ANALYSIS OF RAW MATERIALS, SOAP, AND GLYCERINE.
_Fats and Oils--Alkalies and Alkali Salts--Essential
Oils--Soap--Lyes--Crude Glycerine._
_Raw Materials._--Average figures have already been given in Chapters
III. and VIII. for the more important physical and chemical
characteristics of fats and oils, also of essential oils; the following
is an outline of the processes usually adopted in their determination.
For fuller details, text-books dealing exhaustively with the respective
subjects should be consulted.
FATS AND OILS.
It is very undesirable that any of these materials should be allowed to
enter the soap pan without an analysis having first been made, as the
oil may not only have become partially hydrolysed, involving a loss of
glycerine, or contain albuminous matter rendering the soap liable to
develop rancidity, but actual sophistication may have taken place. Thus
a sample of tallow recently examined by the authors contained as much as
40 per cent. of an unsaponifiable wax, which would have led to disaster
in the soap pan, had the bulk been used without examination. After
observing the appearance, colour, and odour of the sample, noting any
characteristic feature, the following physical and chemical data should
be determined.
_Specific Gravity at 15° C._ This may be taken by means of a Westphal
balance, or by using a picnometer of either the ordinary gravity bottle
shape, with perforated stopper, or the Sprengel U-tube. The picnometer
should be calibrated with distilled water at 15° C. The specific gravity
of solid fats may be taken at an elevated temperature, preferably that
of a boiling water bath.
_Free acidity_ is estimated by weighing out from 2 to 5 grammes of the
fat or oil, dissolving in neutral alcohol (purified methylated spirit)
with gentle heat, and titrating with a standard aqueous or alcoholic
solution of caustic soda or potash, using phenol-phthalein as indicator.
The contents of the flask are well shaken after each addition of alkali,
and the reaction is complete when the slight excess of alkali causes a
permanent pink coloration with the indicator. The standard alkali may be
N/2, N/5, or N/10.
It is usual to calculate the result in terms of oleic acid (1 c.c. N/10
alkali = 0.0282 gramme oleic acid), and express in percentage on the fat
or oil.
_Example._--1.8976 grammes were taken, and required 5.2 c.c. of N/10 KOH
solution for neutralisation.
5.2 × 0.0282 × 100
------------------ = 7.72 per cent. free fatty acids,
1.8976 expressed as oleic acid.
The free acidity is sometimes expressed as _acid value_, which is the
amount of KOH in milligrammes necessary to neutralise the free acid in 1
gramme of fat or oil.
In the above example:--
5.2 × 5.61
---------- = 15.3 acid value.
1.8976
The _saponification equivalent_ is determined by weighing 2-4 grammes of
fat or oil into a wide-necked flask (about 250 c.c. capacity), adding 30
c.c. neutral alcohol, and warming under a reflux condenser on a steam or
water-bath. When boiling, the flask is disconnected, 50 c.c. of an
approximately semi-normal alcoholic potash solution carefully added from
a burette, together with a few drops of phenol-phthalein solution, and
the boiling under a reflux condenser continued, with frequent agitation,
until saponification is complete (usually from 30-60 minutes) which is
indicated by the absence of fatty globules. The excess of alkali is
titrated with N/1 hydrochloric or sulphuric acid.
The value of the approximately N/2 alkali solution is ascertained by
taking 50 c.c. together with 30 c.c. neutral alcohol in a similar flask,
boiling for the same length of time as the fat, and titrating with N/1
hydrochloric or sulphuric acid. The "saponification equivalent" is the
amount of fat or oil in grammes saponified by 1 equivalent or 56.1
grammes of caustic potash.
_Example._--1.8976 grammes fat required 18.95 c.c. N/1 acid to
neutralise the unabsorbed alkali.
Fifty c.c. approximately N/2 alcoholic potash solution required 25.6
c.c. N/ acid..
25.6 - 18.95 = 6.65 c.c. N/1 KOH required by fat.
1.8976 × 1000 / 6.65 = 285.3 Saponification Equivalent.
The result of this test is often expressed as the "Saponification
Value," which is the number of milligrammes of KOH required for the
saponification of 1 gramme of fat. This may be found by dividing 56,100
by the saponification equivalent or by multiplying the number of c.c. of
N/1 alkali absorbed, by 56.1 and dividing by the quantity of fat taken.
Thus, in the above example:--
6.65 × 56.1 / 1.8976 = 196.6 Saponification Value.
The _ester_ or _ether value_, or number of milligrammes of KOH required
for the saponification of the neutral esters or glycerides in 1 gramme
of fat, is represented by the difference between the saponification and
acid values. In the example given, the ester value would be 196.6 - 15.3
= 181.3.
_Unsaponifiable Matter._--The usual method adopted is to saponify about
5 grammes of the fat or oil with 50 c.c. of approximately N/2 alcoholic
potash solution by boiling under a reflux condenser with frequent
agitation for about 1 hour. The solution is then evaporated to dryness
in a porcelain basin over a steam or water-bath, and the resultant soap
dissolved in about 200 c.c. hot water. When sufficiently cool, the soap
solution is transferred to a separating funnel, 50 c.c. of ether added,
the whole well shaken, and allowed to rest. The ethereal layer is
removed to another separator, more ether being added to the aqueous soap
solution, and again separated. The two ethereal extracts are then washed
with water to deprive them of any soap, separated, transferred to a
flask, and the ether distilled off upon a water-bath. The residue, dried
in the oven at 100° C. until constant, is the "unsaponifiable matter,"
which is calculated to per cent. on the oil.
In this method, it is very frequently most difficult to obtain a
distinct separation of ether and aqueous soap solution--an intermediate
layer of emulsion remaining even after prolonged standing, and various
expedients have been recommended to overcome this, such as addition of
alcohol (when petroleum ether is used), glycerine, more ether, water, or
caustic potash solution, or by rotatory agitation.
A better plan is to proceed as in the method above described as far as
dissolving the resulting soap in 200 c.c. water, and then boil for
twenty or thirty minutes. Slightly cool and acidify with dilute
sulphuric acid (1 to 3), boil until the fatty acids are clear, wash with
hot water free from mineral acid, and dry by filtering through a hot
water funnel.
Two grammes of the fatty acids are now dissolved in neutral alcohol
saturated with some solvent, preferably a light fraction of benzoline, a
quantity of the solvent added to take up the unsaponifiable matter, and
the whole boiled under a reflux condenser. After cooling, the liquid is
titrated with N/2 aqueous KOH solution, using phenol-phthalein as
indicator, this figure giving the amount of the total fatty acids
present. The whole is then poured into a separating funnel, when
separation immediately takes place. The alcoholic layer is withdrawn,
the benzoline washed with warm water (about 32° C.) followed by neutral
alcohol (previously saturated with the solvent), and transferred to a
tared flask, which is attached to a condenser, and the benzoline
distilled off. The last traces of solvent remaining in the flask are
removed by gently warming in the water-oven, and the flask cooled and
weighed, thus giving the amount of unsaponifiable matter.
_Constitution of the Unsaponifiable Matter._--Unsaponifiable matter may
consist of cholesterol, phytosterol, solid alcohols (cetyl and ceryl
alcohols), or hydrocarbons (mineral oil). Cholesterol is frequently
found in animal fats, and phytosterol is a very similar substance
present in vegetable fats. Solid alcohols occur naturally in sperm oil,
but hydrocarbons, which may be generally recognised by the fluorescence
or bloom they give to the oil, are not natural constituents of animal or
vegetable oils and fats.
