ACCURACY OF NEUMANN’S METHOD. 87 
Hach precipitate obtained thus contains the equivalent 
of 11°28 mg. of P.O; (standard solution D). Wesee from 
the formula of Hundeshagen for the ammonium phospho- 
molybdate that for each molecule of Pb;(PO.). 24 molecules 
of PbMoO. should be formed. The weight of lead phosphate 
corresponding to the above amount of P.O; is 0°01128 x 
811/142 = 0°0644 gm. (P.O; = 142, Pb;(PO:). = 811); 
the weight of lead molybdate formed at the same time 
should be 0°01128 x 24 x 367°1/142 = 0°699 gm. (PbMoO. 
= 3671). The total weight of the precipitate should 
therefore be 0°0644 + 0°699 = 0°7634 gm. The weights 
of the precipitates of lead phosphate and lead molybdate 
actually obtained are given in the following table :— 
Table V. 
Weights of lead phosphate and lead molybdate equivalent to 11°28 mq. 
Ofer Om, 
Phosphomolybdate Weight of Lead Salts from 
precipitate. Portion A. Portion B. wileoul Gn To iessblvis 
27 0°8075 gm. 0:8048 gm, 0°8062 gm. 
28 0:8048 ,, 0:8049 ,, O80 49 ee 
29 0°8085 ,, 0:8050 ,, OFS0G1s ys 
30 Orsris: 5 0:8095_,, OrslOr 
The mean weight for the whole series is 0°8071 gm. If 
0°0644 gm., the weight of the lead phosphate formed, be 
subtracted from this we get 0°7427 gm. instead of 0°699 gm. 
as the weight of the lead molybdate formed. This number 
is 6°257% in excess of that required by the formula of 
ammonium phosphomolybdate used by Neumann, and 
instead of (NH.),;.PO..12 MoO;.2 HNO; would give us 
(NH.)3.PO..12°75 MoO;.2HNO;. This result isnot far from 
that of Hissink and van der Waerden mentioned above; they 
found the molecular proportion of MoO, to be 12°65. 
Assuming the excess of molybdenum to be present as molyb- 
