect common to all pure yellow liquids. It is

not needful to make the titration so slowly as has been advised--the

permanganate may be dropped in steadily with vigorous stirring, so

long as there is large excess of indigo, but as soon as the bottom

of the basin can be seen through the solution, it must be added very

cautiously, one or two drops at a time, and with occasional pauses, to

allow time for its complete mixture through so large a mass of fluid.

The titration is repeated twice, and the results added together and

denoted by _a_. Then take 50 _c.c._ of the infusion, and add 28·6

_c.c._ of the gelatin solution of Nelson's gelatin of 2 _grm._ to 100

_c.c._ After shaking, the mixture is saturated with salt, which brings

the volume up to 90 _c.c._, and 10 _c.c._ of the dilute sulphuric acid

(containing 1 vol. of concentrated acid in 10) and a teaspoonful of

pure kaolin are added. It is best to do this in a flask in which it

can be well shaken, after which, filtration may be at once proceeded

with, although it is safer to let it stand an hour or two: (the flask

may be cleansed with caustic soda solution). 10 _c.c._ of this filtrate

(= 5 _c.c._ of the original infusion) are employed for a second pair

of titrations, which are added as before, and the result denoted _b_.

If, further, _c_ be the quantity of permanganate required to oxidise

10 _c.c._ of decinormal oxalic acid, and 10 _grm._ of the tanning

material have been employed to make 1 _litre_ of infusion, _c_ : (_a_ -

_b_) :: 6·3 : _x_, where _x_ is the percentage of tannin expressed in

terms of crystallised oxalic acid. If it be desired to calculate the

gallic acid and non-tannin substances contained in the infusion, the

value in permanganate of the indigo alone must be determined. Calling

this _d_, as _c_ is to (_b_ - _d_), so is 6·3 to the percentage of

non-tannins in terms of oxalic acid, and for the present it is best

invariably to calculate results in this way, since we do not actually

know the relation of any single tannin to permanganate, even Neubauer's

number for gallotannic acid being probably too high, according to the

recent investigations of Councler and Schroeder,[M] and Oser's for

quercitannic being at most only approximate. It happens, moreover, that

this last equivalent (62·36 _grm._ of quercitannic acid = 63 _grm._

of crystallised oxalic acid) does not differ from that of oxalic acid

more than the ordinary limits of error of such estimation, and the

substitution is therefore of no commercial importance, while it is

surely better to employ a standard which is easily and exactly verified

than one which is certain to be modified by further research, and so

to run the risk either of having our results made useless for future

comparison, or of establishing a false or arbitrary equivalent. What

is wanted for practical purposes is not the absolute weight of tannins

in the various materials, but only a means for the relative comparison

of two samples of the same material; cross comparisons of different

tannins being simply delusive. If, however, it is necessary at any time

to give actual percentages of gallotannic acid, it is probably best

to stick to Neubauer's number for the present, as it is in general

use. Neubauer states that 63 _grm._ of oxalic acid consume as much

permanganate as 41·37 _grm._ of gallotannic acid. Tshekawa found 41·688

as the equivalent for tannin from Japanese gall nuts (Chem. News,

xlii. 274). Councler and Schroeder on the other hand give only 34·3

_grm._ Simand gives 61·1 _grm._ as the equivalent of quercitannic acid.

Commercial "pure tannin" always gives results higher than the truth, as

the gallic acid which it contains consumes more permanganate than an

equal weight of tannin, or even than the tannin which would yield it

if boiled with acid. When this is done the equivalent used should be

definitely stated, or it will certainly lead to confusion. Neubauer's

equivalent is only properly applicable to gall nuts, and possibly to

sumach and myrabolans. For oak bark Oser's number or that of oxalic

acid is most likely nearly correct; and this may also be approximately

true of oak wood and valonia, but as respects all other materials we

have no information whatever, and the oxalic equivalent is as likely to

be right as any other. (Compare note, p. 128.)

[Footnote M: From researches by von Schroeder, published since the

above was penned, it seems that the permanganate consumed by tannin is

largely influenced by the way in which the titration is conducted, see

p. 128.]

