Mixing Hypo Clearing Agent - sodium bisulfite vs metabisulfite

[Doc W]

I am mixing up my own hypo clearing agent. I understand that for film, it is good to add sodium bisulfite whereas HCA for paper is better without it. However, I can't find sodium bisulfite locally. I have read that sodium metabisulfite is pretty much the same thing. Can I substitute it for sodium bisulfite?

 

[john_s]

For our purposes they are the same. See this link:

Dead Link Removed

 

[Ian Grant]

Pure Sodium Bisulphite is an unstable compound only available in Analytical grade and very expensive.

However in the US Sodium Metabisulphite is usually sold as Sodium Bisulphite it's a slightly lower grade form of Metabisulphite than is sold in Europe etc, has less free SO2 which gives a marginally less acidic pH in solution. It probably has more to do with how it's manufactured.

In most cases including HCA Bisulphite is interchangeable with Metabisulphite, however in some developers where a small amount of Metabisulphite is used as a preservative the purity & freshness of the Metabisulphite is quite critical to the longevity of the developer in storage. One the free SO2 is depleted the developer oxidises.

Ian

 

[Doc W]

Thanks very much, folks. This was very helpful.