The presence of cholesterol and phytosterol may be detected by
dissolving a small portion of the unsaponifiable matter in acetic
anhydride, and adding a drop of the solution to one drop of 50 per cent.
sulphuric acid on a spot plate, when a characteristic blood red to
violet coloration is produced. It has been proposed to differentiate
between cholesterol and phytosterol by their melting points, but it is
more reliable to compare the crystalline forms, the former crystallising
in laminæ, while the latter forms groups of needle-shaped tufts. Another
method is to convert the substance into acetate, and take its melting
point, cholesterol acetate melting at 114.3-114.8° C., and phytosterol
acetate at 125.6°-137° C.
Additional tests for cholesterol have been recently proposed by
Lifschütz (_Ber. Deut. Chem. Ges._, 1908, 252-255), and Golodetz (_Chem.
Zeit._, 1908, 160). In that due to the former, which depends on the
oxidation of cholesterol to oxycholesterol ester and oxycholesterol, a
few milligrammes of the substance are dissolved in 2-3 c.c. glacial
acetic acid, a little benzoyl peroxide added, and the solution boiled,
after which four drops of strong sulphuric acid are added, when a
violet-blue or green colour is produced, if cholesterol is present, the
violet colour being due to oxycholesterol ester, the green to
oxycholesterol. Two tests are suggested by Golodetz (1) the addition of
one or two drops of a reagent consisting of five parts of concentrated
sulphuric acid and three parts of formaldehyde solution, which colours
cholesterol a blackish-brown, and (2) the addition of one drop of 30 per
cent. formaldehyde solution to a solution of the substance in
trichloracetic acid, when with cholesterol an intense blue coloration is
produced.
_Water._--From 5 to 20 grammes of the fat or oil are weighed into a
tared porcelain or platinum dish, and stirred with a thermometer, whilst
being heated over a gas flame at 100° C. until bubbling or cracking has
ceased, and reweighed, the loss in weight representing the water. In
cases of spurting a little added alcohol will carry the water off
quietly.
To prevent loss by spurting, Davis (_J. Amer. Chem. Soc._, 23, 487) has
suggested that the fat or oil should be added to a previously dried and
tared coil of filter paper contained in a stoppered weighing bottle,
which is then placed in the oven and dried at 100° C. until constant in
weight. Of course, this method is not applicable to oils or fats liable
to oxidation on heating.
_Dregs, Dirt, Adipose Tissue, Fibre, etc._--From 10 to 15 grammes of the
fat are dissolved in petroleum ether with frequent stirring, and passed
through a tared filter paper. The residue retained by the filter paper
is washed with petroleum ether until free from fat, dried in the
water-oven at 100° C. and weighed.
If the amount of residue is large, it may be ignited, and the proportion
and nature of the ash determined.
The amount of impurities may also be estimated by Tate's method, which
is performed by weighing 5 grammes of fat into a separating funnel,
dissolving in ether, and allowing the whole to stand to enable the water
to deposit. After six hours' rest the water is withdrawn, the tube of
the separator carefully dried, and the ethereal solution filtered
through a dried tared filter paper into a tared flask. Well wash the
filter with ether, and carefully dry at 100° C. The ether in the flask
is recovered, and the flask dried until all ether is expelled, and its
weight is constant. The amount of fat in the flask gives the quantity of
actual fat in the sample taken; the loss represents the water and other
impurities, and these latter may be obtained from the increase of weight
of the filter paper.
_Starch_ may be detected by the blue coloration it gives with iodine
solution, and confirmed by microscopical examination, or it may be
converted into glucose by inversion, and the glucose estimated by means
of Fehling's solution.
_Iodine Absorption._--This determination shows the amount of iodine
absorbed by a fat or oil, and was devised by Hübl, the reagents required
being as follows:--
(1) Solution of 25 grammes iodine in 500 c.c. absolute alcohol; (2)
solution of 30 grammes mercuric chloride in 500 c.c. absolute alcohol,
these two solutions being mixed together and allowed to stand at least
twelve hours before use; (3) a freshly prepared 10 per cent. aqueous
solution of potassium iodide; and (4) a N/10 solution of sodium
thiosulphate, standardised just prior to use by titrating a weighed
quantity of resublimed iodine dissolved in potassium iodide solution.
In the actual determination, 0.2 to 0.5 gramme of fat or fatty acids is
carefully weighed into a well-fitting stoppered 250 c.c. bottle,
dissolved in 10 c.c. chloroform, and 25 c.c. of the Hübl reagent added,
the stopper being then moistened with potassium iodide solution and
placed firmly in the bottle, which is allowed to stand at rest in a dark
place for four hours. A blank experiment is also performed, using the
same quantities of chloroform and Hübl reagent, and allowing to stand
for the same length of time.
After the expiration of four hours 20 c.c. of 10 per cent. solution of
potassium iodide and 150 c.c. water are added to the contents of the
bottle, and the excess of iodine titrated with N/10 sodium thiosulphate
solution, the whole being well agitated during the titration, which is
finished with starch paste as indicator. The blank experiment is
titrated in the same manner, and from the amount of thiosulphate
required in the blank experiment is deducted the number of c.c. required
by the unabsorbed iodine in the other bottle; this figure multiplied by
the iodine equivalent of 1 c.c. of the thiosulphate solution and by 100,
dividing the product by the weight of fat taken, gives the "Iodine
Number".
_Example._--1 c.c. of the N/10 sodium thiosulphate solution is found
equal to 0.0126 gramme iodine.
0.3187 gramme of fat taken. Blank requires 48.5 c.c. thiosulphate.
Bottle containing oil requires 40.0 c.c. thiosulphate.
48.5 - 40.0 = 8.5, and the iodine absorption of the fat is--
8.5 × 0.0126 × 100
------------------ = 33.6.
0.3187
Wijs showed that by the employment of a solution of iodine monochloride
in glacial acetic acid reliable iodine figures are obtained in a much
shorter time, thirty minutes being sufficient, and this method is now in
much more general use than the Hübl. Wijs' iodine reagent is made by
dissolving 13 grammes iodine in 1 litre of glacial acetic acid and
passing chlorine into the solution until the iodine is all converted
into iodine monochloride. The process is carried out in exactly the same
way as with the Hübl solution except that the fat is preferably
dissolved in carbon tetrachloride instead of in chloroform.
_Bromine absorption_ has now been almost entirely superseded by the
iodine absorption, although there are several good methods. The
gravimetric method of Hehner (_Analyst_, 1895, 49) was employed by one
of us for many years with very good results, whilst the bromine-thermal
test of Hehner and Mitchell (_Analyst_, 1895, 146) gives rapid and
satisfactory results. More recently MacIlhiney (_Jour. Amer. Chem.
Soc._, 1899, 1084-1089) drew attention to bromine absorption methods and
tried to rewaken interest in them.
The _Refractive index_ is sometimes useful for discriminating between
various oils and fats, and, in conjunction with other physical and
chemical data, affords another means of detecting adulteration.