A few results are given below, not as showing the relative values of

the materials, which, of course, cannot be directly compared by any

analytical process, but for comparison with those obtained by other

methods and modes of calculation:--

─────────────────────────────────┬──────────────────┬──────────────────┐

│ │ Other Bodies │

│ Tannin │ Oxidised │

│ (as Oxalic Acid).│ (as Oxalic Acid).│

─────────────────────────────────┼──────────────────┼──────────────────│

Spent Liquor │ O·12 │ 11·0 │

Valonia (good Smyrna). Sample 1 │ 29·1 │ 2·3 │

" " Sample 2 │ 30·7 │ 2·1 │

" " Sample 3 │ 30·5 │ 1·9 │

Hungarian Larch Extract. Sample 1│ 14·78 │ 1·95 │

" " Sample 2│ 18·08 │ 2·33 │

Chestnut-wood Extract, 25° B. │ 25·53 │ 3·68 │

Pegu Cutch │ 63·59 │ 2·45 │

─────────────────────────────────┴──────────────────┴──────────────────┘

It is proved by experiment that kaolin removes nothing which is

oxidised by permanganate, but simply facilitates the precipitation

and filtration; and it is often found useful to clarify the original

infusions and liquors before the first titration. On the other hand,

there is no doubt that the salt and acid of Löwenthal's method

precipitate of themselves a large proportion of certain tannins. In

the case of cutch this amounted, in the analysis given, to 67 per cent.

of the whole. There is, however, good reason to believe that this would

also have been absorbed, or at least removed from solution by hide in

the process of tanning. This is shown by the analysis of the spent

liquor above given, which originally contained the tannins of oak bark,

valonia, myrabolans, gambier, hemlock, and oak wood extracts, &c., to

the extent of 10 to 15 per cent., but which was reduced by contact

with hide to 0·12 per cent. That a portion had not been absorbed but

decomposed is proved by the large accumulation of oxidisable impurities

(equal to 11 per cent. of oxalic acid); at the same time this example

shows that the method is capable of estimating a very small portion of

tannin in presence of much gallic acid and other analogous substances.

It is worth remark that such spent liquors become very pale in colour,

and also that the filtrates, freed from tannin by precipitation, are

nearly colourless, thus proving that the colouring matters present in

tanning materials are of the nature of tannins, at least as regards

their precipitability by hide and gelatin.

Simand (Dingl. Polyt. Jour., ccxlvi. 133) has recommended instead of

precipitation with gelatin, the use of the gelatinous tissue of bones

to remove the tannin. For this purpose porous bones, such as horn

piths, are coarsely powdered, and after treatment with dilute soda

solution to remove the fat, are steeped in weak hydrochloric acid

till all the calcareous matter is dissolved. They are then thoroughly

washed, ground wet through a steel mill, washed again and dried at

a low temperature; the tannin is removed more quickly than by raw

hide, and the amount of gelatinous matter dissolved by cold water is

a very trifling one. This method, or that with purified hide-powder,

is to be recommended for scientific research, since no element capable

of precipitating substances other than those absorbed by the hide

is introduced, while it is not certain in all cases that saturation

with salt and acidification may not remove other constituents of the

liquor besides tannins. It has, however, for technical purposes the

great disadvantage of requiring a much longer time for absorption of

the tannin than is the case with gelatin solution, and of the process

being much more difficult of execution. If hide-powder be employed, it

must be moistened with a small quantity of water before adding to the

infusion, and this water must be taken into account in the quantity

of the filtrate employed for the titration of the "non-tannin." The

digestion with the hide- or bone-powder must be continued till the

filtered liquid does not give the faintest clouding with a drop of

clear gelatin solution, and it is always very difficult to be sure that

the tannin is so completely removed as with gelatin and salt. Hide-

or bone-powder may be employed to determine the actual weight of any

unknown tannin absorbable by hide, by evaporating equal quantities of

the original infusion and of that freed from tannin by digestion with

the powder; the difference giving the tannin absorbed. The evaporation

must be conducted as far as possible in absence of air, for instance

in _vacuo_, or in a current of carbonic dioxide, and the residues both

dried at 212° F. (100° C.) so long as they lose weight. The amount of

matter dissolved from an equal quantity of the hide- or bone-powder by

water must also be ascertained and taken into the calculation.

