294 Report of the Department of Chemistry of the 



is not the action of neutral citrate but rather the action of an acid 

 citrate. There seemed to be no theoretical reason why a solution 

 of sodium citrate should not be just as effective a solvent and it 

 possesses two distinct advantages. It is a more stable salt and as the 

 base in it is not volatile the solution would remain neutral through- 

 out the whole operation. All trouble in securing a neutral solution 

 would be eliminated, for a solution of citric acid could be neutralized 

 with sodium hydroxide, using phenolphthalein as an indicator, or 

 the neutral crystals of sodium citrate could be dissolved in water, 

 and the solution made up to the required volume. 



In order to learn what the action of a solution of sodium citrate 

 might be, one was made which was of the same molecular concen- 

 tration as the Official 1 ammonium citrate solution, i. e., 314 grams 

 crystallized sodium citrate, (CeHsOeNasVH H 2 0, per liter. This 

 solution was used to determine the amounts of insoluble and reverted 

 phosphoric acid in several fertilizers, Thomas slag, ground bone, 

 ground rock phosphate, dicalcic-phosphate, CaHP0 4 , and tricalcic- 

 phosphate, Ca 3 P 2 8 . The results, together with those obtained by 

 the use of the Official citrate solution, are given in the table. In 

 connection with these figures, it is noticeable that the differences 

 between the figures obtained with the two solutions are, in most 

 cases, of the same magnitude as the variations in the figures obtained 

 by different chemists working upon the same sample. 2 It is also 

 interesting to know that Samples 5, 10 and 11, which show the 

 largest differences, all contain bone. The duplicate determinations, 

 in all cases, showed closer agreement with sodium citrate solution 

 than with the Official citrate solution. 



The Official method directs that the flask in which the reaction 

 takes place should be loosely stoppered, during the time it is being 

 maintained at 65° C, in order to prevent evaporation. The use of 

 stoppers often results in the loss of a determination through the 

 breaking of a flask. It is suggested that the flask be closed with a 

 one-hole rubber stopper carrying an empty calcium chloride tube, 

 300 mm. in length, which will serve as a condenser. The use of 

 such a condenser will not interfere with the shaking and it furnishes 

 a vent which prevents the breaking of the flask. 



The last column of the table shows the amounts of ammonia given 

 off during the half hour of treatment with ammonium citrate solu- 

 tion prescribed by the Official method. This ammonia was caught 

 in standard acid by means of an air current which was passed through 

 the Erlenmeyer flask in which the solvent action was taking place. 

 These figures seem to bear some relation to the difference given in the 

 preceding column. By noticing the large amounts of ammonia 

 given off by the Thomas slag, rock phosphate and ground bone 



1 U. S. Dept. Agr., Bur. Chem. Bull. 107 (revised). 



2 Jour. Indust. Eng. Chem., 3:118 and 5:957. The differences between the 

 extremes in these two cases are 1.23 per ct. and 0.90 per ct. respectively. 



