.360 ])i:ti:rmination of piiospffORic oxide. 



unobtainable. The sulphuric acid used for the back titration was 

 standardised against the sodium hydroxide, and was also checked 

 against sodium carbonate prepared from the bicarbonate. 



Weighed cjuantities of the phosphates were dissolved in 

 nitric acid or in water, made up to definite volume, and aliquots 

 of each solution taken for the determinations. As a check on 

 the calculated amount of phosphoric oxide in the pure substances, 

 determinations were also made by the method of precipitation 

 with ordinary ammonium nitrate-molybdate followed by solution 

 of the precipitate in dilute ammonia and reprecipitation as 

 ammonium magnesium phosphate which was finally weighed as 

 magnesium pyrophosphate in the usual way. This method was 

 also used to determine the phosphoric oxide in the fertiliser 

 samples used. 



The theoretical amounts of phosphoric oxide used are given 

 in the second column of the following table, while in the third 

 column the amounts determined by the molybdate-magnesium 

 method as outlined above are given. In the fourth column are 

 the amounts of strictly tenth-normal sodium hydroxide used in 

 the titration, while the fifth and sixth columns give the factors 

 as calculated from the theoretical and determined amounts of 

 phosphoric oxide respectively. .All the results are the mean of at 

 least two agreeing determinations. 



Phosphoric Oxide. P'actor calculated. 



Phosphate X , 10 From 



used. Deter- Sodium From Deter- 



Theory. mined. Hydroxide. Theory. mination. 



gr. gr. CCS. 

 CaH'PO^ 



2H2O 0.00825 0.008.^1 29.10 0.000283' 0.000285* 



CaHPO. 



2H2O 0.00825 0.00822 29.10 0.000283' 0.000285' 



CaHPO, 



2H2O 0.00893 0.00895 3140 0.000284* 0.000285* 



KHjPO* 0.00738 0.00741 25.90 0.000284* 0.000284' 



CaHPO, 



2H«0.. 0.01032 0.01031 ,'^6.30 0.000284' 0.000284* 



CaHPO, 



2H2O 0.02063 0.02067 72.50 0.000284' 0.000285^ 



Mixed Fertiliser — 0.00867 30.22 — 0.000286* 



Mixed Fertiliser — o.oo6r4 21.35 — 0.000287* 



Water extract of 

 Superphos- 

 phate — 0.03405 119.00 — 0.000286' 



The mean factor as calculated from the amounts of phosphoric 

 oxide theoretically present is 0.000284-, while that calculated 

 from the amounts of phosphoric oxide found by the gravimetric 

 method is 0.000285*. The mean ratio of sodium hydroxide to 

 phosphoric oxide is 49.8: i, a close approximation to the theo- 

 retical ratio of 50: i deduced from Hundeshagen's formula. It 



