METHOD OF GILL 461 



tecting the presence of nitrates, 78 a method afterwards worked 

 out by Hooker. 79 



395. Phenylsulfuric Acid Method. Rideal also proposes a 

 variation of the method described by Hooker, which consists in 

 the substitution of phenylsulfuric acid for carbazol. 80 



The solutions required are: 



(a) A standard solution of potassium nitrate containing 0.7215 

 gram of the pure crystallized salt in a liter of water. 



(b) Phenylsulfuric acid (acid phenyl sulfate), prepared by 

 dissolving 15 grams of pure crystallized phenol in 92.5 cubic 

 centimeters of pure, redistilled sulfuric acid free from nitrates 

 and diluted with seven and one-half cubic centimeters of water. 



The process is conducted as follows: 



A known volume of the sample, from 25 to 100 cubic centi- 

 meters, according to its richness in nitrates, is evaporated to 

 dryness in a porcelain dish, one cubic centimeter of phenylsul- 

 furic acid, one cubic centimeter of pure water and three drops 

 of strong sulfuric acid added, and the mixture gently warmed. 

 A yellow color shows the presence of nitrates. Dilute to about 

 25 cubic centimeters with water and add ammonia in slight ex- 

 cess. Pour into a narrow nessler tube, adding the washings, 

 and make up to 100 cubic centimeters. Imitate the color of the 

 solution with the standard potassium nitrate treated with the 

 same reagents. 



The phenylsulfuric acid should be prepared some time before 

 use, as the fresh solution imparts a greenish tint to the yellow 

 of the ammonium picrate formed. 



396. Method of Gill. The phenyl sulfate process, as described 

 by Leffman, is conducted as follows: 81 



Solutions Required. Acid phenyl sulfate (Phenoldisulfonic 

 Acid} : Thirty-seven grams of strong sulfuric acid are added 

 to three grams of pure phenol and heated for six hours in, not 

 upon the water bath, and preserved in a tightly stoppered bottle. 



7 * Zeitschrift fur Hygiene, 1887, 2 : 163. 



19 Journal of the Franklin Institute, 1889, 127 : 61. 



80 Chemical News, 1889, 60 : 261. 



81 Examination of Water for Sanitary and Technical Purposes, 5th 

 Edition, 1903 : 50. 



