416 EXAMINATION OF DRINKING-WATER. 



in this manner. The latter is especially the case if the wells in use lie near water- 

 closets and manure-heaps, so that the products of decomposition can filter through 

 into the reservoir for the water. 



Qualitative Demonstration. (i) A fairly large amount of water is evaporated 

 in a porcelain dish to dryness, and is then subjected to greater heat. If large 

 amounts of organic matter are present a discoloration between brown and black 

 takes place. If the matters contain nitrogen the odor of 'burning hair appears 

 at the same time. Good water, so treated, yields but a faint brown color. Micro- 

 scopic examination may also be made to determine the presence of microorganisms 

 in water. About i cu. cm. of water is allowed to evaporate upon a slide having 

 an up-turned edge and kept in a place free from dust, and the dry spot is examined. 

 (2) A solution of gold-pota % ssium chlorid added to water produces, after stand- 

 ing for a long time, a black muddy precipitate. (3) A solution of potassium per- 

 manganate added to the water placed under cover is gradually decolorized, with 

 the formation of a brown muddy deposit. The precipitates from 2 and 3 are 

 the more abundant the greater the amount of organic substances present in the 

 drinking-water. 



Quantitatively the amount of organic substances is determined, according to 

 Kubel, as follows. Two solutions are required: A, containing 0.63 gram of 

 pure crystalline oxalic acid in i liter of distilled water; B, containing 0.33 

 gram of potassium permanganate in i liter of purest distilled water. For the 

 determination of the efficiency of the latter 100 cu. cm. of distilled water are 

 placed in a wide-necked bottle of 300 cu. cm. capacity, together with 5 cu. cm. 

 of dilute sulphuric acid (i volume of acid to 3 volumes of water) and heated to 

 boiling. Into this from 3 to 4 cu. cm. of solution B are allowed to flow from a buret 

 provided with a glass stop-cock. The mixture is boiled for ten minutes, the 

 heat is then removed and 10 cu. cm. of solution A are added. Finally, the fluid, 

 which has become colorless, is mixed with solution B until a faint red tint appears. 

 The number of cubic centimeters used corresponds to 6.3 mgm. of oxalic acid, 

 which are present in the 10 cu. cm. of solution A, and contains exactly 3.16 mgm. 

 of potassium permanganate, or 0.8 mgm. of oxygen available for oxidation, 

 which is necessary for the transformation of the 6.3 mgm. of oxalic acid into 

 carbon dioxid. 



In order to test a given water for the amount of organic matter present, 

 100 cu. cm. of the sample are placed in a flask of 300 cu. cm. capacity, 5 cu. cm. 

 of dilute sulphuric acid are added and so much of solution B that the fluid becomes 

 an intense red and remains so even when heated. After five minutes' boiling, 

 10 cu. cm. of solution A are added. The fluid, thus made colorless, is then titrated 

 with solution B until a faint red tint appears. 



For purposes of calculation as many cubic centimeters of solution B as are 

 necessary for the oxidation of 10 cu. cm. of solution A are subtracted from the 

 total number of cubic centimeters of solution B used in the experiment. The 

 difference in cubic centimeters is multiplied by 3.16 : x if the proportion of potas- 

 sium permanganate, by 0.8 :x if the proportion of oxygen, necessary for the 

 oxidation of the organic substances present in 100,000 parts of water is desired 

 (x represents the number of cubic centimeters of solution B that corresponds to 

 10 cu. cm. of solution A). 



Example. Nine and nine-tenths cubic centimeters of solution B correspond to 

 10 cu. cm. of solution A. After acidulation with sulphuric acid, 100 cu. cm. of the 

 water under examination is mixed with 15 cu. cm. of solution B and boiled. The 

 red fluid is decolorized by the 10 cu. cm. of solution A. To restore a faint red 

 tint 4.4 cu. cm. of solution B must be added. Estimation: 15 + 4.4 = 19.4; 

 19.4 9.9 = 9.5. Therefore, for the oxidation of the organic Substances in 

 100,000 parts of water (9.5 X 3-i6) : 9.9 = 3.008 of potassium permanganate, or 

 (6.5 X 0.8) : 9.9 = 0.77 part of oxygen are necessary. Bad drinking-water, espe- 

 cially when it contains much organic matter, should never be used in its native 

 state, but particularly not at a time when epidemics of typhoid fever, of cholera 

 or of dysentery prevail or threaten. It should be urgently advised that the 

 water be thoroughly boiled previously, as by this means the germs of infection 

 are destroyed. The resulting insipid taste can be readily improved by means of 

 effervescent powder, sugar or fruit- juice. 



