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.
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