METHODS OF EXAMINING TANNING MATTERS
Whereas the evaluation of vegetable tanning matters necessitates
determinations of their practical applicability in addition to
qualitative and quantitative analyses, the latter two determinations are
of practically no value when dealing with synthetic tannins. The way in
which tanning matters obtained by chemical means exert their action, in
addition to the intensity with which they convert pelt into leather, is
the only criterion of their quality for practical (tanning) purposes;
both may be demonstrated by experimental tests.
When dealing with the natural tanning materials it is desirable to know
their contents of actual tanning matter, from which their special
qualities as tanning agents may be deduced. Where the vegetable tanning
materials have already been converted into extracts, it is essential to
establish the identity of the original material used by the qualitative
reactions of the extract in addition to the quantitative estimation of
actual tannin contents. It is frequently necessary to examine whether
the extract in question has been actually prepared from the material
giving the extract its name, or whether the extract has suffered the
addition of other extracts of tanning materials of but low quality. Such
determinations may be undertaken by microscopical observations and by
means of qualitative and quantitative reactions; for this purpose many
colour reactions and precipitation methods are available in addition to
the determination of the molybdenum figure (Lauffmann),[Footnote:
Collegium, 1913, 10.] the alcohol and ethyl acetate figures and
microscopical examination (Grasser).[Footnote: Ibid., 1911, 349.] Of
other adulterants tending to reduce the quality of extracts may be
mentioned sugars, mineral salts, and coal-tar dyes; [Footnote: Grasser,
_Collegium_, 1910, 379.] for the determination of these, the special
literature should be consulted. [Footnote: Grasser, "Handbuch
f. gerbereichem. Laboratorien" (Leipzig, 1914);
Procter-Paessler, "Gerbereichem. Untersuchungen" (Berlin, 1901).]
Two methods are devised for the purpose of quantitatively determining
the tannin contents, both of which employ hide powder, and which are
known as the "shake method" and the "filter bell method" respectively:
the former is adopted as the official method of the "International
Association of Leather Trades' Chemists" (I.A.L.T.C.). [Footnote: And
also by the Society of Leather Trades' Chemists.-_Transl._]
The original method, [Footnote: _Leather Manufacturer_, 1894, No. 9
J.S.C.I.,1894, 494.] worked out in the laboratory of the Yorkshire
College (now the University of Leeds), essentially consists in
introducing 6-9 gm. of hide powder in a shaker, washing it at least
twice with distilled water and carefully squeezing out the powder in a
linen cloth between each washing. 100 c.c. of the solution to be
examined, which may not contain more than 1 per cent, total solids, are
introduced into the shaking bottle which is then weighed. About
one-third of the washed hide powder is then added, and the bottle shaken
ten to fifteen minutes; another third is then added and, after shaking,
the third portion. The bottle plus contents is now weighed, and the
amount of hide powder introduced ascertained by difference of the two
weighings. The liquid is then filtered through filter paper, 50 c.c. of
the clear filtrate evaporated in a basin, dried and weighed. The residue
in the original solution is then obtained by multiplying the former by
100 (plus weight of water added with hide powder), and dividing by 100.
This method was closely investigated by a large number of leather
trades' chemists, was considerably improved, and in its final form
presented a method of the highest degree of accuracy; the method was
therefore adopted as _The Official Method of Tanning Analysis_ by the
I.A.L.T.C., which body, at the same time, gave precise instructions as
to the details of the method. The latest instructions, which are
reprinted below, permit of any method of analysis which observes the
following conditions:--
1. The solution for analysis must contain between 3.5 and 4.5 gm. of
tanning matter per litre, and solid materials must be extracted so that
the greater part of the tannin is removed at a temperature not exceeding
50° C.
2. The total solubles must be determined by the evaporation of a
measured quantity of the solution previously filtered till optically
clear, both by reflected and transmitted light. This is obtained when a
bright object such as an electric light filament is distinctly visible
through at least 5 cm thickness, and a layer of 1 cm. deep in a beaker
placed on a black glass or black glazed paper appears dark and free from
opalescence when viewed from above. Any necessary mode of filtration may
be employed, but if such filtration causes appreciable loss when applied
to a clear solution, a correction must be determined and applied as
described in paragraph 6.
Filtration shall take place between the temperatures of 15° C. and 20°
C. Evaporation to dryness shall take place between 98.5° C. and 100°
C. in shallow, flat-bottomed basins, which shall afterwards be dried
until constant at the same temperature, and cooled before weighing for
not less than twenty minutes in air-tight desiccators over dry calcium
chloride.
3. The total solids must be determined by drying a weighed portion of
the material, or a measured portion of its uniform turbid solution, at a
temperature between 98.5° C. and 100° C. in shallow, flat-bottomed
basins, which shall afterwards be dried until constant weight at the
same temperature, and cooled before weighing for not less than twenty
minutes in air-tight desiccators over dry calcium chloride.
"Moisture" is the difference between 100 and the percentage of total
solids, and "insoluble" the difference between "total solids" and "total
solubles."
4. _Non-Tannins._--The solution must be detannised by shaking with
chromed hide powder till no turbidity or opalescence can be produced in
the clear solution by salt-gelatine solution. The chromed powder must be
added in one quantity equal to 6.0-6.5 gm. of dry hide powder per 100
c.c. of the tanning solution, and must contain not less than 0.2 per
cent. and not more than 1 per cent. of chromium calculated on the dry
weight, and must be so washed that in a blank experiment with distilled
water, not more than 5 mg. of solid residue shall be left on evaporation
of 100 c.c. All water contained in the powder should be determined and
allowed for as water of dilution.
5. _Preparation of Infusion_.--Such a quantity of material shall be
employed as to give a solution containing as nearly as possible 4 gm. of
tanning matter per litre, and not less than 3.5 or more than 4.5
gm. Liquid extracts shall be weighed in a basin or beaker and washed
with boiling water into a litre flask, filled up to the mark with
boiling water, and well mixed and rapidly cooled to a temperature of
17.5° C., after which it shall be accurately made up to the mark, again
well mixed, and filtration at once proceeded with. Sumac and myrabolam
extracts should be dissolved at a lower temperature.
Solid extracts shall be dissolved by stirring in a beaker with
successive quantities of boiling water, the dissolved portions being
poured into a litre flask, and the undissolved being allowed to settle
and treated with further portions of boiling water. After the whole of
the soluble matter is dissolved, the solution is treated similarly to
that of a liquid extract.
Solid tanning materials, previously ground till they will pass through a
sieve of sixteen meshes per square centimetre, are extracted in Koch's
or Procter's extractor with 500 c.c. of water at a temperature not
exceeding 50° C.; the extraction is then continued with boiling water
till the filtrate amounts to 1 litre. It is desirable to allow the
material to soak for some hours before commencing the percolation, which
should occupy not less than three hours, so as to extract the maximum of
tannin. Any remaining solubles in the material must be neglected or
reported separately as "difficultly soluble" substances.
The volume of liquid in the flask must, after cooling, be accurately
made up to 1 litre.
6. _Filtration_.--The infusion shall be filtered till optically clear
(_vide_ 2). No correction for absorption is needed for the Berkefeld
candle, or for S. and S. 590 paper [Footnote: Schleicher and Schüll,
Düren (Rheinland), Germany.] if a sufficient quantity (250-300 c.c.) is
rejected before measuring the quantity for evaporation, and the solution
may be passed through repeatedly to obtain a clear filtrate.
If other methods of filtration are employed, the average correction
necessary must be determined in the following manner:--About 500 c.c. of
the same or a similar tanning solution is filtered perfectly clear, and
after thorough mixing 50 c.c. is evaporated to determine "Total Soluble
A." A further portion is now filtered in the exact method for which the
correction is required (time of contact and volume rejected being kept
as constant as possible), and 50 c.c. is evaporated to determine "Total
Soluble B." The difference between "A" and "B" is the correction sought,
which must be added to the weight of the total solubles found in
analysis. An alternative method of determining correction, which is
equally accurate and often more convenient, is to filter a portion of
the tanning solution through the Berkefeld candle till optically clear,
which can be generally accomplished by rejecting 300 or 400 c.c., and
returning the remaining filtrate repeatedly; and at the same time to
evaporate 50 c.c. of the clear filtrate obtained by the method for which
correction is required, when the difference between the residues will be
the correction sought. An average correction must be obtained from at
least five determinations. It will be found that this is approximately
constant for all materials, and amounts in the case of S. and S. 605,
150 c.c. being rejected, to about 0.005 gm., and where 2 gm. of kaolin
are employed in addition to 0.0075 gm. The kaolin must be previously
washed with 75 c.c. of the same liquor, which is allowed to stand
fifteen minutes and then poured off. Paper 605 has a special absorption
for a yellow colouring matter often contained in sulphited extracts.
