30 



University of California Publications in Agricultural Sciences [Vol. 1 



TABLE VI 



Effects of Aluminum Cream and Bone Black 



Sufficient alnmimiiii cream to 

 clear solutiou. Five minutes 

 exposure 



Twice the amount of aluminum 

 cream used above. Exposed one 



and one-half hours 



SufBcient bone black to clear 

 and decolorize solution 



The data in Tables Y and VI are clearly very striking. The 

 enormous losses of nitrates sustained through the use of a satur- 

 ated solution of alum, varying quantities of aluminum cream and 

 bone black, make these substances entirely unfit for use as 

 precipitants for clay, or organic matter, or both, when nitrates 

 are to be determined. While bone black occasions the largest 

 losses, and potash alum the smallest, of any of the substances 

 above described, the losses of nitrates brought about through the 

 use of all the precipitants are too great to permit of their con- 

 tinuance in a method for nitrate determinations which is none 

 too accurate under the best of conditions. It is therefore evident 

 that nitrates are lost not merely through the loss of nitric acid, 

 as is the case w^here salts are used, but that there is a loss of 

 nitrates mechanically through adsorption on the part of the 

 colloidal material of the precipitant, as must be the case where 

 such substances as aluminum cream and bone black are used. 

 The large amounts of colloids possessed by these substances, -with 

 the accompanying large surface areas, evidently prevent some of 

 the nitrate in solution from going through the filter. 



On casting about for a method to precipitate clay or organic 

 matter. Ave first tried the Briggs filter pump, but found that open 

 to two objections. First, the losses of nitrates through what we 

 look upon as adsorption on the part of the clay filter, though not 

 very large, were nearly equal to those induced by small amounts 

 of sulfates. Second, Avhile the filter pump yields a clear solu- 

 tion, it does not serve to decolorize solutions. After several fur- 



