QUANTITATIVE DETERMINATIONS

determined by titration with a standard solution of a base or
carbonate, so may the sulphate of hydrogen (sulphuric acid) be
determined. It is obvious that both determinations, as well
as all other acidimetric determinations, are measurements of
ionizable hydrogen alone, and that they can be calculated only to
this hydrogen itself or, if other acids are known to be absent, to
the acid present—in this case sulphuric acid. Such titrations
could not properly be regarded as determinations of the acid
radical, since salts of the essential acid are almost invariably pres-
ent in small and variable quantities.

Determination of Sulphuric Acid: Potassium Hydroxide Method.—Fifth-
normal potassium hydroxide is prepared and standardized as in the deter-
mination of hydrochloric acid, page 59. The sample of sulphuric acid,
being non-volatile, may be weighed in a flask or beaker, if an accurate
balance of sufficient capacity is at hand, or it may be measured and the
specific gravity determined, the weight then being calculated. The dilution
and titration are carried out exactly as directed on pages 57 and 59. The
calculation of grams per cubic centimeter, per cent by weight and normality
differ from that for hydrochloric acid only in the equivalent weights used.
Sulphuric acid, being a dibasic acid, has a hydrogen equivalent of 2 and its
equivalent weight is one-half of its molecular weight.

CALCIUM

Gravimetric, by Weighing Calcium Oxide.—If a neutral or
basic solution of a calcium salt is treated with a soluble oxalate,
as ammonium oxalate, a reaction like the following occurs:

(NH4)2C204 -* CaC204 + 2NH4C1. (1)

After filtering and washing the calcium oxalate this is ignited:

CaC2O4 -> CaCOs + CO ; (2)

» CaO + C02. (3)

The calcium oxide is then weighed.
The method is applicable only to soluble calcium salts and to
calcium oxide, hydroxide or carbonate. The last three com-
pounds dissolve in hydrochloric acid, with formation of water or
carbonic acid as byproducts, and carbonic acid is expelled by
heating. Calcium phosphate must be given a preliminary
treatment to .separate phosphoric acid, as otherwise the phos-
phate will reprecipitate as soon as the solution is made basic.