METHOD OF GILL 463 



i. e., the sample contains one and one-half milligrams of nitro- 

 gen as nitrate per liter. 



The ammonium picrate solution keeps very well, especially in 

 the dark. A good plan, therefore, is to make up a standard solu- 

 tion equivalent to 10 milligrams of nitrogen as nitrate per liter, 

 to which the color obtained from the sample may be directly 

 compared. 



The results obtained by this method are quite accurate. Care 

 should be taken that the same quantity of phenoldisulfonic acid be 

 used for the sample and for the comparison liquid, otherwise dif- 

 ferent tints instead of depths of tints are produced. 



With subsoil, fertilized and other solutions probably containing 

 much nitrate, 10 cubic centimeters of the sample are sufficient 

 for the test, but with river and spring waters, 25 to 100 cubic 

 centimeters may be used. When the organic matter is sufficient 

 to color the residue, it will be well to purify the sample by addi- 

 tion of aluminum hydroxid and subsequent filtration, before 

 evaporating. This method may also be used to determine small 

 quantities of nitrates when the amount is too small for estima- 

 tion by the ferrous chlorid or reduction processes. 



Mason calls attention to the fact that chlorids interfere with 

 the delicacy of the process, giving readings decidedly lower 

 than the truth. 82 The method is so easy and convenient, how- 

 ever, that Mason was led to add salt to the comparison stand- 

 ards rather than to abandon the process. In the conduct of this 

 method the chlorin in the water to be examined is first to be 

 determined and the standard solution is treated with sodium 

 chlorid in sufficient quantity to afford the same quantity of 

 chlorin as in the sample. The results are found to be very satis- 

 factory. If the chlorin be less than 10 parts per million it does 

 not interfere with the determination. The solutions are pre- 

 pared as follows: 



Phenol-sulfonic acid 



Sulfuric acid, pure and concentrated 370 grams 



Pure phenol 30 grams 



These reagents are placed in a flask and kept surrounded by 

 88 Examination of Water, 3rd Edition, 1906 =50. 



