WATEE 19 



Nitrous and Nitric Acids.—Y^hen nitrogenous bodies 

 break up in the process of putrefaction and decomposi- 

 tion, they become reduced to their simple elements. The 

 nitrogen in them combines with hydrogen to form 

 ammonia, while later on the ammonia by uniting with 

 oxygen becomes oxidized to nitrous and finally to nitric 

 acid. ^ The nitrous acid unites with bases to form nitrites, 

 the nitric acid forms nitrates. The determination of 

 nitrites and nitrates in drinking-water is of the greatest 

 importance, as a means of ascertaining the presence of 

 nitrogenous substances undergoing change, which are 

 presumably of animal origin, as vegetable matter yields 

 but little of these acids to water. 



Nitrous acid is formed before nitric acid; if therefore 

 nitrites are found in the water they are regarded as 

 indicating recent contamination, while nitrates suggest the 

 contamination is older. Both, of course, may be found in 

 the same sample of water, indicating that pollution is still 

 occurring. 



To detect nitrous acid add a solution of potassium iodide 

 and starch to the water, and then dilute sulphuric acid. 

 If nitrous acid is present an immediate blue colour is 

 produced. It is well to make a comparative experiment 

 with distilled water. The starch and iodide solution keeps 

 badly so should be freshly prepared. 



The most delicate test for nitrous acid is metaphenylene- 

 diamine. A few drops of this with dilute sulphuric acid 

 produces a yellow colour, more or less immediate ; if there 

 be only one part of nitrous acid in 10,000,000 parts of 

 water the colour will develop in half an hour. 



The test for nitric acid in the absence of nitrous is brucine. 

 A solution of this is added to the water under examination, 

 followed by the cautious addition of a few drops of strong 

 sulphuric acid, the test-tube being inclined so as to let the 

 acid run down the side, and fall to the bottom without 

 mixing. At the junction of the acid and water a pink 

 and yellow coloured zone forms, the depth varying with 

 the amount of acid present. This test is very delicate, 



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