Watts’ ‘Dictionary of Chemistry’ quotes the proportions below:—
| Thames, at Kew | 1·21 |
| Thames, at London Bridge | 6·36 |
| Loch Katrine | 0·56 |
| Rhine, at Basle | 0·15 |
| Rhine, at Bonn | 1·45 |
| Lake of Geneva | 0·67 |
| Elbe, near Hamburg | 3·94 |
| Loire, at Orleans | 0·29 |
The amount of chlorine contained in sewage is stated to range from 6·5 to 21·5 parts, the average being 11·54 parts.[123] It is generally considered that a proportion in excess of 5 parts in 100,000 parts of a drinking water, which is not liable to be affected by mineral admixture, is to be ascribed to organic contamination.
5. Ammonia, free and albuminoid.—It has already been mentioned that the decomposition of the nitrogenous organic impurities present in polluted water results in the production, first, of ammonia, then of nitrites and nitrates, and, as it is commonly asserted that the deleterious character of water is mainly due to the putrefactive processes taking place, which are probably directly proportionate to the quantity of ammonia produced, it is evident that the determination of this compound is of considerable importance. The proportion of albuminous and allied constituents in a sample can, moreover, be measured by the quantity of ammonia produced when the water is boiled with an alkaline solution of potassium permanganate. Upon the foregoing facts, Messrs. Wanklyn, Chapman, and Smith[124] have based a method for the determination of the sanitary quality of potable waters, which is in very general use. It involves, first, an estimation of the ammonia generated upon distilling the water with sodium carbonate (“free” ammonia); second, the quantity given off by boiling with alkaline potassium permanganate (“albuminoid” ammonia). In case the water tested is contaminated with urea, which is not improbable, this compound will be decomposed into ammonia by the treatment with sodium carbonate. The following solutions are employed in the execution of the test:—
Ammonium Chloride.—Dissolve 1·5735 grammes of the dry and pure salt in 1 litre of distilled water. When required for use, dilute 100 c.c. of the solution to 1 litre; 1 c.c. of this diluted solution contains ·00005 gramme of NH3.
Pure Sodium Carbonate.—The ordinary pure reagent is freed from any ammonia possibly contained by heating it in a platinum capsule.
Pure distilled Water.—This is obtained as directed on p. 204.
Nessler’s Reagent.—This is a strong alkaline solution of mercury biniodide. It may be prepared by first dissolving 62·5 grammes of potassium iodide in 250 c.c. of hot distilled water (reserving 10 c.c. of the solution), and adding a concentrated solution of mercury bichloride, with constant shaking, to the remainder, until a permanent precipitate remains undissolved; this is then brought in solution by means of the 10 c.c. of iodide solution, set aside, and the addition of mercury bichloride is carefully continued until a slight precipitate reappears. A concentrated solution of potassium hydroxide (200 grammes dissolved in water) is now added, and the volume of the whole made up with distilled water to 1 litre. The solution is then allowed to subside, after which it is decanted and preserved in a well-stoppered bottle.
Permanganate solution.—Dissolve 8 grammes of potassium permanganate and 200 grammes of potassium hydroxide in 1 litre of water, and boil to expel any ammonia present.
The estimation of free and albuminoid ammonia is made as follows:—100 c.c. of the water to be examined are introduced into a glass retort, which connects with a Liebig’s condenser, and has previously been thoroughly cleansed by boiling with distilled water; one gramme of pure sodium carbonate is added, and the water distilled until 40 c.c. have passed over, the distillate being separately collected in four 10 c.c. cylinders or tubes. About 10 c.c. of the alkaline solution of potassium permanganate is then added to the remaining contents of the retort, and the distillation continued almost to dryness. The second distillate is likewise collected in fractions of 10 c.c. each. It is advisable to so regulate the process of distillation, that only about 10 c.c. pass over in the space of eight minutes. The two sets of distillates are then separately tested by adding 0·5 c.c. of the Nessler solution to each cylinder, well stirring the mixture, and setting it aside for at least five minutes. A series of comparison tubes (10 c.c. in capacity) are prepared by adding ·001, ·003, ·005 up to ·01 gramme of ammonium chloride, and filling to the 10 c.c. mark with pure distilled water; 0·5 c.c. of the Nessler reagent being added to each. The degree of coloration exhibited in the cylinders containing the two sets of distillates is then matched by the comparison cylinders.