Nitrates and Nitrites result from the oxidation of animal matter. Vegetable substances, under like conditions, yield none or but mere traces of these compounds. The presence of nitrates is a most unfavorable sign in a shallow well or river water, because the conditions to which these waters are subjected are so variable that there is a constant liability of the purifying processes diminishing, and allowing the sewage, now only represented by innoxious nitrates, to appear in its dangerous, unoxidised condition.
Dr Frankland takes the sum of the nitrogen existing in the water as ammonia and as nitrites and nitrates, as a sort of measure of the minimum amount of animal or sewage matter destroyed. The amount due to sewage or animal matter is considered to be all over ·032 part per 100,000 (or ·022 gr. per gallon), which is the average of ‘inorganic nitrogen’ natural to unpolluted rain water. Dr Frankland also expresses this ‘previous sewage or animal contamination,’ in terms of London sewage containing 10 parts of nitrogen in 100,000 parts of liquid, by multiplying the above-named corrected sum by 10,000. Thus, a water containing 1 part per 100,000 (·7 gr. per gall.) of ‘inorganic nitrogen’ would have a ‘previous sewage or animal contamination’ of 9680 parts per 100,000, for it would have required 100,000 {(1 - ·032)/10} = 9680 parts of London sewage to produce an amount of nitrogen equal to that found by analysis. A water which contains over 20,000 parts of previous sewage contamination (1·5 grains of
inorganic nitrogen) is said to be dangerous. All other waters containing more inorganic nitrogen than in rain are said to be ‘doubtful’ except springs and deep well waters containing less than 10,000 parts of previous sewage contamination per 100,000, and such shallow wells and running water which from their source may be taken to be free from sewage.
Organic matter.—There is no method by which the actual weight of organic matter can be determined, still less is it possible to say how much is likely to be actually injurious organic matter, but there are several means of measuring the proportionate amount of organic contamination.
Dr Frankland determines the amount of carbon and nitrogen in the organic matter. The smaller the amount of these elements the better the water, and the less the amount of nitrogen, especially in proportion to organic carbon, the less chance of animal matter. A good drinking water will not have more than ·2 parts in 100,000 (·14 gr. per gall.) of carbon, or ·03 part of organic nitrogen in 100,000 parts (·02 gr. per gall.) of the water. The amount of putrescent matter may be estimated by the amount of oxygen consumed in destroying it. Dr Tidy (‘Chem. Soc. Jour.,’ January, 1879) considers that, speaking generally, waters requiring ·05 part per 100,000 (·035 gr. per gall.) to be of great organic purity; ·15 part (·1 gr. per gall.) waters of medium purity; waters of doubtful purity, from ·15 to ·21 part per 100,000 (·15 gr. per gallon). Impure waters, all above ·15 gr. per gall.
The proportion of albuminous substances present is measured by Mr Wanklyn by the amount of ammonia set free by alkaline permanganate. A water containing over ·15 part per million albuminoid ammonia condemns a water absolutely (‘Wanklyn’s Water Analysis,’ 4th edit., p. 54); ·10 part per million with little free ammonia, or ·05 part albuminoid ammonia with much free ammonia, is ‘suspicious.’ A water with less than ·05 part albuminoid ammonia belongs to the class of very pure waters.
Of course the above data are not hard and fast lines, but serve as aid to a judgment which may be modified by other circumstances connected with the analysis, and the source of the water.
Methods of Analysis. Total solid residue.—1000 grains are evaporated to dryness in a platinum dish over a water bath and residue dried in an oven at 212° F. for an hour, or until the weight is constant. The increase in weight of the platinum vessel multiplied by 70 gives the number of grains of total solid residue per gallon.
Hardness is determined by a solution of soap of which 320 grain-measure will soften a water of 16° of hardness. Each degree of hardness represents an amount of soap-destroying matter equivalent to 1 grain of chalk per gallon. 1000 measured grains of the
water are measured into a narrow-mouthed six or eight ounce stoppered bottle, then well shaken, and the air sucked out by means of a piece of glass tube. The standard soap solution is now run in 10 grains at a time, shaking well between each addition until there is formed over the whole surface a lather which, when the bottle is placed upon its side, shall last just five minutes. The number of grain-measures used will indicate the hardness of the water by reference to Table A. Should, however, the permanent lather not be formed before 320 measures of soap solution have been added, a second trial must be made, in which only 500 grain-measures of the water are taken, to which a like amount of recently-boiled distilled water is added. The degree of hardness now obtained must be multiplied by 2. With very hard waters it is necessary to dilute still further, say 250 grains to 750 of distilled, and multiplying the result by 4. If the number of soap-measures does not correspond with any degree on the table, observe which numbers it falls between. The degree corresponding to the lower of these soap volumes will be the whole number in the answer; the fraction will be the difference between the observed number of measures and the next lower on the table, divided by the difference (given in column 3) between the figure above and below it. Thus, if 14 measures were used the hardness would be 6·2°, 13·6 measures being equivalent to 6 degrees, and the fraction being {14 - 13·6}/{13·6 - 11·6} = 4⁄20 = ·2.