S. K. TALAPATRA, S. C. RAY AND K. C. SEN 245
(d) The course of chemical reaction following the solution of the preci-
pitate in an excess of standard alkali.
Pemberton precipitated the phosphate at a temperature of 100°C. Accord-
ing to Kilgore [1894], at this high temperature there is danger of the deposi-
tion of molybdic acid and he suggested a temperature between 55° and 60°C as
the ideal for precipitation. Subsequent workers like Neumann [1902-03] and
Richard and Godden [1924], however, preferred to precipitate between 70° to
80°C when the precipitation is done in the presence of excess of nitric acid.
The official A. 0. A. C. method now advocates a temperature between 25°C to
30°C.
In the original method of Pemberton as also in the subsequent modification
of Neumann and, Richard and Godden, the precipitation was done in a medium
containing not only an excess of HNO 3 but also 7 to 12 per cent of NH 4 NO 3 .
According to Hundeshagen [1924], when ammonium molybdate is added
to a nitric acid solution of an ortho-phosphate the yellow precipitate of ammo-
nium phospho-molybdate has a composition (NH 4 ) 3 PO 4 , 12MoO 3 , 2HNO 3 ,
H 2 O. The acid and water are probably in loose combination and are quickly
and completely removed by heating at 150°C. As the calculation of the
amount of phosphate in the unknown solution is based on the assumption that
the ratio of (PO 4 ): (MoO 3 ) = 1 : 12, in the volumetric method of estimating
phosphate it is essential that the conditions of precipitation be so adjusted that
the precipitate must correspond to the formula of Hundeshagen. The subse-
quent washing of the precipitate has a dual aim. Firstly, to remove the acid
that is superficially adhering to the precipitate and secondly to remove the acid
which is in combination with the precipitate. Richard and Godden have
removed the entire acid, i.e. the portion adhering to the precipitate and the
other in combination with it, by washing with neutral aqueous ammonium
nitrate followed by washing with cold water. According to these authors the
resulting precipitate left in the filter paper has the composition (NH 4 ) 3 PO 4 ,
12MoO 3 . Kolthoff and Sandell [1937] advocates that the precipitate of ideal
composition, i.e. one in which the ratio of PO 4 to MoO 3 — 1 : 12 and which
contains no nitric acid in any form, can be obtained by washing with 1 per cent
solution of KNO 3 until the final washing gives no acid reaction with methyl
orange. In our laboratory as well as in the laboratory of the Imperial Agri-
cultural Chemist (Pusa), the same procedure as that of Kolthoff is practised
but the precipitate is washed with a three per cent solution of KNO 3 . In all
the methods discussed, the washing with ammonium nitrate or potassium
nitrate solution is always preceded by washing with a dilute solution of
HNO 3 in order to remove any molybdic oxide that may have been deposited
with the precipitate.
After the ideal precipitate (NH 4 ) 3 PO 4 , 12MoO 3 has been obtained, the
next operation is to dissolve the precipitate in a known volume of standard
alkali.
It seems that workers are divided in their opinion as regards the course of
reaction that follows during the solution of the precipitate in the alkali.
Some of the typical reactions suggested by different workers are as follows :—
(1) 2(NH 4 )JPO 4 .
24MoO 3
+ 46NaOH = (NH 4 ) 4 H 2 P 2 O 8 +
(NH 4 ) 2 MoO 4 + 23 Na 2 MoO 4 + 22 H 2 O (Pemberton).