7. Hide powder shall be of a woolly texture, thoroughly delimed,
preferably with hydrochloric acid. It shall not require more than 5
c.c. or less than 2.5 c.c. of decinormal NaOH or KOH to produce a
permanent pink colour with phenolphthalein on 6.5 gm. of the dry powder
suspended in water. If the acidity does not fall within these limits it
must be corrected by soaking the powder before chroming for twenty
minutes in ten to twelve times its weight of water, to which the
requisite calculated quantity of standard alkali or acid has been
added. The hide powder must not swell in chroming to such an extent as
to render difficult the necessary squeezing to 70-75 per cent. of water,
and must be sufficiently free from soluble organic matter to render it
possible in the ordinary washing to reduce the total solubles in a blank
experiment with distilled water below 0.005 gm per 100 c.c. The powder,
when sent out from the maker, shall not contain more than 12 per
cent. of moisture, and shall be sent out in air-tight tins.
The detannisation shall be carried out in the following manner:--
The moisture in the air-dried powder is determined, and the quantity
equal to 6.5 gm. actual dry powder is calculated, which will be
practically constant if the powder be kept in an air-tight vessel. Any
multiple of this quantity is taken according to the number of analyses
to be made, and wet back with approximately ten times its weight of
distilled water. Two grammes per 100 of dry powder of crystallised
chromic chloride, CrCl_3.6aq., is now dissolved in water and made basic
with 0.6 gm. of Na_2CO_3 by the gradual addition of 11.25 c.c. of normal
Na_2CO_3, thus making the salt correspond to the formula
Cr_2Cl_3(OH)_3. In laboratories where analyses are continually being
made, it is more convenient to employ a 10 per cent stock solution, made
by dissolving 100 gm. of Cr_2Cl_6.6aq. in a little distilled water in a
litre flask and very slowly adding a solution containing 30 gm. of
anhydrous sodium carbonate, with constant stirring, finally making up to
the mark with distilled water and well mixing. Of this solution 20
c.c. per 100 gm., or 1.3 c.c. per 6.5 gm. of dry powder, should be
used. This solution is added to the powder, and the whole churned for
one hour. At the end of the one hour the powder is squeezed in linen to
free it as far as possible from the residual liquor, and washed and
squeezed repeatedly with distilled water, until, on adding to 50 c.c. of
the filtrate one drop of 10 per cent. K_2CrO_4 and four drops of
decinormal silver nitrate, a brick-red colour appears. Four or five
squeezings are usually sufficient. Such a filtrate cannot contain more
than 0.001 gm. of NaCl in 50 c.c.
The powder is then squeezed to contain 70-75 per cent, of water, and the
whole weighed. The quantity Q containing 6.5 gm. dry hide is thus found,
weighed out, and added immediately to 100 c.c. of the unfiltered tannin
infusion along with (26.5-Q) of distilled water. The whole is corked up
and agitated for fifteen minutes in a rotating bottle at not less than
60 revs. per minute. It is then squeezed through linen, the fitrate
stirred and filtered through a folded filter of sufficient size to hold
the entire filtrate, returning till clear. Sixty c.c. of the filtrate
is then evaporated and calculated as 50 c.c., or the residue of 50
c.c. multiplied by 6/5. The non-tannin filtrate must give no turbidity
with a drop of a solution of 1 per cent, gelatine and 10 per cent,
common salt. [Footnote: It is convenient for technical purposes to
employ the commercially obtainable chromed hide powder as prepared, for
instance, by the German Experimental Station at Freiberg, Saxony.]
One gramme of kaolin, freed from all soluble matter, may be added to the
filtrate, or it may be used by mixing it with the hide powder in the
shaking bottle.
The analysis of used liquors and spent tans shall be made by the same
methods as are employed for fresh tanning materials; the liquors being
diluted, are concentrated by boiling _in vacuo_, or in a vessel so
closed as to restrict access of air, until the tanning matter is if
possible between 3.5 and 4.5 gm. per litre, but in no case beyond a
concentration of 10 gm. per litre of total solids, and the weight of
hide powder used shall not be varied from 6.5 gm.
The results shall be reported as shown by the direct estimation, but it
is desirable that in addition efforts shall be made, by determination of
acids in the original solution and in the non-tannin residue, to
ascertain the amount of lactic and other non-volatile acids absorbed by
the hide powder, and hence returned as "tanning matters."
In the case of tanning materials it must be clearly stated in the report
whether the calculation is on the sample with moisture as received, or
upon some arbitrarily assumed percentage of water; and in that of
liquors whether the percentage given refers to weight or to grammes per
100 c.c., and in both cases the specific gravity shall be reported.
All analyses reported must be the average result of duplicate
determinations, which must agree in the case of liquid extracts within
0.6 per cent, and of solid extracts within 1.5 per cent, or the analysis
shall be repeated until such agreement is obtained.
All reports shall be marked: Analysed in accordance with the rules of
the S.L.T.C. (I.A.L.T.C.)--when the analyses have been carried out
according to the method described above.
As has been repeatedly emphasised in this treatise, the synthetic
tannins form a special class of substances, and the results obtained by
either of the two hide-powder methods do not give figures which are
always comparable to those of the natural tannins. An example of the
inapplicability of the methods where synthetic tannins are concerned is
illustrated by the behaviour towards hide powder of them when partly
neutralised to varying degrees: commercial Neradol D of acidity 1 gm.=
10 c.c. N/10 NaOH contains 33 per cent. tanning matters, completely
neutralised Neradol D, which exerts no true tanning action on pelt,
still contains 20 per cent tanning matter when analysed according to the
Official Method; a difference hence exists regarding the adsorption by
hide powder of a tannin and the adsorption of the latter by hide. As,
however, we are unable to make a distinction between these two different
properties by using hide powder only, we are also unable to draw the
factor into account.
Another source of error is the swelling influence on hide powder by
acids; for instance, an acid extract of vegetable tannins would show
higher tannin contents in the analysis than would the same extract when
less acid. The free sulphonic acid, however, is the active principle in
synthetic tannins, and since the latter always contain other acids (of
organic and inorganic origin) devoid of tannoid character, a source of
error is thus introduced, which we cannot eliminate by the present
method of analysis.
Of other methods of estimating the quality of a tanning material or
tanning extract the _determination of solubility_, _ash_, _colour_, and
_weight-giving properties_ in addition to the _firmness imparted to the
leather_ by the particular material are of importance. As regards the
synthetic tannins they are as a rule very soluble and it will generally
be found sufficient to subject them to the ordinary qualitative
examination. The ash determination in synthetic tannins, on the other
hand, is not of such value as in the case of natural tanning
extracts. From their composition we know that synthetic tannins contain
considerable quantities of mineral salts, the presence of some of which
on the one hand emphasises their pickling effect, and that on the other
hand the property of dissolving phlobaphenes exhibited by the synthetic
tannins is closely connected with their salt contents.
A colour determination of synthetic tannins is not of much importance,
since synthetic tannins nearly always impart a white or light brown
colour to the hide. In those cases only where coloured decomposition
products appear as a result of intermediary reactions, may the former
impart greyish or dirty colorations of little beauty to the hide. This
is easily ascertained by lightly tanning a pelt.
The determination of the weight and solidity-giving properties is
important both for leathers tanned with vegetable tanning extracts and
for those treated with synthetic tannins, but the results obtained when
using animalised cotton are not directly convertible into figures
required for practical purposes. Comparative figures are better
obtained by actually tanning pieces of pelt on as practical a scale as
is possible, and testing the weights and tensile strengths of the pieces
as against those of the original pelts, whereby in the former case the
yield (pelt --> leather) is obtained.
