Amount of Sulfur Dioxide Generated by Sodium Metabisulfite in Acidic Solutions

A. This is an interesting question to which I hope you will receive an answer, but I think it unlikely that the answer will be simple!

The equations for neutralization of hex chrome with bisulphite are widely printed and accepted as:
1a). Na2S2O5+H2O => 2NaHSO3
1b). 3NaHSO3+2H2CrO4+3H2SO4 => Cr2(SO4)+3NaHSO4+5H2O

These equations show no SO2 released. However, this seems quite contrary to experience, so the equations are probably not totally accurate and complete. It is interesting to look at the printed and accepted equations for neutralization of hex chrome with sulfur dioxide, which are:
2a). SO2+H2O <=> H2SO3
2b). 3H2SO3 => Cr2(SO4)3+5H2O

In this version we see the reversible equilibrium equation between SO2 gas and H2SO3 in equation 2a. I'm no chemist, but I think equation 1a is probably not telling the whole story, such that when Na2S2O5 is mixed with acidified water rather than neutral water, there is an additional reaction going on that is releasing sulfur dioxide. In fact, my experience is that the lower the pH, the more objectionable the fuming, reinforcing my belief that acid is reacting with the metabisulphite to release sulfur dioxide. Hopefully someone will offer us their equation for this. But my interim conclusion is that you will find the amount of sulfur dioxide released is proportional to the amount of sulfuric acid added, not the amount of chrome neutralized.

Although the texts say that the neutralization is usually done at pH 2.5-3.0, in order to proceed at reasonable speed, and is often impractical above pH 4.0, this may not be true for a batch reaction, and the fuming will be greatly reduced at a higher pH.

Although you understandably want to measure or predict the release of SO2 as an intermediate goal, the real object is to get it as close to zero as possible. In this light, maybe you could do some lab experiments of adding the metabisulfite to water, adding the diluted bisulphite to the chromic acid, and finally adding a stoichiometric amount of sulfuric acid.

A. While it is quite obvious that SO2 is given off when sodium metabisulphite is acidified, you can smell it even without much, if any acidification the real thing is how much metabisulphite is required to reduce how much chrome. You really don't want to acidify metabisulphite and drive off SO2 as it is likely to bring on an asthma attack.

The Electroplating Engineering Handbook [on AbeBooks or eBay or Amazon affil links] suggests 1.43 lb for each pound of chrome against .96 lb of sulfur dioxide. From this you can calculate the amount of effective SO2 if you wish As you can see from Ted's reactions, sulfuric acid or H+ ions are part of the reaction so if you add metabisulphite without acid the free H+ ions are quickly used up, the pH rises and the reaction stops. At pH 5 there is no reaction, lower to 3 and there is instantaneous reduction so you must continuously add acid if you want to operate in the 3-4 range.

A. There is no doubt that SO2 is generated while treating Cr+6 with SBS. However, it is likely a very small amount as the accepted odor threshold for SO2 is about 0.5 ppm or 1.3 mg/m3. Safe working limits are approached around 5.0 ppm or 13 mg/m3.

Compared to the pounds, and pounds of SBS that are typically used to treat Cr+6, the amount of SO2 being evolved from solution should relatively insignificant. Just happens that very little SO2 can make a very, very big stink.

I would also expect that the rate of SO2 evolution would be effected by other factors such as temperature, rate of ventilation, etc.

A. You will need to control the pH more carefully in order to minimize the amount of sulfur dioxide that is generated. The pH should be < 3, but not much less. At a pH of 2.5, there will be a lot of SO2 released, so you should try for a pH of 2.8-2.9.

Lyle Kirman