43 2 AGRICULTURAL ANALYSIS 



unit value in nitrogen given in the first "gram" column of the 

 table ; viz., 0.000036008. It is thus seen that the 5.86 cubic cen- 

 timeters of indigotin solution are equivalent to 0.000035859 gram 

 of nitrogen per cubic centimeter. The water under examination, 

 therefore, contains nine and six-tenths parts of nitrogen as nitric 

 acid per million. 



Attention must also be paid in standardizing indigotin solu- 

 tions to the initial temperature. A rise in the initial temperature 

 will be attended by a diminution in the quantity of indigotin oxi- 

 dized. Experiments with a room temperature of 10 and a 

 room temperature of 20, being the initial temperatures of the 

 experiments, showed that at the higher temperature the amount 

 of indigotin consumed was about five per cent, less when the 

 strong solutions of nitrate were employed. The indigotin solution, 

 therefore, must be standardized at the same temperature at which 

 the determinations are made. 



If 20 cubic centimeters of the standard nitrate solution em- 

 ployed be used in setting the indigotin solution, this stand- 

 ard will enable the operator to determine nitric acid up to 17.5 

 parts of nitrogen per million in water or soil extracts. 



The presence of an abundance of chlorids in the water under 

 examination tends to diminish the content of nitric acid found, 

 and also tends to introduce an error, which is sometimes of a 

 plus and sometimes of a minus quantity, according to the strength 

 of the nitric acid present. The reaction is shortened in weak 

 solutions by the presence of chlorids, and the quantity of indigotin 

 consumed is consequently increased. The error introduced by 

 chlorids is usually of an insignificant nature. 



On account of the interference of organic matters with the 

 reaction of indigotin it is not of much use in the examination of 

 nitrates washed out of soils, although in some cases the results 

 may be quite accurate. This method must, therefore, be con- 

 sidered as applicable, in general, only to waters or soil extracts 

 which contain little or no organic matter. 



In analytical work pertaining particularly to agriculture, the use 

 of the indigotin method for determining nitric acid has been 

 largely employed, both in the analyses of soil extracts and drain- 