Its capability as a tanning agent may be ascertained by submitting the
synthetic tannin to an actual test tannage. The latter is carried out
by introducing the dilute extract into open glass jars, holding about
400 c.c. at a width of about 8 cm. [Footnote: Accumulator jars are
excellent for the purpose.--_Transl_.] The concentration of the
solution is chosen according to acidity and salt contents of the
synthetic tannin, the most suitable being 1.5°-2.5° Bé. A piece of bated
pelt is suspended in the liquor in such a way that the pelt is
completely surrounded by liquor, without, however, being creased or
touching the bottom. If the pelt were creased during tannage, the
wrinkles would become fixed and would show in the finished leather. Thus
an unfair judgment of the extract would be delivered, since similar
results are produced by liquors which are either too concentrated or are
not properly composed, and naturally this property of an extract would
be greatly to its disadvantage.
The various stages of tannage may be judged from various standpoints
when examining the pelt as tannage proceeds. On the one hand, the
surface of the but slightly porous pelt is altered so as to present a
more porous appearance, which is now rendered more capable of absorbing
liquids. On the other hand, a similar alteration takes place _within_
the pelt, to the extent to which the tanning matter has penetrated it.
How far the penetration has proceeded is easily determined by utilising
the different adsorption of coal-tar dyes by untanned and tanned pelt
(see p. 121). An indicator for those synthetic tannins, which are
derived from the phenols, is ferric chloride, which only colours those
parts of the pelt which have been penetrated by the synthetic tannins;
clearer and better results are, however, obtained when the dyestuffs
referred to above are employed.
As soon as the tanning matter has completely penetrated the pelt, the
total time of tannage is noted, and the velocity with which the tanning
matter converts the pelt into leather at that particular concentration
is thus obtained. The tannage completed, the leather must be well washed
in running water to remove excess of synthetic tannin and then dried. On
examining the dry leathers, the colour may then be observed, and a cut
will give an idea of the tensile strength and the length of fibre of the
leather. The tensile strength is, however, not of much value in such a
barely tanned leather and cannot be compared with that obtained in
leathers tanned on a practical scale. The length of fibre is, however,
of some importance, since a special feature of finished leathers tanned
with synthetic tannins is the beautifully long fibre--a property which
manifests itself when the leather is torn and in which an expression of
the quality of the synthetic tannin may be found.
Similarly, tanning experiments combining synthetic and natural tannins
may be carried out, the most interesting features of these being the
different proportions in which the two products are mixed. Such
experiments may be done, for instance, by preparing 2° Bé. solutions of
each extract and then mixing them in proportions of, say, 10:90, 20:80,
30:70, etc. Here it is again possible to infer the _tanning intensity_
of the synthetic tannin from the concentration and the time used for
tannage.
A further determination of the quality of a synthetic tannin is the
capability of the latter of dissolving or precipitating the natural
tannins. As is well known, synthetic tannins frequently possess the
practically important property of rendering natural tannins easily
soluble in water. In some cases, however, synthetic tannins appear to
solubilise natural tannins in concentrated solutions; when, however, the
latter are diluted, the natural tannin is precipitated with varying
completeness, the reason of which is often the presence of excessive
acid or the presence of such salts as have no phlobaphene-solubilising
properties.
For practical purposes this determination may be carried out by mixing,
in different proportions, concentrated tannin solutions and the
synthetic tannin; heating the mixture on the water bath for a short
time, cooling and finally diluting 10, 20, and 30 gm. of the mixture to
100 c.c., which are then left in measuring cylinders for twelve to
twenty-four hours; the amount deposited will then be an indication of
the solubilising or precipitating effect exhibited by the synthetic
tannin.
Other properties of the synthetic tannins connected with their practical
application will be discussed in Part II. of this treatise.
PART II
SYNTHETIC TANNINS: THEIR INDUSTRIAL PRODUCTION AND APPLICATION
With regard to their _industrial production_, but few synthetic tannins
are, to-day, of practical and commercial interest. In addition to
simplicity in the method of manufacture a certain degree of purity of
the raw materials constitutes the criterion of their suitability. The
methods of manufacture, of which nearly all are the property of the
B.A.S.F., have been so worked out that the production of synthetic
tannins presents no difficulties on a practical scale. Cresols,
naphthalenes, and higher hydrocarbons are used as starting materials in
the production of synthetic tannins; the former substances or their
oxidation products are sulphonated by means of concentrated sulphuric
acid, and the tanning matter produced by condensing the sulphonic acids
with formaldehyde. The crude synthetic tannin thus obtained has yet to
be diluted and partly neutralised before it can be applied in practice,
and this is carried out by mixing the crude product with strong caustic
lye. By these means the high acidity is reduced to a suitable degree
learned from experience on the one hand; on the other hand, the salts of
the sulphonic acids form valuable components of the commercial synthetic
tannins.
The first product placed on the market was named _Neradol D_; this
represents the condensation product of cresolsulphonic acid. The second
synthetic tannin was _Neradol N_, which represents the condensation
product of naphthalenesulphonic acid; when diluted and neutralised to
the same extent as is done in the case of Neradol D, the product is
named _Neradol N D_. The latest synthetic tannin has been called
_Ordoval G_, the starting material of which is a still higher
hydrocarbon.
The tannoid-chemical properties of these synthetic tannins have been
exhaustively determined by the author, who employed Neradol D, which is
most suitable for such a purpose, and the investigations relating to it
will now be treated fully in the following chapters. The two other
synthetic tannins exhibit very similar properties, but their few
characteristics shall be shortly dealt with.
The condensation product obtained by the method described on p. 55 forms
a viscous, dark coloured mass, the analysis of which by the shake method
gives the following figures:-
Tanning matters 62.6 per cent.
Non tannins 6.4 "
Insolubles 0.0 "
Water 31.0 "
---------------
100.0 per cent.
Acidity: 1 gm. = 40 c.c. N/10 NaOH.
According to its chemical constitution, this product may
be considered to be dinaphthylmethanedisulphonic acid.
Samples of this crude, strongly acid material were partly
neutralised, and the following figures obtained on analysis:--
Acidity. Tanning Soluble Water.
Matters. Non-tans.
Per Cent. Per Cent. Per Cent.
1 gm. = 35 c.c. N/10 NaOH 61.8 7.0 31.2
1 " = 30 " " 58.9 7.1 34.0
1 " = 25 " " 50.1 7.9 42.0
1 " = 20 " " 42.2 8.9 48.9
1 " = 15 " " 37.4 10.4 52.2
1 " = 10 " " 31.6 13.6 54.8
1 " = 5 " " 26.3 16.6 57.1
Experimental tanning tests which were carried out with the various
partly neutralised samples yielded leathers which, on an average, were
nearly white, but which in comparison with a leather tanned with Neradol
D appeared rather more greyish and were much harder.
A solution of the half-neutralised substance (1gm. = 20 c.c.
N/10 NaOH) gives the following reactions:---
Gelatine--Precipitate, partly soluble in excess tannin solution.
Ferric chloride-----No coloration.
Barium chloride-----Precipitate, insoluble HNO_3.
Bromine water-----No reaction.
Silver nitrate-----No reaction.
Aniline hydrochloride----Precipitate, dissolves when solution
is heated.
This condensation product is very soluble in water, but insoluble in
most solvents, excepting methyl and ethyl alcohols. The above reactions
show the similarity of this dinaphthyl derivative to the dicresyl
derivative, and the absence in the former of characteristic reactions
with iron salts is mainly accounted for by its lack of phenolic groups.
The absence of this reaction does not, of course, influence the tannoid
character of dinaphthylmethanedisulphonic acid in the least, and is of
no importance in practice, since the various stages of tannage may be
demonstrated by means of a solution of indigotine.
From a technical point of view the absence of this reaction is
advantageous to this extent, that it eliminates the exceedingly great
care to avoid the contact of tan liquors and tanned pelt with iron
particles which has to be observed when tannins of phenolic character
are employed.
