22 University of Calif orn ia Puhlications in Agricultural Sciences [Vol. 1 



able quantities of nitrates are to be found. So far as the clay- 

 coagulating substances are concerned, it has alwaj^s been a com- 

 mon practice in soil work to employ varying amounts of a satur- 

 ated solution of alum to obtain a clear soil solution, and more 

 recently it has been proposed by investigators who have studied 

 the method under discussion to use aluminum cream for the pur- 

 pose in place of alum. For decolorizing solutions both aluminum 

 cream and bone black have been used. The methods for both 

 clay coagulation and decolorization are obviously essential in most 

 soil work, since ordinary filtration, without the use of such 

 agents, can rarely be depended on to yield a clear, colorless soil 

 solution, even if time be no object. The employment of the 

 Pasteur Chamberland filter to remove clay has been found by 

 direct investigation to involve well-defined losses of nitrates. 



It is not our purpose here to enter into a lengthy review of 

 other investigations bearing on the subject in hand, but into a 

 brief discussion of the more important ones which show the ques- 

 tions still remaining unsolved or bring out certain results with 

 which ours do not agree. 



In 1894 GilP carried out a series of painstaking investiga- 

 tions which, briefly, indicate (1) that for purposes of accuracy 

 the phenoldisulphonic acid employed in the nitrate determination 

 must be carefully prepared to insure a uniform compound for 

 use as a standard; (2) that chlorine induces losses of nitric acid 

 both when the solution containing nitrate is evaporated on the 

 water bath and when the residue is treated with the reagent; 

 (3) that NajCOg added to the nitrate-containing solution to pre- 

 vent escape of nitric acid during evaporation induces losses of 

 nitrates varying in quantity from four to six per cent; (4) that 

 alumina may be used to precipitate colloidal material for obtain- 

 ing a clear solution ; (5) that silver sulphate, if free from nitrate, 

 may be employed to precipitate chlorine, thus removing an im- 

 portant interfering agent. 



More recently Chamot and his coworkers^ have prosecuted an 

 even more thoroughgoing investigation than the preceding, in 

 which the most emphasis has been placed, however, on the mode 

 of preparation of the tripotassium salt of nitrophenoldisulphonic 



1 Jo«r. Am. Chem. Soc. vol. 16, p. 122. 1894. 



