278 The Soluhllity of Calcium Phosphates in Citric Acid 



'reverted' phosphate in superphosphate. As a measure of ready 

 availabiUty in basic slag a 2 % solution of citric acid has now super- 

 seded ammonium citrate." 



To test some of these statements, and to add to our knowledge on 

 the subject, it was decided to purchase or to prepare Tricalcium 

 phosphate, and then to ascertain its behaviour towards the prescribed 

 citric acid solution. Citric acid was chosen, as in modern practice this 

 solvent has taken the place of ammonium citrate, and the results are 

 similar so far as my experiments have gone. 



In the analytical work, connected with the experiments about to 

 be described, lime had been determined by three methods: 



(a) Adding sulphuric acid and evaporating till a considerable crop 

 of crystals of gypsum has formed. Cool, add alcohol, filter, wash, 

 ignite, and weigh as CaS04^. 



(6) Dissolve in hydrochloric acid and add ammonia till precipitate 

 just begins to form. Redissolve this in a drop of hydrochloric acid, 

 add ammonium oxalate in excess and then acetate of soda. Allow to 

 settle, filter, wash, dry, ignite, and weigh as CaO^. I find that when 

 this method is followed, the lime precipitate always contains minute 

 quantities of phosphoric acid, which is greater the larger the amount of 

 lime used and precipitated. For accurate determinations this must be 

 determined and deducted. It has been found preferable to precipitate 

 the lime with excess of ammonium oxalate in a solution to which acetic 

 acid has been added. When this is done mere traces of phosphoric acid 

 occur in the lime precipitate. 



(c) (lirard's method or Reynoso's modification of this, of removing 

 the phosphoric acid in nitric acid solution by means of tin-foil, filtering 

 and precipitating the lime by ammonium oxalate^. It was found that 

 the filtrate from the stannic acid and phosphoric acid compound contains 

 small amounts of tin which must be removed by sulphuretted hydrogen 

 before precipitating the lime. Phosphoric acid has been determined by 

 precipitating with molybdate reagent. Dissolving the precipitate in 

 ammonia and precipitating with magnesia mixture, following the 

 method adopted in The Fertilisers and Feeding-stuffs (Methods of 

 Analysis) Regulations 1908^. 



^ Wiley, Priiiciplcs and practice of Agricultural Analysis, Vol. ii, p. 21. 

 - Fresenius, Quantitative ChewAcal Analysis, Vol. i, p. 188. 



* Crookes, Select methods of Chemical Analysis, p. 499. 



* Fertilisers and Ftedimj Stuffs Regulations, 1908. Board of Agriculture. Fisheries 

 Leaflet No. 18, p. 17. 



