EXAMINATION OF DRIXKIXG-WATER. 415 



(poor in calcium) or hard (rich in calcium). After this calcium-precipitate has 

 settled, the clear fluid is poured off and mixed with a solution of sodium phos- 

 phate and a little ammonia; the crystalline precipitate that now forms indicates 

 the presence of magnesia. 



The feebler the reactions for sulphuric acid, chlorin, calcium and magnesium, 

 the better is the water. Good drinking-water, should, moreover, contain only 

 traces of nitrates, nitrites and ammonium-compounds, as their presence points to 

 organic substances containing nitrogen in a state of decomposition. 



Nitricacid is indicated when 100 cu. cm. of water are acidulated with two or three 

 drops of concentrated sulphuric acid, some bits of zinc are added and then a solu- 

 tion of iodin, zinc and starch, and a blue tint appears. The following test is exceed- 

 ingly sensitive : some fragments of brucin sulphate along with a drop of the water 

 to be examined are to be placed in a watch-glass; then a few drops of concentrated 

 sulphuric acid are added. A rose-red color appears. Diphenylamin sulphate 

 mixed with a few drops of concentrated sulphuric acid yields in the presence of 

 nitrates, even when in great dilution, a blue color. This test is, therefore, recom- 

 mended for the demonstration of well-water in milk. 



Demonstration of^nitrous acid: To 100 cu. cm. of water a few drops of pure 

 concentrated sulphuric acid and a solution of zinc iodid and starch are added: 

 a blue color appears. In addition naphthionic acid and pure /3-naphthol, thor- 

 oughly mixed in a mortar, are recommended as a reagent. To 10 cu. cm. of the 

 fluid to be examined for nitrites two drops of a concentrated solution of hydro- 

 chloric acid and as much of the mixture mentioned as can be taken up on the 

 point of a knife are added and the whole is thoroughly shaken. If ammonia is 

 added in a layer on top of this mixture a red ring appears. This test has a sensi- 

 tiveness of i : 100 millions. 



Ammonium-compounds in considerable amount render the water suspicious. 

 To 150 cu. cm. of water 0.5 cu. cm. of solution of sodium hydrate and i.o cu. cm. 

 of solution of sodium carbonate are added and the precipitate is allowed to settle. 

 Of the supernatant clear fluid a column 15 cm. high is introduced into a narrow 

 graduated cylinder and mixed with Nessler's reagent (a solution of mercuric 

 iodid and potassium iodid in an excess of potassium hydroxid) : Traces of ammo- 

 nia in water thus yield a color between yellow and red, large amounts a brown 

 precipitate of mercuric -ammonium iodid. 



The contamination of water by decomposed animal substances will be recog- 

 nized by the amount of contained nitrogen. In most cases it is sufficient to 

 determine the amount of nitric acid. For this two solutions are necessary: A, 

 containing 1.871 gin. of potassium nitrate to a liter of water; i cu. cm. of this 

 contains i gram of nitric acid; B, a dilute solution of indigo; i part of pulverized 

 indigotin is slowly added with stirring to 6 parts of fuming sulphuric acid; the 

 mixture is allowed to settle, the blue liquid is poured into 40 times its amount 

 of distilled water and then filtered. Finally, the fluid is diluted with distilled 

 water until it begins to be transparent in layers of from 12 to 15 mm. thick. To 

 test the efficiency of B, i cu. cm. of A is mixed with 24 cu. cm. of water, some 

 table-salt and 50 cu. cm. of concentrated sulphuric acid are added, and so much 

 of B is now allowed to flow from a buret until a faint green tint appears. The 

 number of cubic centimeters of B used corresponds to i mgm. of nitric acid. 

 Twenty-five cubic centimeters of the water to be examined are mixed with 50 

 cu. cm. of concentrated sulphuric acid and titrated with B until the green color 

 appears. This titration must, however, be repeated, and in the second observa- 

 tion the number of cubic centimeters of indigo-solution be permitted to flow in a 

 stream; a somewhat larger amount of fluid may be required to produce the green 

 color. The number of cubic centimeters of solution B thus used (in proportion 

 to the previously ascertained strength) indicates the amount of nitric acid present 

 in 25 cu. cm. of water. In well-water as much as 10 mgm. nitric acid is found 

 in the liter. 



Hydrogen sulphid is recognized, apart from its odor, through the brown color 

 imparted to a piece of filter-paper that has been saturated in an alkaline solution 

 of lead, and is held over the water boiling in a flask. If hydrogen sulphid is 

 present in combination in the water, some sodium hydroxid and a dilute solution 

 of sodium nitro-prussid are added; a reddish- violet color appearing 



It is of the greatest significance with respect to the excellence of drinking- 

 water that it should be free from putrefying or decomposing organic matter. _ The 

 latter, in conjunction with the lower organisms always to be found in it, 

 when ingested with drinking-water, expose the body to serious dangers, as a 

 number of infectious diseases,, especially cholera and typhoid fever, can be spread 



