2 Na₂S₂O₃ + I₂ = 2 NaI + Na₂S₄O₆.
Suppose that 20 c.c. of thiosulphate solution were required to decolourise the iodine liberated in 250 c.c. of a sample of water, while the distilled water required 25 c.c. Then 25 c.c. thiosulphate represents 10 c.c. of the permanganate solution = ·001 grains of available oxygen.
25-20 = 5
As 25 c.c. = ·001 grm. O, 5 c.c. = 5 ∕ 25 of ·001 = ·0002 grm.
This is the amount of O absorbed by 250 c.c. of the sample.
Therefore the amount of O absorbed by 100,000 c.c. of the sample. = ·08 grm.
It is usual to make a similar determination of the amount of oxygen absorbed in fifteen minutes.
The Interpretation of Results of analysis is more difficult than the analysis. A single analysis may be misleading, unless the source of the water is known. Constancy in composition or analysis is almost as important a criterion of purity as the actual character of the constituents. A knowledge of the source is essential in interpreting results of analysis, as the chemical composition of water varies with its source. The following rules are only approximately correct, and are subject to the above general considerations. The total dissolved solids in river-water are usually 10 to 30 parts in 100,000. Shallow well-water may contain from 30 to 200 parts or even more, and deep well-water from 20 to 70 parts.
Saline Ammonia in water is commonly of animal origin, ammonia (NH₃) being one of the first products of decomposition of nitrogenous animal refuse. Upland surface water usually contains about ·002 parts per 100,000, but it may reach ·008 or more if the land over which the water passes has been manured. Shallow well-water may be free from ammonia, or this may be very excessive in amount. Deep well-water may contain no ammonia or any amount up to ·1 per 100,000. Its presence is suspicious if the albuminoid ammonia is above a trace, or if the oxygen absorbed is appreciable in amount. Generally water is suspicious if saline ammonia is up to ·01 per 100,000. Albuminoid Ammonia indicates the amount of organic nitrogenous matter present in the water. It should not exceed ·005 parts per 100,000, while at the same time the saline ammonia should not usually exceed ·01 per 100,000. For Oxygen consumed the following table of the weight of oxygen required for 100,000 parts of water is given by Clowes and Coleman:—
| UPLAND SURFACE WATER. | WATER FROM OTHER SOURCES. | |
|---|---|---|
| Water of Great purity | Not exceeding ·1 | Not exceeding ·05 |
| Medium purity | From ·1 to ·3 | From ·05 to ·15 |
| Doubtful purity | From ·3 to ·4 | From ·15 to ·20 |
| Impure | Exceeding ·4 | Exceeding ·20 |