In a chemical and technological evaluation of this tanning matter, all
those details apply which will be described when discussing Neradol
D. The most important advantage possessed by this tanning matter, from a
commercial point of
view, is the lower price which it owes to the greater ease with which
naphthalene may be obtained.
By treating the non-condensed crude product with barium chloride, a
product completely devoid of sulphuric acid is easily obtained; the
contents of sulphuric acid calculated as BaSO_4 is about 9.5 per
cent. This value is higher than that found by Neradol D, and may be
explained by the fact that a slight excess of sulphuric acid is
necessary for the preparation of [Greek: b]-naphthalenesulphonic acid.
Comparative tanning tests using products containing sulphuric acid and
products free from sulphuric acid (neutralised to the same degree of
acidity) yielded leathers which were very similar; the liquor containing
no sulphates yielded slightly softer leather than that obtained from a
liquor containing sulphates.
An experiment was also carried out, using a liquor containing the tannin
completely neutralised with caustic soda and subsequently acidified with
acetic acid till the acidity of 1 gm. = 10 c.c N/10 NaOH; here, again,
no essential difference could be detected in the leather as compared
with that from a liquor containing sulphates.
One of the most striking properties of this tanning matter is its
solubilising effect on natural tannins and the phlobaphenes; this
property may mainly be compared to the similar one of other condensed
sulphonic acids in their behaviour towards natural tannins.
If, therefore, natural tannins are mixed with this product and the
solution used for tanning purposes, the resultant leather will possess a
dark colour owing to the presence of solubilised phlobaphenes; if, on
the other hand, a dark coloured leather, which has been tanned with
natural tannins, is washed over with a 5° Bé solution of this synthetic
tannin, or immersed for some time in the solution, the leather assumes a
lighter colour owing to the phlobaphenes being dissolved and removed
from the leather by the synthetic tannin.
The presence of Neradol ND in leathers is detected by methods to be
described under Neradol D (_cf_. p. 108). The oxyazo reaction only
succeeds when the solution has been boiled with a few drops of
hypochlorite solution, quickly cooled and excess of ammonia added. When
applying the indophenol reaction, the solution must be treated as
follows: 3-4 drops of hypochlorite solution is added, and the solution
heated for a short time; or 5-6 drops hypochlorite solution may be
added, and the solution left for some time, in which case the heating
may be omitted. The solution is then made distinctly ammoniacal, 1-2
drops of dimethyl-_p_-phenylenediamine solution and a layer of alcohol
poured on the top. In most cases a blue coloration will appear; the
addition of 1-2 drops of potassium ferricyanide solution with formation
of a blue coloration indicates the presence of Neradol ND without fail.
The fact that a product possessing tanning properties may be obtained by
condensing [Greek: b]-naphthalenesulphonic acid makes it interesting to
investigate the behaviour of a non-condensed [Greek:
b]-naphthalenesulphonic acid towards pelt. The following solutions were
allowed to act upon pelt for twelve days:--
(1) Concentrated solution of [Greek: a]-naphthalenesulphonic acid (10° Bé).
(2) " " [Greek: b]- " " (6° Be.)
(3) " " 2,7- " " (18° Bé.).
Solution 1 swells the pelt to a considerable extent without, however,
solubilising it. Solution 2 produces a similar effect. Solution 3
dissolves the pelt appreciably on the first day; after six days,
solubilisation is complete. The reason of this different behaviour of
the mono- and disulphonic acids is mainly to be sought in their
difference of solubility; the monosulphonic acids are not very soluble,
and are only capable of giving solutions measuring 10° and 6° Bé,
respectively, whereas the disulphonic acid yields an 18° Bé solution, in
addition to which the much higher acidity of the latter quickly
gelatinises the pelt.
As regards the capability of the naphthalenesulphonic acids of
dissolving phlobaphenes, the following results were obtained:--solid
Argentine quebracho extract was mixed with--
5 percent, [Greek: a]-naphthalenesulphonic acid: opaque sol.,
large quantity of insolubles.
10 " " " lesser " "
20 " " " no insolubles.
30 " " " "
5 " [Greek: b]-naphthalenesulphonic acid: opaque sol.,
lesser quantity of insolubles.
10 " " " "
20 " " " clear solution, no insolubles
30 " " " "
5 " 2,7-naphthalenedisulphonic acid: opaque sol.,
large quantity of insolubles.
10 " 2,7 " " as above.
20 " 2,7 " " slightly opaque, some insolubles.
30 " 2,7 " " nearly clear solution, no insolubles.
It is hence clear that the [Greek: b]-sulphonic acid possesses
phlobaphene-solubilising qualities greater than those of the [Greek:
a]-sulphonic acid or the disulphonic acid; the Greek: b]-sulphonic acid
was therefore made the subject of Ger. Pat., 181,288 (8th February
1917).
The synthetic tannin, _Ordoval G_, is the formaldehyde condensation
product of higher hydrocarbons (mainly _retenes_), and is a partly
neutralised product containing no sulphuric acid. The author's analysis
gave the following figures:--
Tanning matters 10.7 per cent.
Soluble non-tannins 16.4 "
Insolubles 0.0 "
Water 73.0 "
Acidity: 1 gm. = 4 c.c. N/10 NaOH.
Density: 23° Be.
Ordoval G is completely soluble in water and glacial acetic acid. Only
its organic constituents are soluble in alcohol, ethyl acetate, and
acetone, whereby a dark coloured crystalline mass separates. Ordoval G
is insoluble in benzene.
The aqueous solution of Ordoval G gives the following reactions:--
Gelatine Moderate flocculent precipitate.
Ferric chloride Darkish coloration.
Potassium dichromate No reaction.
Aniline hydrochloride Dark brown precipitate.
Formaldehyde hydrochloric acid No precipitate.
Bromine water No reaction.
Zinc acetate Very slight opalescence.
Barium chloride Slight opalescence.
Its capability of solubilising and consequent saving of natural tannins
is shown by the fact that 100 kilos of vegetable tanning material may be
substituted by 40 kilos of Ordoval G and the material in question in
order to obtain the entire tanning intensity of the latter.
In one respect--that of its salts--Ordoval G differs from the Neradols;
whereas the chromium and aluminium salts of the latter possess no such
tannoid properties as will make the resultant leather exhibit any of the
characteristics of either tannage, it is possible to carry out combined
tannage with a mixture of Ordoval G and metallic salts. Tanning
experiments carried out with the chromium, iron, aluminium, and calcium
salts of Ordoval G yielded leathers which possessed proportionate
characteristics of either kind of tannage to the extent to which either
material was present. This combination tannage seems to be assured of a
great future; especially may a combination tannage of iron salts and
Ordoval G eventually entirely replace chrome tannage.
The detection of Ordoval G in leather is carried out as follows: 10
gm. of leather are boiled with 150 c.c. of acetic acid, a solution of 25
gm. of CrO_8 in 25 c.c. of a 50 per cent, solution of acetic acid
gradually added, and the mixture boiled for three hours, till the
leather is decomposed and the solution has assumed a brown instead of
the original light yellow colour. The solution is then evaporated, the
residue dissolved in 600 c.c. hot water, and the chromium precipitated
with a 40° Bé. solution of caustic soda. The solution is filtered and
cooled, and a little hydrosulphite is added to 20 c.c. of the cold
alkaline filtrate; in the presence of Ordoval G, a red colour will
appear (oxanthranolsulphonic acid).
Brief mention must be made of the so-called _Corinal_ [Footnote: Swiss
Pat, 78,282, 78,797, 79,39.] a synthetic tannin placed upon the market
by Chem. Fabrik Worms A.-G., in Worms-on-the-Rhine. It is a viscous,
brown fluid, containing the aluminium salts of the tannoid acids. The
latter are formaldehyde-condensation products of sulphonated tar oils,
or the hydroxylated derivatives of the latter. The density being 33° Bé,
it contains 28.1 per cent. tanning matters, 13 per cent. soluble
non-tannins, and 10.8 per cent. inorganic matter (3.2 per cent. Al_2O_3
and 7.6 per cent. Na_2SO_4.