Where a great number of samples have to be tested expeditiously, the
Abbé refractometer or the Zeiss butyro-refractometer may be recommended
on account of the ease with which they are manipulated. The most usual
temperature of observations is 60° C.
The _Titre_ or setting point of the fatty acids was devised by Dalican,
and is generally accepted in the commercial valuation of solid fats as a
gauge of firmness, and in the case of tallow has a considerable bearing
on the market value.
One ounce of the fat is melted in a shallow porcelain dish, and 30 c.c.
of a 25 per cent. caustic soda solution added, together with 50 c.c. of
redistilled methylated spirit. The whole is stirred down on the water
bath until a pasty soap is obtained, when another 50 c.c. of methylated
spirit is added, which redissolves the soap, and the whole again stirred
down to a solid soap. This is then dissolved in distilled water, a
slight excess of dilute sulphuric acid added to liberate the fatty
acids, and the whole warmed until the fatty acids form a clear liquid
on the surface. The water beneath the fatty acids is then syphoned off,
more distilled water added to wash out any trace of mineral acid
remaining, and again syphoned off, this process being repeated until the
washings are no longer acid to litmus paper, when the fatty acids are
poured on to a dry filter paper, which is inserted in a funnel resting
on a beaker, and the latter placed on the water-bath, where it is left
until the clear fatty acids have filtered through.
About 10-15 grammes of the pure fatty acids are now transferred to a
test tube, 6" × 1", warmed until molten, and the tube introduced through
a hole in the cork into a flask or wide-mouthed bottle. A very accurate
thermometer, graduated into fifths of a degree Centigrade (previously
standardised), is immersed in the fatty acids, so that the bulb is as
near the centre as possible, and when the fatty acids just begin to
solidify at the bottom of the tube, the thermometer is stirred round
slowly. The mercury will descend, and stirring is continued until it
ceases to fall further, at which point the thermometer is very carefully
observed. It will be found that the temperature will rise rapidly and
finally remain stationary for a short time, after which it will again
begin to drop until the temperature of the room is reached. The maximum
point to which the temperature rises is known as the "titre" of the
sample.
ALKALIES AND ALKALI SALTS.
Care should be bestowed upon the sampling of solid caustic soda or
potash as the impurities during the solidification always accumulate in
the centre of the drum, and an excess of that portion must be avoided or
the sample will not be sufficiently representative. The sampling should
be performed expeditiously to prevent carbonating, and portions placed
in a stoppered bottle. The whole should be slightly broken in a mortar,
and bright crystalline portions taken for analysis, using a stoppered
weighing bottle.
_Caustic Soda and Caustic Potash._--These substances are valued
according to the alkali present in the form of caustic (hydrate) and
carbonate.
About 2 grammes of the sample are dissolved in 50 c.c. distilled water,
and titrated with N/1 sulphuric acid, using phenol-phthalein as
indicator, the alkalinity so obtained representing all the caustic
alkali and one-half the carbonate, which latter is converted into
bicarbonate. One c.c. N/1 acid = 0.031 gramme Na_{2}O or 0.040 gramme
NaOH and 0.047 gramme K_{2}O, or 0.056 gramme KOH.
After this first titration, the second half of the carbonate may be
determined in one of two ways, either:--
(1) By adding from 3-5 c.c. of N/10 acid, and well boiling for five
minutes to expel carbonic-acid gas, after which the excess of acid is
titrated with N/10 soda solution; or
(2) After adding two drops of methyl orange solution, N/10 acid is run
in until the solution acquires a faint pink tint.
In the calculation of the caustic alkali, the number of c.c. of acid
required in the second titration, divided by 10, is subtracted from that
used in the first, and this difference multiplied by 0.031, or 0.047
gives the amount of Na_{2}O or K_{2}O respectively in the weight of
sample taken, whence the percentage may be readily calculated.
The proportion of carbonate is calculated by multiplying the amount of
N/10 acid required in the second titration by 2, and then by either
0.0031 or 0.0047 to give the amount of carbonate present, expressed as
Na_{2}O or K_{2}O respectively.
An alternative method is to determine the alkalinity before and after
the elimination of carbonate by chloride of barium.
About 7-8 grammes of the sample are dissolved in water, and made up to
100 c.c., and the total alkalinity determined by titrating 20 c.c. with
N/1 acid, using methyl orange as indicator. To another 20 c.c. is added
barium chloride solution (10 per cent.) until it ceases to give a
precipitate, the precipitate allowed to settle, and the clear
supernatant liquid decanted off, the precipitate transferred to a filter
paper and well washed, and the filtrate titrated with N/1 acid, using
phenol-phthalein as indicator. The second titration gives the amount of
caustic alkali present, and the difference between the two the
proportion of carbonate.
When methyl orange solution is used as indicator, titrations must be
carried out cold.
Reference has already been made (p. 39) to the manner in which the
alkali percentage is expressed in English degrees in the case of caustic
soda.
_Chlorides_ are estimated by titrating the neutral solution with N/10
silver nitrate solution, potassium chromate being used as indicator. One
c.c. N/10 AgNO_{3} solution = 0.00585 gramme sodium chloride.
The amount of acid necessary for exact neutralisation having already
been ascertained, it is recommended to use the equivalent quantity of
N/10 nitric acid to produce the neutral solution.
_Sulphides_ may be tested for, qualitatively, with lead acetate
solution.
_Aluminates_ are determined gravimetrically in the usual manner; 2
grammes are dissolved in water, rendered acid with HCl, excess of
ammonia added, and the gelatinous precipitate of aluminium hydrate
collected on a filter paper, washed, burnt, and weighed.
* * * * *
_Carbonated Alkali (Soda Ash)._--The total or available alkali is, of
course, the chief factor to be ascertained, and for this purpose it is
convenient to weigh out 3.1 grammes of the sample, dissolve in 50 c.c.
water, and titrate with N/1 sulphuric or hydrochloric acid, using methyl
orange as indicator. Each c.c. of N/1 acid required represents 1 per
cent. Na_{2}O in the sample under examination.
A more complete analysis of soda ash would comprise:--
_Insoluble matter_, remaining after 10 grammes are dissolved in warm
water. This is washed on to a filter-paper, dried, ignited, and weighed.
The filtrate is made up to 200 c.c., and in it may be determined:--
_Caustic soda_, by titrating with N/1 acid the filtrate resulting from
the treatment of 20 c.c. (equal to 1 gramme) with barium chloride
solution.
_Carbonate._--Titrate 20 c.c. with N/1 acid, and deduct the amount of
acid required for the Caustic.
_Chlorides._--Twenty c.c. are exactly neutralised with nitric acid,
titrated with N/10 AgNO_{3} solution, using potassium chromate as
indicator.
_Sulphates._--Twenty c.c. are acidulated with HCl, and the sulphates
precipitated with barium chloride; the precipitate is collected on a
filter paper, washed, dried, ignited, and weighed, the result being
calculated to Na_{2}SO_{4}.
_Sulphides and Sulphites._--The presence of these compounds is denoted
by the evolution of sulphuretted hydrogen and sulphurous acid
respectively when the sample is acidulated. Sulphides may also be tested
for, qualitatively, with lead acetate solution, or test-paper of sodium
nitro-prusside.
