210 ANNUAL RECORD OF SCIENCE AND INDUSTRY. 



schonbein's test for nitrates. 



Storer lias explained at length the reaction proposed by 

 Schoubein for the detection of minute traces of nitrates. 

 Two modes of applying the test were originally given. In 

 the one, dilute sulphuric acid and iodo-starch paste were 

 added directly to the nitrate solution, and the mixture was 

 stirred with a zinc rod. In the other and better mode, the 

 nitrate was first reduced by means of zinc or cadmium, and 

 then the solution was acidulated with sulphuric acid and the 

 iodo-starch paste added. He finds, contrary to the opinion 

 of Carius, that no objection seems to lie against this test on 

 the score of delicacy; but that it has a fatal defect in the 

 forms in which it is commonly used, in that mere water, en- 

 tirely free from any nitrates or nitrites, will, on being treated 

 with zinc or cadmium as in the process of testing, react upon 

 iodo-starch as if these compounds were present. This color- 

 ation, thus produced, the author shows is due to the hydro- 

 gen peroxide which is formed by the action of the metal 

 itself upon the water a reaction observed by Schonbein 

 himself. Hence, whenever the degree of coloration of the 

 iodo-starch obtained in testing for a nitrate by this method 

 is less intense than the tint obtainable from 0.000187 gramme 

 of potassium nitrate in 50 cubic centimeters of water, it is diffi- 

 cult to decide whether the coloration may not be wholly due 

 to hydrogen peroxide. Having thus shown the defect, the 

 author set himself at work to remedy it, and to devise a mod- 

 ification of the process which, while preserving the delicacy 

 of the test, should yet be easily applied. The very simple 

 plan was adopted of acidulating the solution very slightly, 

 before boiling the nitrate in it, with metallic cadmium. Un- 

 der these circumstances no hydrogen peroxide is produced, 

 while the reduction of the nitrates goes on quite as well. 

 The only precaution necessary is to prevent the loss of any 

 of the nitrous acid, which is easily accomplished by attaching 

 to the flask a small inverted Liebig condenser during the 

 boiling. In his experiments, 0.0001 gramme of nitrogen 

 pentoxide, placed in the solution as potassium nitrate, in 50 

 c. c. of water containing two drops of dilute sulphuric acid, 

 gave, after boiling for five minutes, a reaction in less than fif- 

 teen minutes. Even 0.00005 gramme gave the reaction in 



