ACCURACY OF NEUMANN’S METHOD. 9} 
precipitation. In Neumann’s method the length of time 
for which the precipitate of ammonium phosphomolybdate 
remains in contact with the mother liquor is not long, about. 
thirty minutes, so, as will be seen from the following 
experiments, this factor is not likely to be the source of 
any considerable error. Two pairs of phosphate estimations. 
were carried out by Neumann’s method on 20 cc. portions 
of standard phosphate solution, the precipitates in one case 
being filtered off after having stood under the mother liquor 
for twenty minutes, in the other case being left in contact 
with the mother liquor for three days. The following are 
the results obtained :— 
Table VITI. 
Influence of length of time between precipitation and filtration on error. 
Estimation. Stood for | P,O, found. | P,O, present. as 
33 20 min. 50:0 mg. +12:1 
34 big2O (58 49:9) 6 11:9 
44°6 mg, 
31 3 days BO, 14°3 
32 ils, KOs .. 12°7 
The increase in the error of the results given by the pre- 
cipitates which have stood the longer time in contact with 
the mother liquor is within the experimental variations to . 
which the method is subject, so that, contrary to what 
Baxter and Griffin found in their case, here variations in 
the length of time of contact between precipitate and 
mother liquor do not affect the composition of the precipi- 
tate. 
4, Temperature of precipitation.—Baxter states that 
the temperature of precipitation has an important influence 
on the excessof molybdenum carried down by the precipitate 
of ammonium phosphomolybdate. When the precipitation 
occurred at room temperature, Baxter found an excess of 
about 17% of molybdenum in the precipitate; in the pre- 