The total quantity of these compounds may be ascertained by acidulating
with acetic acid, and titrating with N/10 iodine solution, using starch
paste as indicator. One c.c. N/10 iodine solution = 0.0063 gramme
Na_{2}SO_{3}.
The amount of sulphides may be estimated by titrating the hot soda
solution, to which ammonia has been added, with an ammoniacal silver
nitrate solution, 1 c.c. of which corresponds to 0.005 gramme Na_{2}S.
As the titration proceeds, the precipitate is filtered off, and the
addition of ammoniacal silver solution to the filtrate continued until a
drop produces only a slight opacity. The presence of chloride, sulphate,
hydrate, or carbonate does not interfere with the accuracy of this
method. The ammoniacal silver nitrate solution is prepared by dissolving
13.345 grammes of pure silver in pure nitric acid, adding 250 c.c.
liquor ammoniæ fortis, and diluting to 1 litre.
_Carbonate of Potash (Pearl Ash)._--The total or available alkali may be
estimated by taking 6.9 grammes of the sample, and titrating with N/1
acid directly, or adding 100 c.c. N/1 sulphuric acid, boiling for a few
minutes, and titrating the excess of acid with N/1 caustic soda
solution, using litmus as indicator. In this case each c.c. N/1 acid
required, is equivalent, in the absence of Na_{2}CO_{3}, to 1 per cent.
K_{2}CO_{3}.
Carbonate of potash may be further examined for the following:--
_Moisture._--From 2-3 grammes are heated for thirty minutes in a
crucible over a gas flame, and weighed when cold, the loss in weight
representing the moisture.
_Insoluble residue_, remaining after solution in water, filtering and
well washing.
_Potassium_ may be determined by precipitation as potassium
platino-chloride thus:--Dissolve 0.5 gramme in a small quantity (say 10
c.c.) of water, and carefully acidulate with hydrochloric acid,
evaporate the resultant liquor to dryness in a tared platinum basin, and
heat the residue gradually to dull redness. Cool in a desicator, weigh,
and express the result as "mixed chlorides," _i.e._ chlorides of soda
and potash. To the mixed chlorides add 10 c.c. water, and platinic
chloride in excess (the quantity may be three times the amount of the
mixed chlorides) and evaporate nearly to dryness; add 15 c.c. alcohol
and allow to stand three hours covered with a watch-glass, giving the
dish a gentle rotatory movement occasionally. The clear liquid is
decanted through a tared filter, and the precipitate well washed with
alcohol by decantation, and finally transferred to the filter, dried and
weighed. From the weight of potassium platino-chloride, K_{2}PtCl_{6},
is calculated the amount of potassium oxide K_{2}O by the use of the
factor 94/488.2 or 0.19254.
_Chlorides_, determined with N/10 silver nitrate solution, and
calculated to KCl.
_Sulphates_, estimated as barium sulphate, and calculated to
K_{2}SO_{4}.
_Sodium Carbonate_, found by deducting the K_{2}CO_{3} corresponding to
the actual potassium as determined above, from the total alkali.
_Iron_, precipitated with excess of ammonia, filtered, ignited, and
weighed as Fe_{2}O_{3}.
SODIUM CHLORIDE (COMMON SALT).
This should be examined for the following:--
_Actual Chloride_, either titrated with N/10 silver nitrate solution,
using neutral potassium chromate solution as indicator, or, preferably,
estimated gravimetrically as silver chloride by precipitation with
silver nitrate solution, the precipitate transferred to a tared filter
paper, washed, dried and weighed.
_Insoluble matter_, remaining on dissolving 5 grammes in water, and
filtering. This is washed, dried, ignited and weighed.
_Moisture._--5 grammes are weighed into a platinum crucible, and heat
gently applied. The temperature is gradually increased to a dull red
heat, which is maintained for a few minutes, the dish cooled in a
desicator, and weighed.
_Sulphates_ are estimated by precipitation as barium sulphate and
calculated to Na_{2}SO_{4}.
_Sodium._--This may be determined by converting the salt into sodium
sulphate by the action of concentrated sulphuric acid, igniting to drive
off hydrochloric and sulphuric acids, and fusing the mass until constant
in weight, weighing finally as Na_{2}SO_{4}.
POTASSIUM CHLORIDE.
This should be examined, in the same way as sodium chloride, for
chloride, insoluble matter, moisture, and sulphate. The potassium may be
determined as potassium platino-chloride, as described under carbonate
of potash.
SILICATES OF SODA AND POTASH.
The most important determinations for these are total alkali and silica.
_Total alkali_ is estimated by dissolving 2 grammes in distilled water,
and titrating when cold, with N/1 acid, using methyl orange as
indicator.
_Silica_ may be determined by dissolving 1 gramme in distilled water,
rendering the solution acid with HCl, and evaporating to complete
dryness on the water-bath, after which the residue is moistened with HCl
and again evaporated, this operation being repeated a third time. The
residue is then heated to about 150° C., extracted with hot dilute HCl,
filtered, thoroughly washed, dried, ignited in a tared platinum
crucible, and weighed as SiO_{2}.
ESSENTIAL OILS.
As already stated, these are very liable to adulteration, and an
examination of all kinds of oil is desirable, while in the case of the
more expensive varieties it should never be omitted.
_Specific Gravity._--As with fats and oils, this is usually taken at 15°
C., and compared with water at the same temperature. In the case of otto
of rose and guaiac wood oil, however, which are solid at this
temperature, it is generally observed at 30° C. compared with water at
15° C.
The specific gravity is preferably taken in a bottle or U-tube, but if
sufficient of the oil is available and a high degree of accuracy is not
necessary, it may be taken either with a Westphal balance, or by means
of a hydrometer.
_Optical Rotation._--For this purpose a special instrument, known as a
polarimeter, is required, details of the construction and use of which
would be out of place here. Suffice it to mention that temperature plays
an important part in the determination of the optical activity of
certain essential oils, notably in the case of lemon and orange oils.
For these Gildemeister and Hoffmann give the following corrections:--
Lemon oil, below 20° C. subtract 9' for each degree below, above 20° C.
add 8' for each degree above.
Orange oil, below 20° C. subtract 14' for each degree below, above 20°
C. add 13' for each degree above.
_Refractive Index._--This figure is occasionally useful, and is best
determined with an Abbé refractometer, at 20° C.
_Solubility in Alcohol._--This is found by running alcohol of the
requisite strength from a burette into a measured volume of the oil with
constant agitation, until the oil forms a clear solution with the
alcohol. Having noted the quantity of alcohol added, it is well to run
in a small further quantity of alcohol, and observe whether any
opalescence or cloudiness appears.
_Acid_, _ester_, and _saponification values_ are determined exactly as
described under fats and oils. Instead of expressing the result as
saponification value or number, the percentage of ester, calculated in
the form of the most important ester present, may be obtained by
multiplying the number of c.c. of N/1 alkali absorbed in the
saponification by the molecular weight of the ester. Thus, to find the
percentage as linalyl acetate, the number of c.c. absorbed would be
multiplied by 0.196 and by 100, and divided by the weight of oil taken.
_Alcohols._--For the estimation of these, if the oil contains much ester
it must first be saponified with alcoholic potash, to liberate the
combined alcohols, and after neutralising the excess of alkali with
acid, the oil is washed into a separating funnel with water, separated,
dried with anhydrous sodium sulphate, and is then ready for the alcohol
determination.
