14 PHYSIOLOGICAL CHEMISTRY. 



organic matters or certain inorganic contaminations have the 

 power of decomposing oxygen salts to secure the oxygen they 

 desire, and the amount of oxygen so taken up becomes a 

 measure of the impurity of the water. Potassium perman- 

 ganate is a salt, which, under certain conditions, gives up its 

 oxygen to waters containing organic bodies in solution and is 

 frequently employed in water analysis for this purpose. An 

 experiment will show one way in which it is used. 



Ex. Measure out about 100 cc. of pure, carefully distilled water, pour 

 it into a clean beaker in which water has just been boiled and add 5 cc. 

 of pure dilute sulphuric acid (i to 3). Place the beaker on wire gauze 

 and heat to boiling. Now add 5 drops of a dilute permanganate solution 

 (300 milligrams to the liter) from a burette or dropping tube and boil five 

 minutes. The pink color persists. 



Repeat the experiment, using 100 cc. of common hydrant water to which 

 a trace of egg albumin or urea has been added, and after running in the 

 permanganate boil again. The color fades out and more may be added. 

 Finally, after sufficient has been added the pink color remains. The num- 

 ber of drops or cubic centimeters used is a measure of the contamination 

 of the water, although often, as in this experiment, a very rough one. 



THE TESTS FOR NITRITES AND NITRATES. Nitrogenous 

 matters undergoing oxidation in water and soil usually give 

 rise, in time, to nitrites and finally to nitrates. These com- 

 pounds are therefore looked for in water as evidence of past 

 contamination. In most instances nitrites, as a less advanced 

 stage of oxidation than nitrates, suggest comparatively recent 

 contamination. The tests are especially interesting in the 

 examination of well and spring water. 



Chemists are acquainted with a number of methods for the 

 detection of traces of nitrogen in the form of nitrites and 

 nitrates, but at the present time certain color reactions are, 

 because of their simplicity, mainly in favor. These are illus- 

 trated by the following tests : 



A reagent for nitrites is prepared by dissolving 0.5 gm. of 

 sulphanilic acid in 150 cc. of acetic acid of 25 per cent strength, 

 and mixing this with a solution of o.i gm. of pure naphthyla- 

 mine in 200 cc. of dilute acetic acid. This mixture keeps very 

 well for a time in the dark. 