Ammoniacal solution of cupric acetate or sulphate has been employed by

several chemists to remove tannin from solutions. N. H. Darton of New

York, who has a large practice in tannin analysis, employs cuprammonic

sulphate in the following manner.

The infusion, for which 20 _grm._ of hemlock bark or a corresponding

quantity of other material must be used, is made by exhausting with

2 or 3 quantities of water successively, first cold, and then with

heat (by placing the flask in a pan of boiling water), each portion of

water being poured off into a _litre_ flask. The last should be almost

colourless. The liquor is thus made up to nearly 1 _litre_, 25 _c.c._

of dilute sulphuric acid (about 1 vol. concentrated in 10) is added,

and the liquor is filtered through a small filter, which is finally

rinsed with a small quantity of water. Liquid ammonia is now added till

the liquor slightly smells of it, and, if any precipitate is formed, it

is filtered off as before; 25 _c.c._ of dilute sulphuric acid is again

added (which should give the liquid an acid reaction), and it is made

up to 1 _litre_. The titration is done as described under Löwenthal's

method, but instead of precipitating with gelatin, 100 _c.c._ is mixed

with 100 _c.c._ of a solution of copper sulphate to which sufficient

ammonia has been added to redissolve the precipitate first formed, and

containing 1-1/4 per cent. of copper sulphate. This is well shaken and

filtered, and the "not-tannin" is determined in the filtrate just as

with gelatin; a little dilute sulphuric acid being added in the basin

to neutralise the ammonia. The writer has examined this method with

regard to a few tanning materials. With valonia (and therefore probably

with oak bark) the preliminary treatment is unnecessary, and copper

precipitation gives results practically identical with the improved

gelatin, while it is less troublesome. On the other hand, a sample of

Miller's Hungarian Larch Extract which gave tannin equal to 18·08 per

cent. (by the gelatin method) gave no precipitate with cuprammonic

sulphate, and hence a result in tannin of _nil_ by Darton's method. It

is worth remark that by the copper method it is therefore possible to

estimate the valonia tannin alone in a mixture of larch and valonia

tannin. Probably this mode of analysis may also be utilised to separate

other tannins. With chestnut extract the results seem satisfactory, as

regards the precipitation of the tannin by copper, the figures agreeing

very closely with those by gelatin, but the preliminary treatment with

sulphuric acid and ammonia precipitates about 75 per cent. of what

is usually reckoned as tannin, leaving 7·53 per cent. of tannin only

instead of 25·53 per cent. as reckoned by the gelatin method; which,

judging by practical results in tanning, can hardly be accepted as

correct. The results of the gelatin method are found to agree fairly

with those of direct absorption by hide-powder, which is strong

confirmation that what is estimated as tannin is what is absorbed

by the hide. It is well known that sulphuric acid precipitates many

tannins, and in an experiment with cutch it was found by the writer

that saturation with salt and the addition of dilute sulphuric acid as

for Löwenthal's process, but without the gelatin, precipitated 67 per

cent. of the total tannin as usually reckoned.

It is obvious that it is impossible by analysis to compare the relative

value of different tannins, such as those of myrobalans and gambier,

or hemlock and valonia. All that analysis can reasonably be expected

to do is to give the relative values of different samples of the

same substance, or at the most, of materials of the same class. All

other comparisons are misleading; and would be so, even if the exact

percentage of each tannin could be calculated; since the commercial and

practical value of different materials does not depend on the quantity

of tannin only, but on the character of the leather it produces, hard

or soft, dark- or light-coloured and heavy- or light-weighing.

A Commission of German technical chemists, under the presidency of

Dr. Councler of Eberswalde, and including Messrs. Eberz, Kathreiner,

Schaun, von Schroeder, and Simand, have recently reported on methods

of tannin estimation ('Bericht über die Verhandlungen der Commission

zur Feststellung einer einheitlichen Methode der Gerbstoffbestimmung,'

Cassel, 1885). After reviewing earlier methods, they recommend the

following modifications of the Löwenthal method, for general adoption.