If there is only a small quantity of ester present, this preliminary
saponification is unnecessary.
The alcohols are estimated by conversion into their acetic esters, which
are then saponified with standard alcoholic potash, thereby furnishing a
measure of the amount of alcohol esterified.
Ten c.c. of the oil is placed in a flask with an equal volume of acetic
anhydride, and 2 grammes of anhydrous sodium acetate, and gently boiled
for an hour to an hour and a half. After cooling, water is added, and
the contents of the flask heated on the water-bath for fifteen to thirty
minutes, after which they are cooled, transferred to a separating
funnel, and washed with a brine solution until the washings cease to
give an acid reaction with litmus paper. The oil is now dried with
anhydrous sodium sulphate, filtered, and 1-2 grammes weighed into a
flask and saponified with alcoholic potash as in the determination of
ester or saponification value.
The calculation is a little complicated, but an example may perhaps
serve to make it clear.
A geranium oil containing 26.9 per cent. of ester, calculated as geranyl
tiglate, was acetylated, after saponification, to liberate the combined
geraniol, and 2.3825 grammes of the acetylated oil required 9.1 c.c. of
N/1 alkali for its saponification.
Now every 196 grammes of geranyl acetate present in the acetylated oil
correspond to 154 grammes of geraniol, so that for every 196 grammes of
ester now present in the oil, 42 grammes have been added to its weight,
and it is therefore necessary to make a deduction from the weight of oil
taken for the final saponification to allow for this, and since each
c.c. of N/1 alkali absorbed corresponds to 0.196 gramme of geranyl
acetate, the amount to be deducted is found by multiplying the number of
c.c. absorbed by 0.042 gramme, the formula for the estimation of total
alcohols thus becoming in the example given:--
9.1 × 0.154 × 100
Per cent. of geraniol = ---------------------- = 70.2
2.3825 - (9.1 × 0.042)
The percentage of combined alcohols can be calculated from the amount of
ester found, and by subtracting this from the percentage of total
alcohols, that of the free alcohols is obtained.
In the example quoted, the ester corresponds to 17.6 per cent. geraniol,
and this, deducted from the total alcohols, gives 52.6 per cent. free
alcohols, calculated as geraniol.
This process gives accurate results with geraniol, borneol, and menthol,
but with linalol and terpineol the figures obtained are only
comparative, a considerable quantity of these alcohols being decomposed
during the acetylation. The aldehyde citronellal is converted by acetic
anhydride into isopulegol acetate, so that this is also included in the
determination of graniol in citronella oil.
_Phenols._--These bodies are soluble in alkalies, and may be estimated
by measuring 5 c.c. or 10 c.c. of the oil into a Hirschsohn flask (a
flask of about 100 c.c. capacity with a long narrow neck holding 10
c.c., graduated in tenths of a c.c.), adding 25 c.c. of a 5 per cent.
aqueous caustic potash solution, and warming in the water-bath, then
adding another 25 c.c., and after one hour in the water-bath filling the
flask with the potash solution until the unabsorbed oil rises into the
neck of the flask, the volume of this oil being read off when it has
cooled down to the temperature of the laboratory. From the volume of oil
dissolved the percentage of phenols is readily calculated.
_Aldehydes._--In the estimation of these substances, use is made of
their property of combining with sodium bisulphite to form compounds
soluble in hot water. From 5-10 c.c. of the oil is measured into a
Hirschsohn flask, about 30 c.c. of a hot saturated solution of sodium
bisulphite added, and the flask immersed in a boiling water bath, and
thoroughly shaken at frequent intervals. Further quantities of the
bisulphite solution are gradually added, until, after about one hour,
the unabsorbed oil rises into the neck of the flask, where, after
cooling, its volume is read off, and the percentage of absorbed oil, or
aldehydes, calculated.
In the case of lemon oil, where the proportion of aldehydes, though of
great importance, is relatively very small, it is necessary to first
concentrate the aldehydes before determining them. For this purpose, 100
c.c. of the oil is placed in a Ladenburg fractional distillation flask,
and 90 c.c. distilled off under a pressure of not more than 40 mm., and
the residue steam distilled. The oil so obtained is separated from the
condensed water, measured, dried, and 5 c.c. assayed for aldehydes
either by the process already described, or by the following process
devised by Burgess (_Analyst_, 1904, 78):--
Five c.c. of the oil are placed in the Hirschsohn flask, about 20 c.c.
of a saturated solution of neutral sodium sulphite added, together with
a few drops of rosolic acid solution as indicator, and the flask placed
in a boiling water-bath and continually agitated. The contents of the
flask soon become red owing to the liberation of free alkali by the
combination of the aldehyde with part of the sodium sulphite, and this
coloration is just discharged by the addition of sufficient 10 per
cent. acetic-acid solution. The flask is again placed in the water-bath,
the shaking continued, and any further alkali liberated neutralised by
more acetic acid, the process being continued in this way until no
further red colour is produced. The flask is then filled with the sodium
sulphite solution, the volume of the cooled unabsorbed oil read off, and
the percentage of aldehydes calculated as before.
_Solidifying Point, or Congealing Point._--This is of some importance in
the examination of anise and fennel oils, and is also useful in the
examination of otto of rose. A suitable apparatus may be made by
obtaining three test tubes, of different sizes, which will fit one
inside the other, and fixing them together in this way through corks.
The innermost tube is then filled with the oil, and a sensitive
thermometer, similar to that described under the Titre test for fats,
suspended with its bulb completely immersed in the oil. With anise and
fennel, the oil is cooled down with constant stirring until it just
starts crystallising, when the stirring is interrupted, and the maximum
temperature to which the mercury rises noted. This is the solidifying
point.
In the case of otto of rose, the otto is continually stirred, and the
point at which the first crystal is observed is usually regarded as the
congealing point.
_Melting Point._--This is best determined by melting some of the solid
oil, or crystals, and sucking a small quantity up into a capillary tube,
which is then attached by a rubber band to the bulb of the thermometer,
immersed in a suitable bath (water, glycerine, oil, etc.) and the
temperature of the bath gradually raised until the substance in the tube
is sufficiently melted to rise to the surface, the temperature at which
this takes place being the melting point.
The melting point of otto of rose is usually taken in a similar tube to
the setting point, and is considered to be the point at which the last
crystal disappears.
_Iodine Absorption._--In the authors' opinion, this is of some value in
conjunction with other data in judging of the purity of otto of rose. It
is determined by Hübl's process as described under Fats and Oils, except
that only 0.1 to 0.2 gramme is taken, and instead of 10 c.c. of
chloroform, 10 c.c. of pure alcohol are added. The rest of the process
is identical.
SOAP.
In the analysis of soap, it is a matter of considerable importance that
all the determinations should be made on a uniform and average sample of
the soap, otherwise very misleading and unreliable figures are obtained.
Soap very rapidly loses its moisture on the surface, while the interior
of the bar or cake may be comparatively moist, and the best way is to
carefully remove the outer edges and take the portions for analysis from
the centre. In the case of a household or unmilled toilet soap, it is
imperative that the quantities for analysis should all be weighed out as
quickly after each other as possible.
_Fatty Acids._--Five grammes of the soap are rapidly weighed into a
small beaker, distilled water added, and the beaker heated on the water
bath until the soap is dissolved.
