158 Austin — Double Ammo7iium Phosphates in Analysis. 



The results of the experiments of Section B, in which the 

 precipitation was made by microcosmic salt in presence of a 

 small amount of ammonium chloride, show an average consti- 

 tution of the precipitate similar to that of the precipitate 

 obtained in A by an equivalent amount of ammonium phos- 

 phate, but the results of both series, A and B, vary considerably 

 from the ideal much more closely approximated in the experi- 

 ments of C, in which precipitation was made in the presence 

 of a large excess of ammonium chloride. With these results 

 those of Dakin are not in accord. Upon examination, how- 

 ever, Dakin's procedure appears to be open to criticism. 



In the first place, it appears that the asbestos employed by 

 Dakin was of the hydrous (serpentine) variety, which disinte- 

 grates when heated and is readily acted on by many reagents. 

 Though previously treated vrith hydrochloric acid, it was, by 

 Dakin's account, perceptibly soluble in a solution of ammonium 

 phosphate. After drying at 100°-105° the crucible and felt 

 lost some decimilligrams on ignition. It was on this account 

 that Dakin weighed precipitate and filter, after drying at 100°- 

 105°, when the double phosphate was to be estimated as such, 

 or after ignition when the zinc pyrophosphate was weighed, 

 and then, dissolving the precipitate in nitric acid, re weighed 

 the crucible and asbestos, and by difference from the former 

 weight obtained the apparent weight of the precipitate. The 

 inevitable loss through disintegration or solubility of the filter 

 in the nitric acid used to dissolve the precipitate in Dakin's 

 procedure, must raise the corresponding apparent weight of 

 the precipitate itself. Any series of results based upon the 

 use of such material must of necessity be imperfect to the 

 extent to which the ignited filter disintegrates or dissolves 

 under the action of the nitric acid used to dissolve the precipi- 

 tate. 



Anhydrous (amphibole) asbestos, the material of which I 

 made use, is insoluble, under the conditions of analytical work, 

 in ordinary reagents including even the strong acids, as has 

 been abundantly shown. ^ It is likewise completely insoluble 

 in ammonium phosphate under the conditions of the work 

 described, as I have found by experiment. Herein lies one 

 cause of difference between Dakin's results and mine. 



A second source of error in Dakin's procedure is found 

 in the fact that precipitates of the double ammonium phos- 

 phates are washed in a one per cent solution of the reagent, 

 ammonium phosphate followed by "redistilled alcohol." Dakin 

 calls attention to the fact that absolute alcohol must not be 



*Gooc]a, Am. Chem, Jour, i, 317. Mar, this Journal, xii, 288; xliii, 521. 

 Browning, this Journal, xliv, 399. Phinney, this Journal, xlv, 468. 