A similar product, containing chrome salts as base, is the so-called
ESCO-EXTRACT, [Footnote: Schorlemmer, _Collegium_, 1917, 124]
manufactured by the Chem. Fabrik Jucker & Co. in Haltingen
(Baden). This product is a dark, reddish-brown fluid, possessing acid
reaction, which strongly precipitates gelatine. Analysed by the filter
method it contains 12-15 per cent. tanning matters, 17-20 per cent.
soluble non-tannins, and 18 per cent. ash, of which 3 per cent. is
Cr2O_3. This synthetic tannin may be employed alone or in conjunction
with other tannins, and yields a leather similar to that obtained by
chrome tannage.
A. Condensation of Free Phenolsulphonic Acid
In practice, the results of condensing phenolsulphonic acid with
formaldehyde are manifold, according to whether these materials are used
in their concentrated or dilute state; whether they interact in the cold
or when heated; or whether their interaction is gradual or rapid.
1. If a moderately dilute solution of phenolsulphonic acid (1:1) is
mixed with one-sixth of its volume of a dilute formaldehyde solution (1
part 30 per cent. HCHO solution plus 3 parts of water) in the cold, with
continuous stirring, the solution remains clear and assumes a brown
colour. When left several hours, a light, white flocculent precipitate
deposits, which increases in quantity on diluting with water. The
solution precipitates gelatine; the flocculent precipitate is easily
soluble in hot caustic soda solution, and, when subsequently neutralised
with acetic acid, precipitates gelatine.
If equal parts of dilute phenolsulphonic acid and dilute formaldehyde
(concentrations as above) are gradually mixed in the cold, whilst
stirring, the mixture soon becomes opalescent, and a flocculent deposit
separates after eighteen to twenty-four hours.
These experiments carried out on the water bath immediately yield
opalescent liquids, from which an insoluble, brown, gluey, and very
sticky mass separates after twenty-four hours; the latter is sparingly
soluble in alkalies, partly so in organic solvents.
2. If a moderately dilute solution of phenolsulphonic acid (1:1) is
gradually mixed with one-sixth of its volume of a concentrated (30 per
cent.) formaldehyde solution in the cold, whilst stirring, slight
opalescence immediately results, and a flocculent deposit separates
after about twenty minutes, which gradually increases in quantity during
the next few hours. If the volume of formaldehyde is increased to the
same as that of phenolsulphonic acid solution, the flocculent deposit
immediately separates, and after twenty-four hours a brown, gluey, and
very sticky mass--of the same solubility as that described in the
previous experiment--is to be found at the bottom of the vessel used.
It should be noted that in both these experiments with concentrated
formaldehyde solution a slight increase in temperature occurs
concurrently with the process of condensation. If the experiments are
carried out on the water bath, a gelatinous mass is instantly formed,
which assumes the colours of grey, dirty light violet and dark violet,
in the order named, and which, whilst left several hours--or when heated
on the water bath--is suddenly converted into the insoluble, brown,
gluey mass above referred to.
3. If, for the purpose of condensation, phenolsulphonic acid to which 10
per cent, of water has been added, is employed, the reaction proceeds
very quickly and energetically. If one-sixth of its volume of
formaldehyde (1:3 of the 30 per cent. solution) is added drop by drop to
a cold solution of phenolsulphonic acid, a reddish, milky solution
results, which assumes a slightly lighter colour on addition of more
formaldehyde and deposits an insoluble flocculent precipitate. If the
solution is kept below 45° C., by artificial cooling, the light colour
is maintained, but a gelatinous precipitate is soon formed, the
viscosity of which increases on stirring, and finally is converted into
an insoluble, tough, gummy mass. If, on the other hand, the mass is
heated at the beginning of the reaction, or if the amount of
formaldehyde is increased and the mass cooled during reaction,
effervescence occurs, and a cheesy, dirty-coloured mass results, which,
on cooling, rapidly becomes solid and yields a very firm, elastic,
rubbery mass, which is absolutely insoluble in water.
4. The condensation proceeds exceedingly violently when concentrated
phenolsulphonic acid is acted upon by one-sixth of its volume of
formaldehyde. If the latter is firstly added drop by drop to the
phenolsulphonic acid, a gel immediately results, the temperature of
which quickly increases on further addition of formaldehyde and suddenly
boils over, yielding a reaction product which, when cooled, forms a
dirty violet, firm, elastic, and rubbery mass, insoluble in alkalies and
hardly affected by organic solvents.
Finally, if the amounts of concentrated phenolsulphonic acid and
formaldehyde stated above are mixed, strong effervescence occurs and
heat is evolved, and a dirty blackish-violet mass is instantly formed
which, on cooling, yields a rather brittle, hard product insoluble in
water.
5. Totally different end-products are, however, obtained when the
addition of formaldehyde (30 per cent.) in the proportion of one-sixth
of the volume of dilute phenolsulphonic acid (1 plus 9 aq.) to the
latter is extended over several hours. In this case a slightly
opalescent liquid is obtained which, when left twelve hours, is
transformed into a brown mass soluble in water, which strongly
precipitates gelatine and possesses tanning properties. Hence direct
tannoid substances are obtained by this method of condensation.
Whereas no direct tanning experiment can be carried out with the
insoluble compact mass obtained in the preparations described above on
account of their absolute insolubility, it is still possible to carry
out tanning experiments with opalescent colloidal solutions in the
following ways:--
(a) If a bated pelt is immersed in a liquid containing a condensation
product obtained by gradually mixing a moderately dilute solution of
phenolsulphonic acid and a dilute solution of formaldehyde, the pelt is
rapidly tanned on the surface. Complete penetration of the substance
does not occur even after several days, since the strong acidity of the
solution causes a strong swelling of the pelt.
(b) If a pelt is shaken for six hours in a shaking apparatus containing
the liquid mentioned under (a), tannage again only takes place on the
surface, penetration being impeded by the strong swelling effect of the
liquid. Repetition of the latter two experiments, with the addition of
15 per cent, common salt, increases the tanning effect to some extent;
the pelt, however, is not tanned through, but the non-tanned layers may
be clearly seen to be pickled.
The tanning effects described above are only exhibited when the
colloidal tan-liquor is present in great excess over the pelt, since the
former obviously only contains small amounts of tanning matter, and even
the presence of common salt does not bring about complete tannage of the
pelt.
In order to prove the presence of "tanning matters" in the liquid
described above, several freshly prepared samples of the latter were
analysed by the shake method of analysis without being first filtered
and the following figures obtained:--
1. 2. 3. 4.
Per Cent. Per Cent. Per Cent. Per Cent.
Tanning matters 6.4 7.7 8.2 9.1
Soluble non-tannins 15.2 17.4 14.5 11.8
These condensation products suspended in water all precipitate gelatine
strongly and leave behind a perfectly clear liquid. In all cases, an
intense blue colour was obtained on adding ferric chloride, a slight
precipitate only was obtained with aniline hydrochloride, and bromine
was rapidly absorbed with the separation of an insoluble white deposit.
The condensation products obtained by the interaction of dilute
solutions of phenolsulphonic acid and formaldehyde at moderately high
temperature, which form slimy masses and are insoluble in water, are
soluble in alcohol. An alcoholic solution of such a product was used in
a tanning experiment, and a piece of pelt immersed in the solution was
tanned through in a few days; the resultant leather being rather firm,
springy, and slightly hard, and the colour was a light brownish-grey.
All those condensation products which are easily or partly soluble in
alcohol dissolve in caustic soda, sodium carbonate, in some cases also
in borax and sodium sulphite. They are rendered soluble with greater
ease when the _freshly prepared_ solution is heated on the water bath
with the alkali; the alkaline solution, neutralised as far as is
possible with acetic acid, yields light brown coloured solutions, the
tanning effects of which have proved very satisfactory. Leathers tanned
in such solutions, however, are rather empty and hard, possess but
little resilience and an uneven, dirty greyish-brown colour.
A sample of such a product, as nearly as possible neutralised with
acetic acid, contained 14.8 per cent. tanning matters, by the shake
method of analysis.