A slight excess of mineral acid is now added, and the whole heated until
the separated fatty acids are perfectly clear, when they are collected
on a tared filter paper, well washed with hot water and dried until
constant in weight. The result multiplied by 20 gives the percentage of
fatty acids in the sample.
A quicker method, and one which gives accurate results when care is
bestowed upon it, is to proceed in the manner described above as far as
the decomposition with mineral acid, and to then add 5 or 10 grammes of
stearic acid or beeswax to the contents of the beaker and heat until a
clear layer of fatty matter collects upon the acid liquor.
Cool the beaker, and when the cake is sufficiently hard, remove it
carefully by means of a spatula and dry on a filtering paper, add the
portions adhering to the sides of the beaker to the cake, and weigh.
The weight, less the amount of stearic acid or beeswax added, multiplied
by 20 gives the percentage of fatty acids.
Care must be taken that the cake does not contain enclosed water.
The results of these methods are returned as fatty acids, but are in
reality insoluble fatty acids, the soluble fatty acids being generally
disregarded. However in soaps made from cocoa-nut and palm-kernel oils
(which contain an appreciable quantity of soluble fatty acids) the acid
liquor is shaken with ether, and, after evaporation of the ethereal
extract, the amount of fatty matter left is added to the result already
obtained as above, or the ether method described below may be
advantageously employed.
Where the soap under examination contains mineral matter, the separated
fatty acids may be dissolved in ether. This is best performed in an
elongated, graduated, stoppered tube, the total volume of the ether,
after subsidence, carefully read, and an aliquot part taken and
evaporated to dryness in a tared flask, which is placed in the oven at
100° C. until the weight is constant.
In a complete analysis, the figure for fatty acids should be converted
into terms of fatty anhydrides by multiplying by the factor 0.9875.
In this test the resin acids contained in the soap are returned as fatty
acids, but the former can be estimated, as described later, and deducted
from the total.
_Total Alkali._--The best method is to incinerate 5 grammes of the soap
in a platinum dish, dissolve the residue in water, boil and filter,
making the volume of filtrate up to 250 c.c., the solution being
reserved for the subsequent determination of salt, silicates, and
sulphates, as detailed below.
Fifty c.c. of the solution are titrated with N/1 acid, to methyl orange,
and the result expressed in terms of Na_{2}O.
Number of c.c. required × 0.031 × 100 = per cent. Na_{2}O.
The total alkali may also be estimated in the filtrate from the
determination of fatty acids, if the acid used for decomposing the soap
solution has been measured and its strength known, by titrating back the
excess of acid with normal soda solution, when the difference will equal
the amount of total alkali in the quantity taken.
The total alkali is usually expressed in the case of hard soaps as
Na_{2}O, and in soft soaps as K_{2}O.
_Free caustic alkali_ is estimated by dissolving 2 grammes of the soap,
in neutral pure alcohol, with gentle heat, filtering, well washing the
filter with hot neutral spirit, and titrating the filtrate with N/10
acid, to phenol-phthalein.
Number of c.c. required × 0.0031 × 50 = per cent. free alkali Na_{2}O,
as caustic.
_Free Carbonated Alkali._--The residue on the filter paper from the
above determination is washed with hot water, and the aqueous filtrate
titrated with N/10 acid, using methyl orange as indicator. The result is
generally expressed in terms of Na_{2}O.
Number of c.c. required × 0.0031 × 50 = per cent. free alkali Na_{2}O,
as carbonate.
_Free Alkali._--Some analysts determine the alkalinity to
phenol-phthalein of the alcoholic soap solution without filtering, and
express it as free alkali (caustic, carbonates, or any salt having an
alkaline reaction).
_Combined Alkali._--The difference between total alkali and free alkali
(caustic and carbonate together) represents the alkali combined with
fatty acids. This figure may also be directly determined by titrating,
with N/2 acid, the alcoholic solution of soap after the free caustic
estimation, using lacmoid as indicator.
The potash and soda in soaps may be separated by the method described
for the estimation of potassium in _Pearl ash_ (page 126).
The potassium platino-chloride (K_{2}PtCl_{6}) is calculated to
potassium chloride (KCl) by using the factor 0.3052, and this figure
deducted from the amount of mixed chlorides found, gives the amount of
sodium chloride (NaCl), from which the sodium oxide (Na_{2}O) is
obtained by multiplying by 0.52991.
The potassium chloride (KCl) is converted into terms of potassium oxide
(K_{2}O) by the use of the factor 0.63087.
_Salt_ may be determined in 50 c.c. of the filtered aqueous extract of
the incinerated soap, by exactly neutralising with normal acid and
titrating with N/10 silver nitrate solution, using a neutral solution of
potassium chromate as indicator. The final reaction is more distinctly
observed if a little bicarbonate of soda is added to the solution.
Number of c.c. required × 0.00585 × 100 = per cent. of common salt,
NaCl.
Chlorides may also be estimated by Volhard's method, the aqueous extract
being rendered slightly acid with nitric acid, a measured volume of N/10
silver nitrate solution added, and the excess titrated back with N/10
ammonium thiocyanate solution, using iron alum as indicator.
_Silicates._--These are estimated by evaporating 50 c.c. of the filtered
extract from the incinerated soap, in a platinum dish with hydrochloric
acid twice to complete dryness, heating to 150° C., adding hot water,
and filtering through a tared filter paper.
The residue is well washed, ignited, and weighed as SiO_{2}, and from
this silica is calculated the sodium silicate.
_Sulphates_ may be determined in the filtrate from the silica estimation
by precipitation with barium chloride solution, and weighing the barium
sulphate, after filtering, and burning, expressing the result in terms
of Na_{2}SO_{4} by the use of the factor 0.6094.
_Moisture._--This is simply estimated by taking a weighed portion in
small shavings in a tared dish, and drying in the oven at 105° C. until
it ceases to lose weight. From the loss thus found is calculated the
moisture percentage.
_Free or Uncombined Fat._--This is usually determined by repeated
extraction of an aqueous solution of the soap with petroleum ether; the
ethereal solution, after washing with water to remove traces of soap, is
evaporated to dryness and the residue weighed.
A good method, which can be recommended for employment where many
determinations have to be performed, is to dissolve 10 grammes of soap
in 50 c.c. neutral alcohol and titrate to phenol-phthalein with N/1
acid. Add 3-5 drops HCl and boil to expel carbonic acid, neutralise with
alcoholic KOH solution and add exactly 10 c.c. in excess, boil for
fifteen minutes under a reflux condenser and titrate with N/1 acid. The
difference between this latter figure and the amount required for a
blank test with 10 c.c. alcoholic KOH, denotes the amount of alkali
absorbed by the uncombined fat.
_Examination of the fatty acids_ as a guide to the probable composition
of the soap:--
From the data obtained by estimating the "titre," iodine number, and
saponification equivalent of the mixed fatty and rosin acids, and the
rosin content, a fairly good idea of the constitution of the soap may be
deduced.
The titre, iodine number, and saponification equivalent are determined
in exactly the same manner as described under Fats and Oils.
