OF ARTS AND SCIENCES. 53 



IV. 



ON THE ESTIMATION OF PHOSPHORIC ACID AS 

 MAGNESIC PYROPHOSPHATE. 



By F. A. Goocii. 



Presented October 8th, 1879. 



The investigation of which this paper is an account was under- 

 taken at the request of Dr. Wolcott Gibbs, for the purpose of find- 

 ing, for use in his work upon the complex inorganic acids, the best 

 mode of proceeding in determining the phosphoric oxide of phos- 

 photungstates and phosphomolybdates by precipitation as ammonio- 

 magnesic phosphate and estimation as magnesic pyrophosphate. 

 During the course of the work it has been found necessary to review 

 much of what has been previously published concerning this method 

 of determining phosphoric acid. 



In a paper upon this subject, Kubel * criticised Fresenius's correc- 

 tion t (subsequently withdrawn t) for the solubility of ammonio- 

 magnesic phosphate in water containing free ammonia and magnesia 

 mixture, and asserted that such correction is not only unnecessary, 

 but that the results of the analysis are of themselves much higher 

 than theory indicates unless the precipitate is, after washing, dissolved 

 and reprecipitated. Kubel' s method was to add hydrochloric acid and 

 then ammonia, or ammonic chloride, or ammonia, to a measured 

 amount of a solution of sodic phosphate, to precipitate with magnesic 

 sulphate mixture, and after twelve hours to filter off and wash with 

 dilute ammonia containing one part of strong ammonia to three of 

 water. A number of analyses made in this way gave amounts of phos- 

 phoric oxide varying from 101.5 to 104.3 per cent, of the real quan- 

 tity present, — the actual weight being in each case 0.1986 gr. An- 

 other set of analyses in which the precipitate was, after washing, 

 dissolved in hydrochloric acid and again thrown down with ammonia, 



* Zeitscbrift fur Anal. Chem., VIII., 125. 

 t Anleitung zur Quant. Anal., 5 Aufl., 333. 

 t Ibid., 6 Aufl. 134. 