B. Condensation of Partly Neutralised Phenolsulphonic Acid
Attempts were made at condensing partly neutralised phenolsulphonic
acid; the latter was obtained by mixing equal quantities of
phenolsulphonic acid and sodium phenolsulphonate (prepared by exactly
neutralising phenolsulphonic acid with a concentrated solution of
caustic soda).
The consequent dilution and decrease in acidity, however, considerably
diminished the velocity of the reaction. Hence, if the half-neutralised
Solution A1 (_cf_. p. 98) is diluted with water, taking equal volumes,
and one-sixth of the volume of dilute formaldehyde (1:3) gradually added
in the cold, condensation is not induced. When heated several hours an
opalescent liquid results from which, however, no flocculent deposits
separate when left for some time. Using a concentrated solution of
formaldehyde (Experiment A2, p. 98) in the cold produces no reaction,
but after heating for a time an opalescent liquid is obtained. Both
liquids give only slight precipitates with gelatine. Excess formaldehyde
does not influence the reaction.
A repetition of Experiment A3 (_cf_. p. 99), using the above
half-neutralised phenolsulphonic acid, similarly required heat to induce
condensation, when a milky liquid of light reddish colour resulted.
Whereas the addition of formaldehyde to non-neutralised concentrated
phenolsulphonic acid caused violent reaction, this proceeded very slowly
in the case of half-neutralised phenolsulphonic acid, resulting in the
formation of a semi-solid mass, which on heating became more viscous,
and finally, when left twenty-four hours, became a solid, compact,
insoluble mass possessing a dirty light violet colour.
Tanning experiments with these opalescent solutions proved them to exert
a rapid penetration on the surface, complete tannage, however, taking
place after eight days only, when a flat, greyish-coloured and rather
hard leather resulted.
C. Condensation of Completely Neutralised Phenolsulphonic Acid
If concentrated phenolsulphonic acid is gradually neutralised with
concentrated caustic soda solution till the former is faintly alkaline,
the sodium salt thus obtained is not so easily condensed with
formaldehyde as is the case with the free acid.
1. If formaldehyde is gradually added to the neutralised phenolsulphonic
acid in the cold, opalescence immediately results; on addition of water,
the liquid assumes a milky appearance. On adding gelatine to this
liquid, a slimy precipitate is thrown down, leaving a slightly
opalescent liquid.
2. If formaldehyde is added to neutralised phenolsulphonic acid whilst
it is heated on the water bath, a slimy mass instantly separates, which
on cooling solidifies and forms a greyish-blue brittle mass, insoluble
in water and but sparingly soluble in alcohol; the alcoholic solution is
capable of converting pelt into leather.
The filtrate from the solidified mass strongly precipitates gelatine,
whereas the insoluble condensation product is soluble in caustic soda;
this alkaline solution also precipitates gelatine and the addition of
acetic acid transforms the mixture into the gel state.
If the insoluble condensation product is dissolved in warm concentrated
sulphuric acid, the solution remains clear upon the addition of water,
but does not precipitate gelatine. If, finally, this solution is
neutralised with caustic soda, the solution remains clear and
precipitates gelatine strongly.
D. Condensation of Cresolsulphonic Acid
Experiments were carried out with the object of condensing _o_-, _m_-,
and _p_-cresolsulphonic acids with formaldehyde in various ways; no
essential differences could be detected as regards the mode of reaction
or the properties of the intermediary and end-products as compared to
those of phenolsulphonic acid. Similarly, condensation of different
samples of crude cresol containing varying quantities of _o_-, _m_-, and
_p_-cresol did not yield end-products sufficiently different to justify
describing them in detail.
E. Relative Behaviour of an Alkaline Solution of Bakelite and Natural
Tannins
Phenolsulphonic acid was condensed with a little formaldehyde, and the
reddish pasty condensation product dissolved in caustic soda. This
alkaline solution of bakelite was exactly neutralised with acetic acid
and mixed with strong solutions of an untreated quebracho extract. It
was observed that the solubility of the quebracho extract was not
increased by this treatment, but the faintly acidic character of the
natural tannin caused the bakelite to be thrown down as an insoluble
precipitate.
Crude phenolsulphonic acid, when added to a solution of the quebracho
extract referred to, does not increase the solubility of the latter,
which even deposits considerable amounts of insoluble tannin particles.
Quite different properties are exhibited by sodium phenolsulphonate,
which completely converts quebracho tannin into a water-soluble
substance, the aqueous solution of which deposits no insolubles. The
partly neutralised condensation product of phenolsulphonic acid and
formaldehyde exhibits similar properties [Footnote: Grasser, _Collegium_,
1913, 521, 478.] (see later).
F. Dicresylmethanedisulphonic Acid (Neradol D) [Footnote: Ger, Pat.,
291, 457; Austr. Pat., 61, 057.]
Neradol D is a viscous liquid, measuring about 33° Bé., which is similar
to extracts of natural tannins. One of its characteristics is its
phenolic odour; it is completely soluble in water, forming a clear,
semi-colloidal solution, but is insoluble in all organic solvents with
the exception of alcohol, glacial acetic acid and ethyl acetate, which
dissolve all but its inorganic constituents. The latter owe their
presence to the neutralisation of the crude Neradol with caustic soda,
and are composed of sodium salts of the sulphonic acid in addition to
Glauber salts.
The aqueous solution of Neradol D shows properties similar to those
exhibited by solutions of natural tannins and reacts as
follows:--[Footnote: Grasser, _Collegium_, 1913, 520, 413.]
Methyl orange Acid reaction.
Barium chloride White precipitate, insoluble in HNO_3.
Ferric chloride Deep blue coloration.
Silver nitrate Slight opalescence.
Bromine water No precipitate.
Formaldehyde hydrochloric acid No precipitate.
Gelatine Complete precipitation.
Aniline hydrochloride Strong precipitate.
The reactions with ferric chloride and gelatine should be especially
noted, since they are analogous to those given by natural tannins. On
the other hand, the reactions with BaCl_2, bromine water and formaldehyde
hydrochloric [Footnote: Stiasny carries out the reaction with
formaldehyde-hydrochloric acid as follows:--50 c.c. of the tannin
solution, plus 5 c.c. concentrated hydrochloric acid and 10
c.c. formaldehyde (40 per cent.) are heated under reflux condenser for
ten minutes; most natural tannins are completely precipitated
(_Collegium_, 1906, 435; 1907, 52 _et_ 188).] acid prove the different
chemical composition of Neradol D as compared to that of the natural
tannins.
The fact that a positive reaction is given with aniline
hydrochloride [Footnote: This reaction is carried out as follows:--5
c.c. of the tannin solution to be examined (about 4 gm. tanning matter
per litre) are shaken violently in a test tube with 0.5 c.c. aniline and
2 c.c. concentrated HCl added. All natural tannins are unaffected by
this treatment, ligninsulphonic and other sulphonic acids cause
opalescence. _Note_.--Employing formic acid in lieu of hydrochloric
acid (Knowles) renders the reaction no more reliable.--_Transl_.] is
very puzzling; none of the natural tannins are precipitated by this
reagent, but only sulphite cellulose on account of its content of
ligninsulphonic acid. One is justified in assuming that there is at
least some connection between the constitution of ligninsulphonic acid
and that of dicresylmethanedisulphonic acid.
Stiasny [Footnote: _Collegium_, 1913, 516, 142.] recommends the
following reaction for the detection of and differentiation between
Neradol D and wood pulp extract:--10 c.c. of a 5 per cent. solution of
the extract to be analysed are violently shaken with 1-2 drops of a 1
per cent. alum solution and about 5 gm. of ammonium acetate. If only
Neradol D is present no precipitate separates even after twenty-four
hours, but if wood pulp be present, a precipitate is thrown down in a
quantity corresponding to the amount of wood pulp present.
The official analysis gives the following figures:
[Footnote: Grasser, _loc. cit._]
Tanning matters 32.5 per cent.
Soluble non-tannins 33.0 "
Insolubles 0.0 "
Water 34.5 "
-------------
100.0 per cent.
Ash 17.0 "
Acidity: 1 gm. = 10 c.c. N/10 NaOH.
Density: 33º Bé.