The presence of rosin may be detected by the Liebermann-Storch reaction,
which consists in dissolving a small quantity of the fatty acids in
acetic anhydride, and adding to a few drops of this solution 1 drop of
50 per cent. sulphuric acid. A violet coloration is produced with rosin
acids. The amount of rosin may be estimated by the method devised by
Twitchell (_Journ. Soc. Chem. Ind._, 1891, 804) which is carried out
thus:--
Two grammes of the mixed fatty and rosin acids are dissolved in 20 c.c.
absolute alcohol, and dry hydrochloric acid gas passed through until no
more is absorbed, the flask being kept cool by means of cold water to
prevent the rosin acids being acted upon. The flask, after
disconnecting, is allowed to stand one hour to ensure complete
combination, when its contents are transferred to a Philips' beaker,
well washed out with water so that the volume is increased about five
times, and boiled until the acid solution is clear, a fragment of
granulated zinc being added to prevent bumping. The heat is removed, and
the liquid allowed to cool, when it is poured into a separator, and the
beaker thoroughly rinsed out with ether. After shaking, the acid liquor
is withdrawn, and the ethereal layer washed with water until free from
acid. Fifty c.c. neutral alcohol are added, and the solution titrated
with N/1 KOH or NaOH solution, the percentage of rosin being calculated
from its combining weight. Twitchell suggests 346 as the combining
weight of rosin, but 330 is a closer approximation.
The method may be also carried out gravimetrically, in which case
petroleum ether, boiling at 74° C. is used for washing out the beaker
into the separator. The acid liquor is run off, and the petroleum ether
layer washed first with water and then with a solution of 1/2 gramme KOH
and 5 c.c. alcohol in 50 c.c. water, and agitated. The rosin is thus
saponified and separated. The resinate solution is withdrawn, acidified,
and the resin acids collected, dried and weighed.
_Halphen's Reaction._--This is a special test to determine the presence
or absence of cotton-seed oil fatty acids in mixtures. Equal parts of
the fatty acids, amyl alcohol, and a 1 per cent. solution of sulphur in
carbon bisulphide, are heated in a test-tube placed in a water-bath
until effervescence ceases, then in boiling brine for one hour or longer
when only small quantities are present. The presence of cotton-seed oil
is denoted by a pink coloration. The reaction is rendered much more
rapid, according to Rupp (_Z. Untersuch. Nahr. Genussm._, 1907, 13, 74),
by heating in a stoppered flask.
Other bodies which it is occasionally necessary to test for or determine
in soap include:--
_Carbolic acid._--Fifty grammes of the soap are dissolved in water and
20 c.c. of 10 per cent. caustic potash added. The solution is treated
with an excess of brine, the supernatant liquor separated, and the
precipitate washed with brine, the washings being added to the liquor
withdrawn. This is then evaporated to a small bulk, placed in a Muter's
graduated tube, and acidified with mineral acid.
The volume of separated phenols is observed and stated in percentage on
the soap taken.
Or the alkaline layer may be rendered acid and steam distilled; the
distillate is made up to a known volume, and a portion titrated by the
Koppeschaar method with standard bromine water.
_Glycerine._--Five grammes of soap are dissolved in water, decomposed
with dilute sulphuric acid, and the clear fatty acids filtered and
washed. The filtrate is neutralised with barium carbonate, evaporated
to 50 c.c., and the glycerol estimated by the bichromate method detailed
under Crude Glycerine.
_Starch_ or _gum_ may be detected by dissolving the soap in alcohol,
filtering, and examining the residue on the filter paper. Starch is
readily recognised by the blue coloration it gives with a solution of
iodine in potassium iodide.
_Sugars_ are tested for by means of Fehlings' solution, in the liquor
separated from the fatty acids, after first boiling with dilute acid to
invert any cane sugar.
_Mercury_ will be revealed by a black precipitate produced when
sulphuretted hydrogen is added to the liquor separated from the fatty
acids, and may be estimated by filtering off this precipitate on a tared
Gooch's crucible, which is then dried and weighed.
_Borax or borates_ are tested for in the residue insoluble in alcohol.
This is dissolved in water, rendered faintly acid with dilute
hydrochloric acid, and a strip of turmeric paper immersed for a few
minutes in the liquid. This is then dried in the water-oven, when if any
boric acid compound is present, a bright reddish-pink stain is produced
on the paper, which is turned blue on moistening with dilute alkali.
The amount of the boric acid radicle may be determined by incinerating
5-10 grammes of soap, extracting with hot dilute acid, filtering,
neutralising this solution to methyl orange, and boiling to expel carbon
dioxide. After cooling, sufficient pure neutralised glycerine is added
to form one-third of the total volume, and the liquid titrated with N/2
caustic soda solution, using phenol-phthalein as indicator. Each c.c. of
N/2 NaOH solution corresponds to 0.031 gramme crystallised boric acid,
H_{3}BO_{3} or 0.0477 gramme crystallised borax,
Na_{2}B_{4}O_{7}·10H_{2}O.
LYES.
The amounts of caustic alkali (if any), carbonated alkali, and salt
present are determined in the manner already described under Alkali and
Alkali Salts. The glycerol content is ascertained by taking 2.5 grammes,
adding lead subacetate solution, and filtering without increasing the
bulk more than is absolutely necessary; the solution is concentrated to
about 25 c.c., and the oxidation with bichromate and sulphuric acid
conducted as described in the examination of Crude Glycerine. The
solution, after oxidation, is made up to 250 c.c., and titrated against
standard ferrous ammonium sulphate solution, the formula for the
calculation being:--
{0.25 - 2.5}
Per cent. of glycerol = { ---} × 40
{ n }
where n equals the number of c.c. of oxidised lyes required to oxidise
the ferrous ammonium sulphate solution.
The estimation of actual glycerol in this is necessarily a matter of
considerable importance, and a very large number of processes, which are
constantly being added to, have been suggested for the purpose.
Hitherto, however, only two methods have been generally adopted, _viz._
the acetin and the bichromate processes. Unfortunately the results
obtained by these do not invariably agree, the latter, which includes
all oxidisable matter as glycerol, giving sometimes considerably higher
results, and it has been suggested that a determination should be made
by both methods, and the average of the two results considered the true
value. This involves a considerable amount of time and trouble, and it
will generally be found sufficient in a works laboratory to determine
the glycerol by one method only in the ordinary course, reserving the
other process for use as a check in case of dispute or doubt.
_Acetin Method._--This consists in converting the glycerol into its
ester with acetic acid, the acetic triglyceride, or triacetin being
formed. This is then saponified with a known volume of standard alkali,
the excess of which is titrated with acid, and the percentage of
glycerol calculated from the amount of alkali absorbed.
From 1 to 1.5 grammes of the glycerine is weighed into a conical flask
of about 150 c.c. capacity, 7 or 8 c.c. of acetic anhydride added,
together with about 3 grammes of anhydrous sodium acetate, and the whole
boiled on a sand-bath under a reflux condenser for one to one and a half
hours, after which it is allowed to cool, 50 c.c. water added, and the
ester dissolved by shaking, and gently warming, the reflux condenser
still being attached as the acetin is very volatile. The solution is
then filtered from a white flocculent precipitate, which contains most
of the impurities, into a larger conical flask, of some 500-600 c.c.
capacity, and after cooling, rendered just neutral to phenol-phthalein
by means of N/2 caustic soda solution, the exact point being reached
when the solution acquires a reddish-yellow tint; 25 c.c. of a strong
caustic soda solution is then added, and the liquid boiled for about
fifteen minutes, the excess of alkali being titrated after cooling, with
N/1 or N/2 hydrochloric acid. A blank experiment is carried out
simultaneously, with another 25 c.c. of the soda solution, and the
difference in the amounts of acid required by the two, furnishes a
measure of the alkali required to saponify the acetin formed, and hence
the amount of glycerol in the crude glycerine may be calculated.
