Contribution from the Chemical Laboratory of Grinnell College 
ELECTROMETRIC TITRATION OF SULFUROUS ACID 
WITH PERMANGANATE 
W. S. HENDRIXSON AND L. M. VERBECK 
The purpose of this work was to determine whether sulfurous 
acid in acid solution is completely oxidized to sulfuric acid by an 
excess of permanganate as stated in the literature. If so the 
excess of permanganate should be easily and accurately deter- 
minable electrometrically with potassium iodide thus giving a sim- 
ple method for determining sulfur dioxide, sulfurous acid or 
sulfites. The results show that with a small or large excess of 
permanganate only about 90 per cent is oxidized to sulfuric acid, 
the remainder of the sulfurous acid probably forming dithionic 
acid. 
The practicability of titrating sulfurous acid with permanganate 
seems so obvious that it is not surprising that it has been attempted 
by many chemists, though the results have many times been shown 
to be too low and discordant. For this fact two causes have been 
assigned, loss of sulfur dioxide and the incomplete oxidation to 
sulfuric acid by permanganate whether in excess or not. Nearly 
forty years ago Honig and Zatzek 1 stated that permanganate did 
not oxidize sulfurous acid completely to sulfuric acid in acid 
solution, but only in neutral or alkaline solution. In titrating 
sulfurous acid with permanganate Dymond and Hughes 2 obtained 
as the average 88.9 per cent of oxygen used as compared with 
theory for complete oxidation. By separate experiments on a 
large scale they definitely proved the formation of dithionic acid 
when they at the same time ran permanganate and sulfur dioxide 
into cold water, keeping the sulfur dioxide in excess. They re- 
garded the formation of the dithionic acid as an essential part of 
the reaction, and due to a stage in the reaction, namely, the 
reduction of Mn0 2 to MnO. Their equation is, 
17 JESOs + 6 KMnOi = 2 K 2 S 2 0c + K 2 S0 4 + dMnSCX + 
6 H 2 S0 4 + 11 H 2 0 
1 Honig and Zatzek, Monatshefte, f. Chem., 4, 738. 
2 Dymond and Hughes, Jour. Chem. S'oc. 71, 314 (1897). 