A comparison of its quantitative analysis to that of a natural tanning
extract is illustrated by the following figures of a chestnut and a
quebracho extract of same density (26º Bé):--
Chestnut Quebracho
Per Cent. Per Cent.
Tanning matters 32.0 34.0
Soluble non-tannins 12.0 8.0
Insolubles 1.5 2.0
Water 54.5 56.0
----- -----
100.0 100.0
Ash 0.4 2.0
This comparison shows that extracts of natural tannins firstly contain
certain amounts of "insolubles," whereas Neradol is completely soluble
in water, forming a clear solution; secondly, natural tanning extracts
contain smaller quantities of soluble non-tannins, consisting of
colouring matter and sugars, in addition to small quantities of mineral
matters (ash). Neradol D contains considerable amounts of soluble
non-tannins, derived from salts of sulphonic and sulphuric acids, again
offering a satisfactory explanation of the high ash. If, therefore, a
mixture of Neradol D and a natural tanning extract was submitted to a
quantitative analysis, the higher non-tannins and the high ash would
indicate the presence of Neradol D, provided that wood pulp or a highly
sulphited extract were not components of the mixture.
The chemical reactions taking place in the preparation of
Neradol D may be expressed thus:-
OH OH OH OH
H ^ H___O_____H ^ H H ^ H__CH_2__H ^ H
| | || | | = | | | | + H_2O
| | CH_2 | | | | | |
H v CH_3 CH_3 v H v CH_3 CH_3 v H
HSO_3 HSO_3 HSO_3 HSO_3
1. Neradol D Reactions
1. The quantitative determination of phenols introduced by Bader,
[Footnote: _Bull. soc. scient., Bucarexi_, 1899, 8, 51.] which consists
in precipitating them as oxyazo compounds, has been modified by Appelius
and Schmidt [FootNote: _Collegium_, 1914, 597.] for the purpose of
detecting Neradol D:--To 50 c.c. of the tannin solution (analytical
strength) 15 c.c. of diazo solution are added, the mixture filtered, if
necessary, and the filtrate made alkaline with caustic soda; in the
presence of Neradol D in sufficient quantity, a blood-red coloration
results. If but little Neradol D be present, the procedure is altered as
follows:--The tannin solution, to which the diazo solution has been
added, is filtered, and the filtrate poured on a piece of filter paper
which is then dried; a solution of caustic soda is spotted on the paper,
when, if Neradol D be present, a red-edged spot will appear.
According to Tschirch and Edner, [Footnote: _Archiv. d. Pharm_., 1907,
150.] the diazo solution is prepared as follows:--5 gm._p_-nitraniline
are introduced into a 500 c.c. measuring flask, 25 c.c. of water and 6
c.c. concentrated sulphuric acid added, the mixture shaken and a
solution of 3 gm. of sodium nitrite in 25 c.c. of water plus 100 c.c. of
water added, and the whole then filled up to 500 c.c. The solution
should be stocked in the dark.
2. A less sensitive reaction for Neradol and wood pulp extract
constitutes that of Appelius and Schmidt employing cinchonine,
[Footnote: _Collegium_ 1914, 597.] while the presence of the substances
in question yields characteristic precipitates.
3. Seel and Sander [Footnote: _Zeits. f. ang. Chem._, 1916, 333.]
recommend the following method of detecting Neradol D:--
_(a) Oxyazo Reaction_.--About 5 c.c. of the tannin solution are rendered
alkaline with caustic soda; after cooling with ice, about half the
volume of alcohol is added. 3-4 drops of diazo solution are then
added. Frequently, this results in the solution assuming a blue
coloration. If not, the solution is acidified with hydrochloric acid,
ether added, and the mixture well shaken. The water is now separated
from the mixture, fresh water added, together with some caustic soda
solution, when, if Neradol D be present, the salt of the colour acid
formed dissolves in the water with a beautiful green or bluish-green
colour. At the place of contact of the water and the ether a
bluish-green ring appears.
The diazo solution is prepared by dissolving _p_-aminophenol or its
hydrochloric in a little dilute hydrochloric acid, cooling in ice and
carefully diazotising in the cold till a slight excess of nitrous acid
is present. It is essential that this solution should be tested before
use, and this is carried out by coupling it with an alkaline phenol
solution; if a dark blue oxyazo colour is formed, the solution may be
used. It must be kept cool by surrounding it with ice.
_(b) Indophenol Reaction_.--To 5 c.c. of the solution to be tested, a
drop of a solution of dimethyl-_p_-phenylenediamine is added, the
solution rendered alkaline with caustic soda and 1-2 drops of a 5 per
cent. solution of potassium ferricyanide added. If Neradol D be present,
a blue colour appears, either immediately or after some time. The
reaction is rendered more sensitive if alcohol is carefully poured on
the solution after it has been rendered alkaline, and potassium
ferricyanide is then added. At the place of contact a blue layer is
formed, which ultimately diffuses into the alcohol.
According to Lauffmann [Footnote: _Collegium_, 1917, 233.] the presence
of natural tannins as well as that of wood pulp diminishes the
sensitiveness of the reactions described above; [Footnote:
_Zeits. f. ang. Chem._, 1916, 333.] this investigator recommends a
modification of these reactions.
2. Electro-Chemical Behaviour of Neradol D
The author's investigations of the electro-osmosis of an aqueous
solution of Neradol D [Footnote: _Collegium_, 1920, 597, 24.] proved
that dicresylmethanedisulphonic acid exhibits anodic migration; hence
this product possesses negative charge and acidic character. The
impurities accompanying the synthetic tannin, _i.e._, salts, free
sulphuric acid, and some phenols, migrated anodic and cathodic
respectively, according to their charges. A Neradol D purified by
electro-osmosis finally yielded a pure solution of
dicresylmethanedisulphonic acid, which precipitated gelatine and
exhibited pronounced tanning effects, but gave a greenish-black
coloration with iron salts. This conclusively proves that the blue
coloration given by Neradol D with iron salts is no characteristic
feature of the _pure_ synthetic tannin, but is caused by the phenolic
impurities accompanying the latter. Especially the first stage of the
electro-osmosis produces a cathodic migration of the phenols, which may
then be detected at a cathode by means of the iron and bromine
reactions.
It is characteristic of a dicresylmethanedisulphonic acid purified by
electro-osmosis that it does not precipitate aniline hydrochloride. It
appears, therefore, that this reaction--which is characteristic of most
synthetic tannins--is again caused by the presence of impurities.
3. The Influence of Salts and Acid Contents on the Tanning Effect of
Neradol D
Chemical analysis of crude Neradol revealed a natural
dicresylmethanedisulphonic acid (the tanning agent) contents of about 68
per cent, which agrees fairly well with the calculated amount. Like
other "strong" and "weak" acids this sulphonic acid exercises a strongly
swelling influence on pelt. Whereas the effect of acid present in
solutions of Neradol D of medium concentration and its tanning effect
both influence the pelt and are fairly well balanced, this is not the
case as regards highly concentrated and very dilute solutions. If, for
instance, a very dilute solution of crude Neradol (about 0.25° Bé.) is
used, the tanning effect of this solution is exceedingly small and does
not show itself till after several hours. The relatively high
dissociation of the acids at this high degree of dilution causes an
extremely rapid and strong swelling of the pelt, which has therefore
absorbed its maximum amount of water (maximum swelling) before the
tanning effect of the sulphonic acid comes into play and by fixing the
surface of the pelt is enabled to prevent the excessive swelling effect
of the acids.
The addition of neutral salts to the tan liquor diminishes the effect of
the acids on pelt (dehydrates the pelt) and prevents "drawing" of the
grain. If, for instance, common salt be added to a solution of crude
Neradol, the original quantity of sulphonic acid present would remain
constant, but the presence of salt would diminish the degree of
dissociation and consequently the swelling. This effect is still more
pronounced when the absolute amount of free sulphonic acid is
diminished. Hence, if crude Neradol is treated with increasing amounts
of caustic soda, a series of products containing increasing quantities
of salt and decreasing concentrations of sulphonic acid is obtained.