_Example._--1.4367 grammes crude glycerine, after treatment with acetic
anhydride, and neutralising, was saponified with 25 c.c. of a 10 per
cent. caustic soda solution.
The blank experiment required 111.05 c.c. N/1 hydrochloric acid.
Flask containing acetin " 75.3 c.c. " "
-----
35.75 c.c. " "
Hence, the acetin formed from the glycerol present in 1.4367 grammes of
the crude glycerine required 35.75 c.c. N/1 caustic alkali for its
saponification, so that the percentage of glycerol may be calculated
from the following formula:--
35.75 × 0.03067 × 100
Per cent. glycerol = --------------------- = 76.3.
1.4367
_Bichromate Method._--This process was originally devised by Hehner
(_Journ. Soc. Chem. Ind._, 1889, 4-9), but the modification suggested by
Richardson and Jaffe (_ibid._, 1898, 330) is preferred by the authors,
and has been practised by them for several years with perfectly
satisfactory results.
Twenty-five grammes of the crude glycerine are weighed out in a beaker,
washed into a 250 c.c. stoppered flask, and made up to the graduation
mark with water. Twenty-five c.c. of this solution are then measured
from a burette into a small beaker, a slight excess of basic lead
acetate solution added to precipitate organic matter, the precipitate
allowed to settle, and the supernatant liquid poured through a filter
paper into another 250 c.c. flask. The precipitate is washed by
decantation until the flask is nearly full, then transferred to the
filter, and allowed to drain, a few drops of dilute sulphuric acid being
added to precipitate the slight excess of basic lead acetate solution,
and the contents of the flask made up with water to 250 c.c. This
solution is filtered, 20 c.c. measured from a burette into a conical
flask of about 150 c.c. capacity, 25 c.c. of a standard potassium
bichromate solution containing 74.86 grammes bichromate per litre added,
together with 50 c.c. of 50 per cent. sulphuric acid, and the whole
placed in a boiling water-bath for one hour, after which it is allowed
to cool, diluted with water to 250 c.c., and this solution run in to 20
c.c. of a 3 per cent. ferrous ammonium sulphate solution until the
latter is completely oxidised, as shown by no blue coloration being
produced when one drop is brought into contact with one drop of a
freshly prepared solution of potassium ferricyanide on a spot-plate. The
ferrous ammonium sulphate solution is previously standardised by
titration with a potassium bichromate solution of one-tenth the above
strength, made by diluting 10 c.c. of the strong solution to 100 c.c.
with water.
The reaction taking place in the oxidation may be represented by the
equation:--
3C_{3}H_{5}(OH)_{3} + 7K_{2}Cr_{2}O_{7} + 28H_{2}SO_{4} =
9CO_{2} + 40H_{2}O + 7K_{2}SO_{4} + 7Cr_{2}(SO_{4})_{3}.
Now the strong potassium bichromate solution above mentioned is of such
a strength that 1 c.c. will oxidise 0.01 gramme glycerine, and 20 c.c.
of the ferrous ammonium sulphate solution should require about 10 c.c.
of the one-tenth strength bichromate in the blank experiment. If it
requires more or less than this, then the amount of ferrous ammonium
sulphate solution which would require exactly 10 c.c. (corresponding to
0.01 gramme glycerine) is calculated, and the oxidised glycerine
solution run into this until oxidation is complete.
The formula for the calculation of the percentage of glycerol then
becomes:--
{0.25 -(250 × 0.01)}
Per cent. of glycerol = { ---------- } × 500,
{ n }
where n equals the number of c.c. of oxidised glycerine solution
required to oxidise the ferrous ammonium sulphate solution.
Example:--
In the blank experiment 20 c.c. ferrous ammonium sulphate solution
required 9.8 c.c. one-tenth strength bichromate solution, so that 20.4
c.c. ferrous solution would equal 10 c.c. bichromate.
20.4 c.c. ferrous solution required 27.8 c.c. of oxidised glycerine
solution before it ceased to give a blue coloration with potassium
ferricyanide.
{0.25 - (250 × 0.01)}
Therefore, per cent. of glycerol = { ------------} × 500
{ 27.8 }
= 80.04 per cent.
Other methods have been suggested for the preliminary purification,
_e.g._, silver oxide, silver carbonate and lead subacetate, and copper
sulphate and caustic potash, but the lead subacetate alone with care
gives satisfactory results.
Other determinations include those of specific gravity, alkalinity,
proportion of salts and chloride, and tests for metals, arsenic, sulphur
compounds, sugar, and fatty acids.
_Specific gravity_ is determined at 15° C., and may be taken in specific
gravity bottle, or with a Westphal balance or hydrometer It usually
ranges from 1.3 to 1.31.
_Alkalinity_, which is usually sodium carbonate, and may be somewhat
considerable if the soap has been grained with caustic alkali, is
determined after dilution with water by titrating with N/2 acid, using
methyl orange as indicator.
_Salts._--These may be determined by gently incinerating 5-6 grammes of
the glycerine, extracting the carbonaceous mass with distilled water,
filtering, and evaporating the filtrate on the water bath. The dried
residue represents the salts in the weight taken.
_Chloride of sodium_ (common salt) may be estimated by dissolving the
total salts in water, adding potassium chromate, and titrating with N/10
silver nitrate solution.
_Copper_, _lead_, _iron_, _magnesium_, and _calcium_ may also be tested
for in the salts, by ordinary reactions.
_Arsenic_ is best tested for by the Gutzeit method. About 5 c.c. is
placed in a test-tube, a few fragments of granulated zinc free from
arsenic, and 10 c.c. dilute hydrochloric acid added, and the mouth of
the tube covered with a small filter paper, moistened three successive
times with an alcoholic solution of mercury bichloride and dried. After
thirty minutes the filter paper is examined, when a yellow stain will be
observed if arsenic is present.
_Sulphates._--These may be precipitated with barium chloride in acid
solution, in the usual way, dried, ignited, and weighed.
_Sulphites_ give with barium chloride a precipitate soluble in
hydrochloric acid. If the precipitate is well washed with hot water, and
a few drops of iodine solution together with starch paste added, the
presence of sulphites is proved by the gradual disappearance of the blue
starch-iodine compound first formed.
_Thiosulphates_ are detected by precipitating any sulphite and sulphate
with barium chloride, filtering, acidifying, and adding a few drops of
potassium permanganate solution, when in the presence of a mere trace of
thiosulphate, the solution becomes cloudy.
_Sulphides._--Lewkowitsch recommends testing for these by replacing the
mercury bichloride with lead acetate paper in the Gutzeit arsenic test.
Any sulphide causes a blackening of the lead acetate paper.
_Sugars_ may be tested for both before and after inversion, by boiling
with Fehlings' solution, when no reduction should take place, if pure.
_Fatty acids_ are detected by the turbidity they produce when the
diluted glycerine is acidified.