92 H. §S. H. WARDLAW. 
cipitate obtained at 50°—60° C. the excess of molybdenum 
was about twice as great. Chesneau also calls attention 
to the effect of temperature on the composition of this pre- 
cipitate. He states that above 65°—70° C. ammonium 
tetramolybdate—(NH.),0.4 MoO,—is formed ina solution 
of ammonium molybdate, and that it is this substance which 
is carried down by the precipitate of ammonium phospho- 
molybdate. Many other authors state that at high tem- 
peratures molybdic acid is thrown down from solutions of 
ammonium molybdate, and that the precipitate of ammo- 
nium phosphomolybdate formed is contaminated with this. 
Precipitation at as low a temperature as possible is there- 
fore generally advised. With the proportions of the reagents 
used by Neumann, however, it was not found possible to 
bring about precipitation within reasonable time at low 
temperatures. When the solutions were kept at room 
temperature no sign of precipitation appeared even after 
standing for two days. When the reagents were mixed at 
40° OC. and maintained at this temperature, only partial 
precipitation had occurred after four hours. Even at 60°. 
—70° C. the precipitation was by no means complete in 
half an hour. It was therefore found necessary, in order 
to ensure complete precipitation, to adhere to the tem- 
perature of precipitation set down by Neumann. 
Summary. : 
1. The values obtained in the estimation of phosphate by 
Neumann’s method were always high, the error increasing 
with the amount of phosphate estimated. For 22 mg. of 
P.O; the mean error was + 47%. 
2. The source of this error is an excess of molybdenum 
carried down in the precipitate of ammonium phospho- 
molybdate. 
3. The error does not depend on the rate of addition of 
the precipitant. 
