ms of Na_{2}O.

Number of c.c. required x 0.031 x 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 x 0.0031 x 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 x 0.0031 x 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