58 JOURNAL OF THE 



ON THE NEUTRALITY OF STANDARD AMMO- 

 NIUM CITRATE FOR THE DETERMINA- 

 TION OF REVERTED PHOS- 

 PHORIC ACID. 



H. B. BATTLE. 



The necessity that the Standard Ammonium Citrate Solution 

 (Specific Gravity = 1.09) should be exactly neutral is emphasized in 

 the Washington Method for the determination of Phosphoric Acid 

 as well as in all succeeding methodsgenerally adopted. The method 

 of the Official Agricultural Chemists, adopted first at Atlanta then 

 at Philadelphia and at Washington, lays especial stress on this 

 point. 



It is stated by Gladding (American Chem. Journal, VI, p. 3,) that 

 the condition of the Ammonium Citrate, whether neutral, ammo- 

 niacal or acid causes varying effect in dissolving the Citrate-Soluble 

 Phosphoric Acid. In a sample of S. C. rock he found — digesting at 

 65°C for 30 minutes with a neutral solution- 1.35 per cent. P0O5 dis- 

 solved, with an ammoniacal solution 1,06 per cent, dissolved, and 

 with an acid solution 2.89 per cent., thus showing that the acid so- 

 lution dissolves 1.54 per cent. P0O5 more than the neutral solution. 

 The same authority elsewhere writes (Am. Chem. Journ., Vol. IV, 

 p. 138) : "To the use of acid solutions of citrate, either acid at the 

 beginning of the digestion or becoming acid during its continuance, 

 are probably due more than to any other cause the discrepancies in 

 this analytical process." In view of this fact especial care should 

 be taken in order that the citrate should be exactly neutral at the 

 time of digestion. 



Yet, and I believe it is not generally known, the Ammonium Ci- 

 trate solution after being neutralized, on standing, gradually be- 

 comes acid or slightly so. To prove this I carefully neutralized sev- 

 eral solutions, and after the lapse of some days, examined them 

 with reference to their neutrality. The solutions were made as laid 

 down by the Atlanta method, using commercial citric acid, neutral- 

 izing first with commercial ammonium carbonate*, then with am- 



*NoTE. — I would like to state here that I have found the weights given by 

 the Atlanta method inaccurate. They should be 470 grms. Citric Acid and 370 

 grms. Am. Garb., using ordinary commercial chemicals in both instances, in- 

 stead of the weights given by the method. 



