Certain precautions should be observed in the collection of samples of water intended for examination. It is indispensable for this purpose to employ scrupulously clean glass stoppered bottles, which are washed out several times with the water previous to being filled. If a well or stream is to be sampled, the bottle should be entirely immersed in the water some distance from the sides of the stream, and, if taken from a pump or pipe, the latter should be cleansed by first running a considerable quantity of the water before charging the bottle.
1. Colour, odour, and taste.—The colour is best determined by filling a glass cylinder, about 2 feet in height, with the sample, placing it upon a white surface and observing the tint produced; or, by the use of a coloured glass tube of the same length, which is provided with glass plates attached at each end, and is filled with the sample and viewed when held towards a sheet of white paper.
As a rule, pure water exhibits a light-bluish tint, a yellowish hue being generally considered a suspicious indication; but it frequently occurs that a perfectly colourless water is bad, and one possessing a decided colour may prove to be at least, fair in quality. The odour of the sample is ascertained by placing a corked bottle, one-half filled with the water, in a warm place (at about 38°) for some time, and then shaking the bottle, withdrawing the stopper and immediately testing the odour. Pure water should be free from much perceptible odour of any kind, and more especially from one of a disagreeable nature. The same remark applies to the taste. Water should be practically tasteless, even when warmed. It frequently happens, however, that a water may be highly contaminated with deleterious organic impurities, and still remain devoid of any marked unpleasant taste. There are few simple tests of any value which will reveal at once the sanitary quality of drinking water. One, sometimes employed, is to fill a clean quart bottle about three-fourths full with the suspected sample, and dissolve in it a teaspoonful of fine granulated white sugar. The bottle is then corked and allowed to remain in a warm place for two days, when, in the presence of sewage contamination, it will become cloudy or milky.[119] According to Wanklyn and Chapman,[120] if a brownish colour or precipitate is produced upon the addition of 1·5 c.c. of Nessler’s reagent (see p. [208]) to 100 c.c. of the water, it should be considered unfit for domestic use.
2. Total solid residue and loss on ignition.—500 c.c. of the water under examination are introduced, in small portions at a time, into a tared platinum dish, and evaporated to dryness over the water-bath, the residue being subsequently dried for three or four hours in an air-bath at 100°. The solid residue obtained, multiplied by 200, represents parts in 100,000: or, by 140, grains per imperial gallon. It is usually considered that, unless the proportion of total solids exceeds 40 grains per Imperial gallon (32 grains per U.S. gallon, or about 56·5 parts per 100,000), the water need not be objected to for drinking purposes on this ground alone. The volatile and organic matters are determined by igniting the solid residue, which is afterwards allowed to cool. It is then moistened with a little carbonic acid water or solution of ammonium carbonate, dried to constancy at 130°, and the organic matter estimated by the decrease in weight. Formerly, this process was chiefly depended upon for determining the proportion of organic substances contained in water. It is open to numerous serious objections, among which are, that it may afford a result either below or above that correctly representing the quantity of organic ingredients present in the sample. The first case takes place when a portion of the organic matter is decomposed during the process of evaporation, and is quite liable to occur; the second case takes place when the water contains nitrates, which would be decomposed upon ignition. The method, however, possesses some value, and is still often resorted to as affording a general idea of the proportion of organic contamination present, the degree of blackening of the solid residue during the process of ignition being, at least, a useful qualitative indication.
3. Organic matter in solution.—A method frequently employed for this determination is based upon the supposition that the amount of potassium permanganate required to oxidise the organic constituents contained in water would serve as a criterion of its sanitary value. It is generally known as the “Forchammer” or “oxygen” process, and, although of undoubted service in comparing the quality of samples of very impure water, it is defective in the following important respects: Different organic substances are not affected to an equal extent by potassium permanganate; albumen, for instance, being far less easily oxidised than other compounds, and the value of the results afforded is vitiated by the presence of certain inorganic bodies, such as nitrites, sulphuretted hydrogen, ferrous salts, etc. It has been stated, that the more deleterious and putrescent organic ingredients of water are those most readily affected by the permanganate solution. As modified and improved by Miller[121] and by Tidy,[122] the process consists substantially in adding an excess of a standard solution of potassium permanganate to a measured quantity of the water under examination (acidulated with sulphuric acid), and then determining the excess of permanganate used by means of sodium hyposulphite and potassium iodide. The following solutions are required:—
Potassium Permanganate.—0·395 gramme of the salt is dissolved in 1 litre of distilled water; 10 c.c. of this solution represent 0·001 gramme of available oxygen.
Sodium Hyposulphite.—One gramme of the salt is dissolved in a litre of water.
Starch solution.—One gramme of starch is triturated with about 20 c.c. of boiling water, and the mixture allowed to stand at rest over night, after which the clear supernatant solution is drawn off.
Pure distilled Water.—This is prepared by digesting 10 litres of distilled water with 10 grammes of potassium hydroxide and 2 grammes of potassium permanganate in a still provided with an inverted condenser at 100° for twenty-four hours, after which the water is distilled, separate portions being frequently tested with Nessler’s solution; the distillate is not reserved for use until this reagent ceases to produce a brownish coloration.
The determination proper is executed as follows:—Two flasks are first thoroughly cleansed by washing with concentrated sulphuric acid, and subsequently with water; 250 c.c. of the water to be examined are introduced into one, and the same volume of the pure distilled water, prepared as above, is placed in the other. 10 c.c. of dilute sulphuric acid (1 part pure acid and 8 parts distilled water) and 10 c.c. of the potassium permanganate solution are now added to each flask, both then being put aside for three hours. Two drops of a 10 per cent. solution of potassium iodide are next added to the flasks, and the amount of iodine liberated (which is equivalent to the quantity of permanganate unacted upon by the water) is determined by titration with the sodium hyposulphite solution. The precise end of the reaction is ascertained by means of a few drops of the starch paste, the hyposulphite being added to each flask until the blue colour produced by the starch disappears. The quantities of solution used in each titration are then read off.