On the Reduction of Ferric Iron. 
13 
1911-12.] 
acid solutions were used, which were found by titration to be approximately 
2 N. and 03 N. respectively. 
Method of Experiment . — In order to make the experiments comparable 
as far as possible, certain conditions were kept constant in each case. For 
each titration, 25 c.c. of the ferric solution were taken, and a measured 
volume of sulphurous acid added. In most cases a measured volume of an 
acid or alkali of known concentration was added, and in all cases sufficient 
water was added to bring the total volume up to 60 c.c. The mixture was 
heated quickly to boiling. When the solution had boiled for ten minutes, 
10 c.c. of dilute sulphuric acid (about 4 N.) were added, and a stream of 
carbon dioxide blown through the boiling liquid until all the sulphur dioxide 
was expelled. The complete expulsion of the sulphur dioxide required 
about twenty minutes. 
The correct titre was found to be 24*8 c.c. 
Reduction was found to be complete under the above conditions when 
25 c.c. of the 2 N. sulphurous acid solution was used, whether the reduction 
occurred after addition of 25 c.c. of ammonia, of 25 c.c. of decinormal hydro- 
chloric acid, or without addition of acid or alkali. 
Addition of 10 c.c. of N. sulphuric acid retarded the reduction, so that 
the titre was 20T c.c. 
Another set of experiments was performed with addition of variedamounts 
of acid and with less sulphurous acid than in the preceding experiments. 
In each case 25 c.c. of the ferric solution and 25 c.c. of 03 N. sulphurous 
acid were used; a measured volume of 4 N. sulphuric acid was added, 
together with sufficient water to bring the total volume to 60 c.c. 
Acid added = none 
(c.c. of 4 N.) 
Titre in c.c. = 24*8 
1 c.c. 2'5 c.c. 
(•2 grm.) (0 - 5 grm.) 
24-8 24*2 
5 c.c. 10 c.c. 
(1*0 grm.) (2 grms.) 
/ 22T 15*8 
1 20-8 17-8 
Reduction was incomplete in ten minutes if the amount of free acid 
exceeded 1 c.c. of 4 N. sulphuric acid. 
Reduction with Sulphur Dioxide Gas . — The effect of acid on the reduc- 
tion appeared to be less than in some earlier experiments, in which sulphur 
dioxide gas had been blown into the solution. A few experiments were 
tried under as nearly as possible the same conditions to ascertain the 
difference between reduction with the gas and with solution. In all cases 
the titre would be 24 - 8 c.c. if reduction were complete. 
I. Sulphur dioxide was passed into the boiling ferric solution for ten 
minutes. Carbon dioxide was then blown through until no 
sulphur dioxide could be detected in the vapour. Titre = 24*6 c.c. 
