INORGANIC ELEMENTS. 13 



hold waste somewhere, and a quantitative test is of prime 

 importance to settle this point. Such a test and a few others 

 will be illustrated below. 



Ex. THE TEST FOR CHLORIDES. A test is often made in this way: 

 Measure out 200 cc. of the water, add to it a few drops of a solution of 

 pure neutral potassium chromate, and then from a burette run in, with 

 constant stirring, solution of tenth normal silver nitrate until a faint red- 

 dish precipitate of silver chromate appears. Each cubic centimeter of the 

 silver solution precipitates 3.54 mg. of chlorine from common salt or other 

 chloride, and when the last trace of chlorine is combined, the silver begins 

 to precipitate the chromate with production of red color. The chromate 

 acts here as an " indicator," as it shows just when the chlorine is all com- 

 bined by beginning to precipitate itself. 



In making this test it is well to take two similar beakers, place them 

 side by side on white paper, pour equal amounts of water in each, add to 

 each the same number of drops of the indicator, and then with one make 

 the actual test by adding the silver solution. Note the amount used to 

 give a light shade and then discharge it by adding a drop of salt solution. 

 Now, with this opalescent or turbid liquid for comparison add silver nitrate 

 to the second beaker until the light yellowish red shade just appears. This 

 reading is usually somewhat more accurate than the first. 



As a result of the decomposition of various nitrogenous 

 matters ammonia is frequently found in natural waters. Its 

 amount is therefore a measure of contamination to some ex- 

 tent, and tests for its presence are always made in sanitary 

 examinations. In practice the test is usually made on a dis- 

 tillate from the water in question, but the following experi- 

 ment will illustrate the behavior of the reagent employed. 



Ex. TEST FOR AMMONIA. Solutions of ammonia or ammonium salts 

 possess the peculiar property of giving a yellowish brown color with what 

 is known as Nessler's reagent (a solution of mercuric-potassium iodide, 

 made strongly alkaline with sodium or potassium hydroxide). With more 

 than traces of ammonia a precipitate is formed. 



To make the test measure out 50 cc. of the water in a large test-tube, 

 or tall narrow beaker, and add to it 2 cc. of the Nessler solution. By 

 placing the beaker on a sheet of white paper and looking down through it, 

 the depth of color can be observed. A few parts of ammonia in one hun- 

 dred million parts of water can be readily seen and measured. 



THE OXIDATOX TESTS. Pure water absorbs free oxygen 

 from the atmosphere but has no tendency to decompose com- 

 pounds to secure it. On the other hand waters containing 