Chemicals employed.

(1) _Permanganate solution._ 10 _grm._ of the purest potash

permanganate are dissolved in 6 _litres_ of distilled water.

(2) _Indigo solution._ 30 _grm._ dry sulphindigotate of soda (Carminum

cærul. opt., "pure Indigotin I" of Gehe & Co., Dresden), air-dry, are

dissolved in 3 _litres_ of dilute sulphuric acid (1 vol. H_{2}SO_{4}

to 3 vols, water), 3 _litres_ of distilled water are added, the whole

is shaken till dissolved, and filtered. In each titration, 20 _c.c._

are used in 3/4 _litre_ of water, and reduce about 10·7 _c.c._ of

permanganate.

(3) _Hide-powder_ must be white and in a fine woolly state of division,

and should yield to cold water no substance capable of reducing

permanganate. Such a powder is prepared by Dr. Both of Berlin,[N] and

by the Vienna Research Station.

[Footnote N: Messrs. Mawson and Swan, of Newcastle, have kindly

undertaken to keep these, and the other reagents mentioned in this

book, in stock for the convenience of English tanners and chemists.]

Mode of Titration.

Instead of adding the permanganate solution drop by drop, to the

mixture of indigo, water, and liquor (as described, p. 121), it is

recommended to add it 1 _c.c._ at a time,[O] vigorously stirring 5-10

seconds after each addition. When the liquid has become bright green,

2-3 drops at a time are cautiously added with stirring, till the liquid

is pure yellow. Either a beaker on a white tile or a white basin may be

used (compare p. 121). It is advantageous in strong sunlight to shade

the window with white tissue-paper.

[Footnote O: It has been noted by several chemists, and especially by

Kathreiner, and later by Prof. von Schroeder, that the quantity of

permanganate reduced by a given amount of tannin varies within rather

wide limits, according to the rate at which the permanganate is added;

and the "1 _c.c._ method" was suggested by Prof. von Schroeder, to

secure uniformity in this particular. It has, however, been found by

the writer, in the course of experiments not yet completed, that the

quantity of permanganate required, was a function not simply of time,

but of the rapidity of diffusion through so large a bulk of liquid;

and by the alternate use of a simple glass rod, and of a specially

constructed perforated stirrer, he was able, while adhering strictly

to Prof. von Schroeder's directions, to obtain results even more

divergent by the "1 _c.c._ method" than could be obtained by the drop

method previously recommended, when properly carried out. Employed in

conjunction with the use of tannin for standardising, as recommended by

the Commission, either method gives perfectly dependable results.

The explanation of the variation is a simple one. The oxidation in

the Löwenthal process should be limited to indigo, and bodies more

oxidisable than indigo, but there exist both ready formed in liquor,

and among these oxidation products many substances which in the absence

of indigo will readily reduce permanganate. When the latter is added

rapidly, and with insufficient stirring, it destroys the indigo and

tannin in contact with it, and proceeds also to oxidise the other

matters present, although in other parts of the beaker indigo and

tannin still exist. Thus more permanganate is reduced than corresponds

to the indigo and tannin, and this is especially so towards the end

of the process, when very little of either remains. The more slowly

the permanganate is added, and the more vigorously it is stirred, the

more closely it will approximate to the theoretical quantity required

merely to oxidise the indigo and tannin. It seems to the writer more

scientific to approach this as nearly as possible, than to attempt to

establish a purely arbitrary standard such as the "1 _c.c._ method;"

but he would rather refrain from committing himself to a definite

opinion till his experiments are complete.]

[Illustration: PL. V.

_E. & F. N. Spon, London & New York._

"INK-PHOTO." SPRAGUE & CO. LONDON.

SOFTENING THE SKINS.]

Standardisation of the permanganate.

