DETEKMIXATIOX OF rilOSI'IIORIC OXIDE. 



203 



If the pli()si)]i()iic oxidei as atcII as the ammonia l)e 

 remoTed by suitable means from the- alkaline solution to 

 Avliicli excess of sodinm hydroxide has been added, which 

 was done by adding' a. solution of calcium chloride, the 

 amount of alkali used should correspond with the equation : 



2 P.O^ . 52 Mo03 + CaCL + 116 NaOH 



= 2 Ca3 (PO,)^ + 12 XaCl + 52 Na.MoO,. 

 The foliowinff results were obtained : — 



It will he- noticed, of course, that the values (b) given in 

 the second columns of the two tables above are calculated 

 on the basis of (a) being equivalent to 101, not 100, mols. 

 of sodium hydroxide. The difference which would be made 

 by taking" the latter experimental fig-ure instead of that 

 indicated by the formula is, however, not great, and, in 

 addition, the determination of the amounts of sodium 

 hydroxide required is probably subject to the same errors 

 iu all three cases. Xeither of the above modifications, the 

 addition of formaldehyde or of calcium chloride or of both, 

 is recommended, as the end point with phenolphthalein is not 

 more sensitive than in the stiaightforward method. 



To return to the difference between the amount of sodium 

 hydioxide found necessary in practice and that required by 

 the formula given to the i)recipitate, while we can at present* 

 offer no satisfactory explanation thereof, the variations found 

 by other workers for modifications of the Peinberton alkali- 

 metric process may be mentioned. Thus Pembertont 

 originally used a factor corresponding to the ratio 



^aOH : PoO^ = 4G-4 : 1 ; 



later the same worker adopted thei ratio 4() : 1, while recently 

 Prescott:!: determined a factor corresponding to a ratio 



XaOH : P,0, = 4T-2 : 1. 



The second of these ratios corresiionds to the formula for 

 ammonium phospho-molybdate given by Hundeshag'en,§ while 



* Mean of several determinations. 

 iJour. Am. Chem. Soc, xv, 382 (1894). 

 X'Tovni. .[(/)■. Science, vi, 111 (1914). 

 § Zcit. Anal Chem., xxviii, 164 (1899). 



