14 
Proceedings of the Royal Society of Edinburgh. [Sess. 
II. Sulphur dioxide was passed into the boiling solution, to which 
10 c.c. of 4 N. sulphuric acid had been added. The excess of 
sulphur dioxide was then expelled by a current of carbon dioxide. 
Titre = 9’4 c.c. 
III. The conditions in this experiment were the same as in II., except 
that the sulphur dioxide was passed into the cold solution, which 
was then heated. Titre =18*5 c.c. 
IV. Sulphur dioxide was passed for ten minutes into the boiling solu- 
tion, to which 10 c.c. of O02 N. ammonia had been added. Titre 
= 24‘8 c.c. and 24*7 c.c. 
These are typical of the results obtained with sulphur dioxide gas, and 
leave little doubt that the difference between reduction by the gas and by a 
solution are to be explained by the non-absorption of sulphur dioxide by a 
hot acid solution. 
Effect of Alkali . — Addition of a small amount of alkali appeared to 
accelerate the reduction, and the effect of varied quantities of alkali was 
therefore tried. Some experiments with sodium sulphite, sodium bisulphite, 
and potassium metabisulphite were also tried for comparison, as these have 
been recommended. 
Addition of alkali caused precipitation of ferric hydroxide, but the 
solution remained acid until all the iron was precipitated. At or about the 
stage when precipitation was complete the solution was neutral to litmus. 
The nearer the neutral point was approached the quicker was the reduction, 
provided that the solution remained acid in reaction towards litmus. If 
the solution, after addition of the sulphurous acid or sulphite solution, was 
even faintly alkaline to litmus, no reduction occurred even on boiling 
for some time. Possibly this explains some discrepancies in work with 
sulphites. If an alkaline sulphite is added to a ferric solution, rapid 
reduction occurs if the mixed solution still reacts acid, but if so much 
sulphite is added that the mixed solution is alkaline, reduction only occurs 
after acid is added to expel the sulphur dioxide, and is therefore often 
incomplete. 
All reducing agents increase in reducing power as the acidity is 
diminished or the alkalinity increased, and this reduction up to a certain 
point conforms to this rule. Neutrality to litmus corresponds (at least 
approximately) to the completion of precipitation, and it is therefore not 
surprising that there should be no reduction when the solution becomes 
alkaline, since there is no longer any iron in solution. 
It becomes, therefore, a matter of indifference in practice whether 
sulphurous acid or a sulphite is used for reduction. The essential for rapid 