To avoid the uncertainty involved in comparing tannin (which reduces

different quantities of permanganate according to the method of

titration) with so dissimilar a reducing agent as oxalic acid, it

is recommended to employ tannin, titrated under precisely the same

conditions as the tanning material, so that whatever method be

employed, the differences will be common to both, and will so be

eliminated. Prof. von Schroeder has shown (Report, p. 45) by careful

experiment, that with the purest samples of tannin the permanganate

value estimated on the total dry substance of the tannin varied by

very little from that of the part of the tannin absorbed by hide as

determined by Hammer's process, but on the average bore the proportion

of 1 : 1·05. The percentage of water in an air-dried tannin must

be estimated by drying a portion at 201°-212° F. (94°-100° C.) and

determining the loss, and a quantity equivalent to 2 _grm._ must be

dissolved in 1 _litre_ of water and 10 _c.c._ titrated with indigo in

the usual way. If the permanganate value thus obtained be multiplied

by 1·05, it will be equivalent to that of 2 _grm._ of chemically pure

tannin. It is only necessary to determine the moisture occasionally, if

the tannin be kept in a well-closed box or bottle.

To ascertain if a tannin is pure enough for this, use a solution made

as above described (it is not necessary to determine the moisture) and

10 _c.c._ are titrated with permanganate in the usual way. 50 _c.c._

are then digested in the cold with 3 _grm._ hide-powder (previously

moistened with distilled water and well squeezed in linen) for 18-20

hours, with frequent shaking, filtered, and 10 _c.c._ again titrated.

If the second titration ("not-tannin") does not exceed 5 per cent.

of the total, it is _good_, but it may be _used_ so long as the

"not-tannin" does not exceed 10 per cent.[P] The purest tannin examined

by Prof. von Schroeder was Schering's Phar. Ger., which may be obtained

of Messrs. Mawson and Swan.

[Footnote P: Gallic acid suggests itself to the writer as being a good

standard, since it behaves with permanganate like tannin, and being

crystalline is easily purified and of definite composition.]

[Illustration: Fig. 20.]

The course of analysis is as follows:--

_Preparation of the infusions._--Extracts are dissolved in hot

water, and if necessary, filtered. Barks and other solid materials

are treated in Prof. von Schroeder's extraction-apparatus (Fig. 20)

(which seems very well adapted for its purpose). This consists of a

perfectly cylindrical vessel of cast-tin, about 12·5 _c.m._ deep and

7 _c.m._ diameter. A strainer covered with fine muslin fits it like a

piston.[Q] The powdered material is placed in the cylinder, and stirred

up with 200 _c.c._ of cold water. At the end of an hour, the piston is

inserted and pressed down gently, the clear liquor is poured off, and

the process is 4 times repeated with hot water, at intervals of 1/2

hour, placing the cylinder in a water-bath. The liquid is made up to 1

_litre_, and, if necessary, filtered (Report, p. 66). The quantity of

material used should be such as to give an infusion of which 10 _c.c._

do not reduce more than 8 _c.c._ permanganate. If it is desired to

determine separately the "easily soluble tannin" (viz. that extracted

by cold water), Real's Press (Fig. 21) is employed, which consists of

a cylinder _a_, through which water may be forced by the pressure of a

column of liquid. The small sieve _d_, covered with a disc of linen,

is placed in _a_, next the tanning material previously thoroughly

moistened with water, and the tap is closed. The press is then filled

with water and left 15 hours under a pressure of 1-1/2 _metres_. The

tap is then opened and 1 _litre_ is allowed to run through in the

course of about 2 hours, and mixed by shaking. The material is finally

exhausted like a new material in von Schroeder's apparatus to extract

the difficultly soluble tannin.

[Footnote Q: Both this apparatus, and the Real's press, may be obtained

from C. Focke, Zinngiesser, Grosse Kirchgasse 3, Dresden.]

[Illustration: Fig. 21.]

The titration is carried out as before described; in each infusion

separately to determine the "not-tannin" 50 _c.c._ are treated with 3

_grm._ hide-powder, and 10 _c.c._ are titrated.

It may be well in conclusion for the writer to state for the

information of the non-chemical reader, that though for purposes of

comparison of the results of different chemists, it is most desirable

to have a standard method of the highest possible perfection; any of

the accepted modifications of the Löwenthal method will give excellent

practical results in careful hands.

A Text-book of Tanning A treatise on the conversion of skins into leather, both practical and theoretical. · 第 6 / 24 章