It is evident, that from the data thus obtained, the amount of ammonia obtained by the first distillation with sodium carbonate (free ammonia), and by the second distillation with alkaline potassium permanganate (albuminoid ammonia), can be determined. It has been previously mentioned that urea evolves ammonia when boiled with sodium carbonate; the amount of ammonia obtained by the first process of distillation will therefore include that actually contained as such in the water, and that generated by the decomposition of any urea possibly present. As the presence of this body is incompatible with a good drinking water, this fact is of little real importance. In case, however, it be desired to make an estimation of the free ammonia really present, 500 c.c. of the water to be tested are treated with 1 or 2 c.c. of calcium chloride solution, then with a slight excess of potassium hydroxide, and the liquid filtered. It is next distilled as directed above, and the remaining contents of the retort made up to 500 c.c. 200 c.c. of the original sample are then subjected to the same treatment with calcium chloride and potassium hydroxide, and filtered. The second solution, which contains all the ammonia originally present in the water, is now tested with Nessler’s reagent, the solution first obtained by diluting the contents of the retort being employed, instead of pure distilled water, for comparison.
The proportions of free and albuminoid ammonia found in the preceding operations are usually expressed in parts per 100,000 of the water. Wanklyn gives the following amounts of free and albuminoid ammonia contained in 100,000 parts of several kinds of water:—
| Deep spring water | not over 0·001 |
| „ „ „ mixed with surface water | 0·005 |
| Filtered water | 0·005 to 0·010 |
| Imperfectly filtered water | 0·01 „0·02 |
| Sewage | 0·30 |
The same authority makes the following classification of potable water, reference being made to parts of albuminoid ammonia present in 100,000 parts:—
| Extraordinary purity | 0 to 0·005 |
| Satisfactory purity | 0·005 „ 0·010 |
| Dirty | over 0·010 |
The presence of any considerable proportion of free ammonia is usually indicative of recent sewage contamination. In the absence of free ammonia, a water need not be rejected unless the albuminoid ammonia exceeds 0·010 part, but a water containing over 0·015 part of albuminoid ammonia should be condemned under all circumstances.
6. Nitrogen as nitrites and nitrates.—It is quite generally accepted that the presence in water of the oxidation products of nitrogen, is to be ascribed to the oxidation of nitrogenous organic matter, unless they are the result of percolation through soil containing nitrates, and, for this reason, considerable importance attaches to the quantitative estimation of the nitrogen present in the state of nitrates, and, in some cases, nitrites. One of the most reliable methods for this determination is the eudiometric process of Frankland, which is based upon that of Crum,[125] and consists in agitating the concentrated water with mercury and strong sulphuric acid, and measuring the volume of nitric oxide formed by the reduction of nitrates and nitrites. Owing, however, to the necessity of employing gas apparatus, this method is not in very general use. Wanklyn’s process is the following:—100 c.c. of the sample are made alkaline with pure sodium hydroxide, evaporated to about one-fourth of its original volume, next made up to 100 c.c. by adding pure distilled water, and introduced into a flask which connects with a U-tube filled with powdered glass moistened with hydrochloric acid. A piece of aluminium foil is then added to the contents of the flask, and the mixture is allowed to stand at rest for six or seven hours. The contents of the U-tube are now transferred to the flask, the latter is connected with a Liebig’s condenser and the liquid distilled. The proportion of ammonia contained in the distillate is determined by Nessler’s reagent as previously described, from which the amount of nitrogen present as nitrates and nitrites is calculated.
Griess[126] has suggested a very useful process for the determination of nitrous acid and nitrites in potable waters. It is executed by placing 100 c.c. of the filtered water in a glass cylinder, and adding a few drops of dilute hydrochloric acid, and 1 c.c. of a solution of sulphanilic acid and naphthylamine hydrochloride. In the presence of nitrites, a beautiful rose-red colour (due to the formation of azobenzol-naphthylamine sulphonic acid), will be produced. The proportion of nitrites contained in the water, is ascertained by simultaneously subjecting a solution of potassium nitrite, of known strength, to the same treatment, and matching the degree of colour obtained, as in the Nessler process. This solution can be prepared by dissolving 0·406 gramme of dry silver nitrite in hot water, and adding a slight excess of potassium chloride. After cooling, the solution is made up to one litre, the silver chloride allowed to settle, and the clear liquid filtered. If 100 c.c. of the filtrate are further diluted to one litre, each c.c. will contain 0·00001 gramme of nitrous acid.
In Ditmar’s method, the residue obtained by the evaporation of the water, is first mixed with pure sodium hydroxide, and placed in a small silver boat. It is next introduced into a combustion tube and burned in a current of hydrogen, the evolved gases being received in an absorption apparatus filled with very dilute hydrochloric acid. In this method the amount of ammonia formed, is likewise estimated by means of Nessler’s solution. The proportion of organic nitrogen is found by deducting the free ammonia present in the water and multiplying the remainder by 14⁄17.
Messrs. Dupré and Hake[127] determine the organic carbon in water essentially as follows:—The residue of the evaporation of the water is obtained in a very thin silver dish, which can be rolled up and introduced into a combustion tube filled three-fourths of its length with cupric oxide. The residue is then burned in a stream of oxygen. The evolved carbonic acid is absorbed in a solution of barium hydroxide, the precipitate formed being collected upon a filter, washed, dried, and weighed; its weight, divided by 19·4, gives the amount of organic carbon present in the sample. The carbonates and nitrates originally contained in the water can be removed by boiling with a saturated solution of sulphurous acid before the preliminary evaporation.