Metal finishing Q&As since 1989

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Sodium Bisulphite fumes turning the chromic acid blue

We use 40% of sodium bisulfide, 2L of sulfuric acid, and 260L ground water to neutralize the chromic trioxide on the surface of brass during etching process.
but what I observed after preparation of new solution that the surrounding plastic components are turning blue and I can smell the fumes. Please be detailed

Shafiuddin A. Mohammed
metal coating shop - Dubai, United Arab Emirates
2007

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Sodium bisulphite, or more usually sodium metabisulphite, is a reducing agent and reacts with with hexavalent chromium to produce the more benign trivalent chromium. It is this that gives you the blue colour, although more usually it is green. I am not sure what you mean by being able to smell the fumes, as you do not give any details of which fumes - however, if the pH is not low enough, you can sometimes smell sulfurous fumes emanating from the metabisulphite bath. The remedy is usually to decrease the pH, but I think with 2N H2SO4 in the bath, it should be low enough - perhaps more detail would be advantageous.

Trevor Crichton
R&D practical scientist
Chesham, Bucks, UK
2007