USE; OF ZINC SULFID AND SODIUM THIOSULFATE 377 



The phenolsulfuric acid used contains only enough phenol 

 to reduce half a gram of saltpeter. 



The sample and acid mixture having been put in the combus- 

 tion flask, the latter is heated and shaken, at intervals, for an 

 hour and the contents cooled. 



The conversion of the nitrates into nitro-phenol compounds is 

 finished in this time, and the next step consists in reducing these 

 bodies to the amido-phenol group. This is accomplished in the 

 cold by nascent hydrogen produced by the addition of zinc dust 

 to the mixture. From one to three grams of the dust are to be 

 used in proportion to the quantity of nitrates originally present. 



The flask should be placed in a cooling mixture and the zinc 

 dust added in small portions to prevent a too violent evolution 

 of hydrogen. After the reduction is ended, the flask is allowed 

 to stand for two hours, after which the combustion, distillation, 

 and titration are accomplished in the usual way. On cooling, 

 after the end of the combustion, the contents of the flask become 

 solid. They may be brought again into the liquid state by shak- 

 ing and gentle warming. 



327. The Salicylic Acid Method. The introduction of the use 

 of salicylic acid as the proper reagent to prevent -the loss of 

 nitrogen when a nitrate :s acted on by sulfuric acid is due to 

 Scovell. It was noticed that the action of phenol was too violent 

 to protect the process from loss of nitrogen. After a careful 

 trial of many organic compounds capable of forming, nitro-com- 

 pounds in those circumstances, salicylic acid was selected as the 

 most promising reagent. 19 Rigid trials by Scovell and others 

 extending over many years have confirmed the propriety of 

 this choice. 20 The method has also been found to be accurate 

 in the presence of chlorids as well as of nitrates. 



328. Use of Zinc Sulfid and Sodium Thiosulfate. During 

 the many analyses made by this modified method, it was 

 noticed that on pure nitrates there was apparently a slight 



19 Thesis for Degree of Doctor of Philosophy, University of Illinois, 

 June, 1906, (Unpublished). 



10 Division of Chemistry, Bulletin 16, 1887 : 51. 



Division of Chemistry, Bulletin 19, 1888 : 47. 



New Jersey Agricultural Experiment Station Report, 1887, 169. 



