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Page 94
The fluid runs from the fatty acids on the filter, which, with the
washings, has been preserved in a sufficiently large beaker glass, is
colored with tincture of litmus, and decomposed with a test alkaline
solution until the blue color appears. The difference of the quantity of
alkali required to neutralize the sulphuric acid, and the quantity of
sulphuric acid used in the first instance, allows a calculation to be
made as to the quantity of effective alkali in the soap, for example:--
23.86 grms. of soap (partly cocoa-nut oil soap).
17.95 " fatty acids with filter.
04.44 " filter.
-----
13.51 grms. of hydrates of fatty acids = 56.62 per cent.
28.00 cub. cent. of the diluted sulphuric acid applied for the
decomposition of the soap, of which 100 cub. cent.
represent 2982 grms. of carbonate of soda.
17.55 cub. cent. of alkaline fluid, which were used for the
saturation of the above acid, and of which 100 cub.
cent. saturate an equal quantity of that acid.
----
10.45 cub. cent. of the sulphuric necessary for the alkali
contained in the soap, representing 0.1823 grms. of
soda = 7.64 per cent.
A determination of the alkali as a sulphate afforded in another portion
of soap 9.57 per cent. of soda, because the sulphate of soda and
chloride of sodium present in the soap gave up their alkali.
The alkaline fluid applied by me was a saccharine solution of lime,
which can be naturally replaced by a solution of soda, and must be if
the chloride of sodium and sulphate of soda mixed with the soap shall be
determined in the following way:--
The fluid again exactly neutralized with alkali is evaporated to
dryness, and the residue gently heated to redness. As in the above
manipulation, the fluid was not heated to the boiling point, the
original chloride of sodium and sulphate of soda are contained in the
weighed residue, besides the soda of the soap and that which has been
added with the sulphuric acid, forming sulphate of soda. A second
exposure to a red heat with sulphuric acid converts the whole residue
into sulphate of soda, and from the increase of weight, by a comparison
of the equivalents of NaCl and NaO, SO_{3} the quantity of the former
may be decided. According to the equivalents which Kopp furnished in
1850, the increase of weight to the chloride of sodium is as 1:4.68. The
original sulphate of soda must be, lastly, found by the subtraction of
the same salt formed plus the calculated chloride of sodium from the
first heated residue.
In practice, it is seldom necessary to proceed with the determination of
the chloride of sodium and sulphate of soda, except with stirred and
cocoa-nut oil soaps; certainly less of the truth is seen if, after the
above determination of the fatty acids and the effective alkali, the
absent per centage of water is introduced in the calculation, than if
the water is reckoned, which is never completely evolved from soap, even
technically prepared at 302� Fahr., and another determination made of
the fatty acids or alkali _en bloc_ the fatty acids, or even the
alkaline contents.
The method here given partakes of the usual imperfections, that the
fatty acids as well as the unsaponified soap are equally estimated, and
the mixed hydrate or carbonate of the alkali as well as the combined
alkali. The presence of the carbonate can be easily recognized by the
foaming of the soap solution, upon the addition of the sulphuric acid.
These imperfections, however, are of little importance.
It must be granted that the minutely correct determination of the
constitution of soap must be always yielded up to those who are
technically conversant with this department of chemistry, the estimation
of free alkali and unchanged fat excluded in, at least, by certain ages
of the soap. Further, a considerable excess of one or another ingredient
soon betrays itself by a corresponding departure in the soap of the
characteristic properties of a good product, and a small excess can be
judged sufficiently exact from the proportion of the alkali, which,
supposing soda present, should not amount to more than 13 per cent. with
a pure cocoa-nut oil soap, not less than 11.5 per cent. with a tallow
soap; but with palm oil and mixed soaps the one or the other limit
approximates.--_Journal f�r Praktische Chemie._
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