Effect of Varying Amounts of Iron.—It is important to know within what limits the quantity of iron in an assay may safely vary from that used in standardising. In the following experiments the conditions as to bulk, acidity, and mode of working were the same as before:—
| Ferrous sulphate solution taken | 1 | c.c. | 10 | c.c. | 20 | c.c. | 50 | c.c. | 100 | c.c. |
| Permanganate required | 1.0 | " | 9.7 | " | 19.6 | " | 48.9 | " | 97.7 | " |
The ferrous sulphate solution is here a little weaker than that of the permanganate of potassium, but the results show that the permanganate required is proportional to the iron present.
Titrations in Hydrochloric Solutions.—These are less satisfactory than those in sulphuric solutions, since an excess of hydrochloric acid decomposes permanganate of potassium, evolving chlorine, and since the finishing point is indicated, not by the persistence of the pink colour of the permanganate, but by a brown coloration probably due to perchloride of manganese. Nevertheless, if the solution contains only from 5 to 10 per cent. of free hydrochloric acid (sp. g. 1.16) the results are the same as those obtained in a sulphuric acid solution. Equal weights (0.1 gram) of the same iron wire required exactly the same quantity of the permanganate of potassium solution (20 c.c.) whether the iron was dissolved in dilute sulphuric or dilute hydrochloric acid. The following series of experiments are on the same plan as those given above with sulphuric acid solutions. A solution of ferrous chloride was made by dissolving 5.01 grams of iron wire in 50 c.c. of dilute hydrochloric acid and diluting to 1 litre. The dilute hydrochloric acid was made by mixing equal volumes of the acid (sp. g. 1.16) and water.
Rate of Atmospheric Oxidation.—20 c.c. of the ferrous chloride solution were acidified with 10 c.c. of the dilute hydrochloric acid and diluted to 100 c.c. This solution was exposed cold in open beakers.
| Time exposed | — | 1 hour | 1 day | 2 days | 3 days |
| Permanganate required | 19.6 c.c. | 19.6 c.c. | 19.5 c.c. | 19.4 c.c. | 19.5 c.c. |
Similar solutions boiled required, before boiling, 20 c.c.; after boiling for one hour, replacing the water as it evaporated, 19.3 c.c.; and after evaporation to a paste and redissolving, 17.0 c.c.
Effect of Varying Temperature.—Solutions similar to the last were titrated and gave the following results:—
| Temperature | 15° | 30° | 50° | 70° |
| Permanganate required | 19.8 c.c. | 19.6 c.c. | 19.5 c.c. | 19.4 c.c. |