The acidity of the Neradols may be determined by titration with N/10
caustic soda; this procedure hence establishes a means of determining
the (unknown) acidities which may be expressed in terms of c.c. N/10
NaOH. The acidity of crude Neradol was found to be--
1 gm. = 50 c.c. N/10 NaOH
_i.e._, 1 gm. of crude Neradol requires 50 c.c. N/10 NaOH for complete
neutralisation; the decrease in acidity causes a decrease in contents of
tanning matters and the quantities of salts increase. The following
table gives the figures obtained by differently neutralised neradols:--
Acidity. Tanning Matters. Na_2SO_4.
Per Cent. Per Cent.
1 gm. = 50 c.c. 68 ...
1 gm. = 40 c.c. 59 4
1 gm. = 30 c.c. 50 8
1 gm. = 20 c.c. 41 12
1 gm. = 10 c.c. 33 17
1 gm. = 5 c.c. 28 20
1 gm. = 0 c.c. 20 ...
Tanning experiments with these different neradols (employing solutions
of 1° Bé. strength) demonstrated that neradols of acidity 50°, 40°, and
30° exerted strong swelling and gave comparatively hard leathers;
neradols of acidity 20°, 10°, and 5° exert no swelling, yield quick
tannage and soft leather. The swelling (hardening) effect of the acid
and the dehydrating (softening) effect of the salts in this case,
therefore, are well balanced, and this fact affords an explanation of
the rapid change from hardening to softening effects exhibited by partly
neutralised Neradol where less acid and a greater quantity of salts
respectively are present.
It may finally be noted that the acidity of Neradol D, 1 gm. = 10
c.c. N/10 NaOH, has been found to be the most suitable one for practical
purposes. The author has, however, successfully employed some neradols
of considerably higher acidities. The acidity above mentioned is
possessed by a Neradol D containing 17 per cent. ash and 30 per cent.
sodium sulphonates and Glauber's salts crystals respectively. This
large quantity of salts present on the one hand effects the rapid pickle
and tanning effect exhibited by Neradol D, on the other hand it also
effects the softness in the leather resulting from its use either alone
or in admixture with natural tannins.
4. Phlobaphene Solubilising Action of Neradols
A special feature of Neradol D is its property of solubilising
phlobaphenes, which may be ascribed to its contents of sulphonic acids
or their salts. In order to demonstrate whether the sulphonic acids and
their salts are capable of solubilising the insoluble or sparingly
soluble anhydrides of the tannins (the phlobaphenes) before and after
condensation, the following experiments were carried out:--
Crude Argentine solid quebracho extract was converted into a highly
viscous liquid by treating it for several hours with water at 100° C.,
and the anhydrides rendered insoluble by diluting the liquid with a
large volume of cold water. The precipitate formed, consisting of
quebracho phlobaphenes, was separated from the liquid by decantation,
and purified by washing it several times with water. Each 10 gm. of this
moist paste were treated in the cold with (_a_) free phenolsulphonic
acid; (_b_) sodium phenolsulphonate; (_c_) crude Neradol and (_d_)
Neradol D, 20 c.c. of water at 45° C. added, and the mixture allowed to
cool slowly; the following solutions resulted:--
(_a_) Opalescent solution, much deposit,
(_b_) Clear solution, no deposit.
(_c_) Nearly clear solution, very little deposit.
(_d_) Clear solution, no deposit.
This clearly proves that free and condensed phenolsulphonic acids as
such are not capable of completely solubilising phlobaphenes, whereas
the sodium salts of free and condensed phenolsulphonic acids possess
this property. The salt contents of Neradol D, therefore, constitute an
advantage in this respect, that not only may Neradol D be mixed with
solutions of any natural tannin without insolubles being thereby
deposited, but it may also be added in large quantities to a tannin
solution with the result that the sparingly soluble and wholly insoluble
constituents (phlobaphenes) are completely brought into solution.
The practical importance of the solubilising effect of Neradol D
relating to solid Argentine quebracho extract is demonstrated in the
following series of investigations carried out by the author:--
[Footnote: _Collegium_, 1913, 478; Austr. Pat., 68, 796.]
Solid Neradol Matters Tanning Abs. Increase
Argentine D. Calc. of Mixture Increase per
Quebracho Found. in Tanning 100 gms.
Extract. Matters. Extract.
Gm. Gm. Per Cent. Per Cent.
100 0 66.0 66.0 ... ...
0 100 32.5 32.5 ... ...
90 10 62.7 64.7 2.0 2.2
80 20 56.1 58.7 2.6 3.3
60 40 52.6 56.9 4.3 7.1
50 50 49.3 55.2 5.9 11.8
30 70 42.6 47.3 4.7 15.6
20 80 39.2 42.3 3.1 15.5
The maximum solubilising effect is exhibited in the mixture of 70 parts
of Neradol and 30 parts of quebracho, with an additional percentage of
tanning matters in the mixture of 15.6 per cent.--a figure which is very
nearly identical with that of the insolubles present in the original
Argentine quebracho extract.
The phlobaphene-solubilising property of Neradol D is closely connected
with the influence of the latter on the colour of leathers tanned with
natural tannins. If, on the one hand, a pelt is tanned with natural
(_i.e._, non-treated) quebracho extract, a rather light coloured leather
results, the fleshy colour of which is characteristic of quebracho. The
dark coloured phlobaphenes present, on account of their insolubility,
will have no influence on the colour of the leather. If, now, the
quebracho extract be treated with sulphite and bisulphite in the usual
way, the phlobaphenes are solubilised, but the reducing effect of the
bisulphite tends to brighten the colour of the otherwise dark coloured
phlobaphenes as well as that of the soluble tannins, and a
reddish-yellow coloured extract results, imparting its own colour to the
pelt. When, on the other hand, the quebracho extract is solubilised by
means of Neradol D, the phlobaphenes are brought into solution without
reduction taking place, and a dark brownish-red extract results, which
imparts a similar colour to the finished leather. This darkening effect
of Neradol D is most conspicuous in the case of mangrove, maletto, and
chestnut, but is absent in the case of algarobilla, dividivi, gambir,
sumac, and valonea. The varying phlobaphene contents of the tannins
easily afford an explanation of the different properties above alluded
to: the mangrove phlobaphenes are dark coloured bodies, those of mimosa,
maletto, and chestnut are of lighter colour, and the last-named tanning
materials enumerated above are either devoid of phlobaphenes or possess
them only as very light coloured bodies. Algarobilla, sumac, gambir,
dividivi, and valonea, on the other hand, are associated with large
amounts of sparingly soluble ellagic acid, known as "bloom" or "mud"
which imparts a light colour to the finished leather, and conveniently
covers the dark colour imparted to the leather by other tanning
materials; for this reason the former are often used in the lay-aways or
in the finishing processes.
Similar effects to those of Neradol D are exhibited by other salts of
sulphonic acids, _e.g._, sodium benzylsulphanilate (Solvenol B.A.S.F., or
solution salt ("Solutionsalz") Hoechst); the author prepared mixtures of
such salts and untreated quebracho extract in order to determine their
solubilising effects, and arrived at the following results:--
30 parts Solvenol plus 70 parts quebracho extract: clear solution,
no deposit.
25 parts Solvenol plus 25 parts quebracho extract: clear solution,
very little deposit
20 parts Solvenol plus 80 parts quebracho extract: nearly clear solution,
very little deposit.
15 parts Solvenol plus 85 parts quebracho extract: slightly opaque solution
some deposit.
Leathers tanned with these mixtures were more or less dark coloured
according to the amounts used of solvenol and the consequent
solubilisation of the phlobaphenes.
A similar effect, though of opposite nature from a tanning standpoint,
is exhibited by sulphonates on certain colloidal dark coloured
substances. A phenolsulphonic acid, which had been overheated during
sulphonation and subsequently condensed (crude Neradol), imparted a
conspicuous greyish-brown colour to the leather; samples of this crude
product were then partly neutralised with varying amounts of alkali, and
these samples (containing increasing quantities of salts) tested for
tannin and colour effects. It was found that the more highly neutralised
samples imparted a darker colour to the solutions, but these dark
products did not deposit the dark impurities on the pelt. One may
therefore assume that tannoid substances are colloidally suspended, and
when converted into true solutions are incapable of being fixed in
insoluble form by the pelt.