a3(C6H507)2 + 6NH4C1,
(1)

Ca3(C6H507)2 -i
h 4NH4C1

+ 2HC1,
(2)

Ca3(C6H507)2 -
\- 2NH4C1

+ 4HC1,
(3)

Ca3(C6H507)2 H
'- 6HC1.
(4)

280 QUANTITATIVE AGRICULTURAL ANALYSIS
acid) as indicator. This method is unreliable because corallin is
not sufficiently sensitive to citric acid or ammonium hydroxide.
In the other method the solution is nearly neutralized and a
small excess of calcium chloride solution in water and alcohol is
added. Calcium citrate, a salt of small solubility, precipitates
as a result of such reactions as the following:
7 + 3CaCl2 ->
5O7 + 3CaCl2
507 + 3CaCl2
2H3C6H507 + 3CaCl2
Equation (1) shows that if only triammonium ("neutral")
citrate is present, no matter how highly this may be hydrolyzed,
the solution will be left neutral to all indicators by the removal
of calcium citrate. According to Eqs. (2), (3) and (4) any acid
citrate or free citric acid will produce free hydrochloric acid,
which may be made evident by the use of indicators. On the
other hand, if the citrate solution contained an excess of ammo-
nium hydroxide this would remain after the precipitation of cal-
cium citrate. According to the result obtained by testing the
filtrate with an indicator, either citric acid or ammonium hydrox-
ide may be added, as necessary, to obtain the proper condition
of equivalent quantities of acid and base. That this solution
is not really neutral and that it does not really contain tri-
ammonium citrate, has already been explained.
Preparation of Ammonium Citrate Solution: Calcium Chloride Method.—
To 370 grn of commercial citric acid, dissolved in 1500 cc of water, add
commercial ammonium hydroxide until nearly neutral, testing with recently
prepared corallin solution. Add water until the specific gravity is about
1.11 at 20°
Prepare a solution of fused calcium chloride, 20 gm to 100 cc, and add
400 cc of 95-per cent alcohol. Make this solution exactly neutral with
tenth-normal ammonium hydroxide or hydrochloric acid, as may be neces-
sary, using freshly prepared corallin solution as a preliminary indicator;
test finally by diluting 2 cc with an equal volume of water and adding methyl
red (cochineal is the official indicator for this purpose). Approximately
50 cc of this solution will precipitate the citric acid from 10 cc of the citrate
solution.