The amount of permanganate consumed is equal to A-B, where A represents the hyposulphite used with the distilled water, and B, that used with the sample under examination, and the proportion of oxygen which is consumed by the water tested, can be calculated by the formula:—
(A - B) a A
in which a is the available oxygen in the added permanganate. For example, if 10 c.c. of permanganate (= 0·001 gramme available oxygen) are added to the 250 c.c. (= ¼ litre) contained in each flask, and the distilled water required 35 c.c., the sample 15 c.c., of the hyposulphite solution, the proportion of oxygen consumed by the ¼ litre of water, would be
(35 - 15) × ·001 35
= ·000571, which represents ·228 parts of oxygen in 100,000 parts of water.
In applying the preceding test, it is requisite that the flasks should be kept at a particular temperature, such as 27°. The presence of putrescent and readily oxidised organic matter or nitrites, which indicates dangerous contamination, is recognised by the absorption of any considerable proportion of oxygen in the space of two minutes. According to Dr. Tidy, 100,000 parts of water of various degrees of purity, absorb the following amount of oxygen in three hours:—
| Part Oxygen. | |
| 1. Great organic purity | 0 to 0·05 |
| 2. Medium purity | 0·05 „ 0·15 |
| 3. Doubtful | 0·15 „ 0·21 |
| 4. Impure | over 0·21 |
4. Chlorine.—The importance attached to the estimation of chlorine in potable waters is derived from the fact that this element enters largely into the food of men and animals, and is thrown off in their excreta. This, naturally, contributes to the sewage contamination to which water is often exposed. Water, however, may take up a certain proportion of chlorides from the geological strata through which it passes, and it is of importance to bear this fact in mind in forming a conclusion as to the significance of the results afforded by this determination. It is, likewise, to be remembered that vegetable organic pollution would escape detection were the quantity of chlorine contained alone taken into consideration. The determination is conveniently made as follows:—50 c.c. of the water are introduced into a beaker, a drop or two of a concentrated and neutral solution of potassium chromate added, and then a standard solution of silver nitrate very cautiously added from a burette, drop by drop, until a faint but permanent red tint is produced. If the silver solution is prepared by dissolving 2·394 grammes of the nitrate in 1 litre of distilled water, the number of c.c. required to cause the reddish coloration directly indicates the parts of chlorine present in 100,000 parts of the water examined. According to Frankland, 100,000 parts of water from various sources contain the following proportions of chlorine:—
| Rainwater | 0·22 |
| Upland surface water | 1·13 |
| Springs | 2·49 |
| Deep wells | 5·11 |