TITRATION OF SULFUROUS ACID 
363 
ing three or four days it was standardized by the iodine method. 
In all titrations recently boiled water was used and the titrating 
vessel was kept full of carbon dioxide. Loss of sulfur dioxide 
was prevented by use of series of bulbs containing iodine and 
potassium iodide solutions in the standardizations, potassium 
permanganate in the titrations with permanganate. On account 
of the rapid change in the concentration of the sulfite solution it 
was restandardized by the iodine method on the same days that 
its concentration was determined with permanganate. Though 
protected in the apparently perfectly working apparatus which 
maintained an outward pressure of hydrogen equal to about 20 cms. 
water pressure, the sulfite solution lost about 1 per cent of its 
reducing power daily. This would require for the average amount 
of solution stored, the access of about 40 cc. of air daily, which 
seems impossible, and the facts incline one to the view of Shene- 
field, Vilbnandt and Withrow 9 that the salt suffers auto-oxidation. 
In carrying out the electrometric determinations with permanga- 
nate a weighed .amount of this solution was placed in the titration 
vessel, an amount of dilute, recently boiled and cooled sulfuric 
acid was added to make the acid concentration about normal at 
the end of the experiment and a rapid stream of purified carbon 
dioxide was run through the solution for several minutes. The 
current of gas was slowed down, the mechanical stirrer was 
started and sufficient sulfite solution was run in to use about half 
of the permanganate in series (1), nearly all in series (2), and 
to the end point in series (3). In series (1) and (2) the excess 
of permanganate was destroyed by a slight excess of known 
potassium iodide and then permanganate was run in till the vol- 
tage showed the end point had been reached. The high degree 
of uniformity attained by essentially the same method in the 
determination of nitrous acid 10 was not reached in the titration 
of sulfurous acid, indicating that the reaction in the latter case is 
SERIES (1) 
TITRATION OF SULFUROUS ACID WITH A LARGE EXCESS OF 
PERMANGANATE 
KMnOi reduc. cc. 
Excess KMnOi cc. 
NasSO* cc. 
H 2 SO 3 to 1 cc. 
1 . 
37.61 
39.64 
24.50 
0.00325 
2. 
38.86 
41.74 
24.95 
0.00329 
3. 
51.27 
30.88 
33.25 
0.00326 
4. 
31.28 
16.73 
20.00 
0.00330 
Average 
0.003275 
9 Shenefield, Vilbrandt and Withrow, loc. cit. 
10 Hendrixson, loc. cit. 
