Ryuji Suzuki used to have an informative article on the confusion between metabisulphite and bisulphite. His site seems to have been streamlined. Below is a paste of what I copied from there. Incidentally, Murray has this right. The potassium salt is heavier than the sodium one, so you need a bit more.
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Bisulfite
From Silvergrain Labs
(Redirected from Metabisulfite)
Sodium bisulfite, NaHSO3, and sodium metabisulfite, Na2S2O5
Sodium bisulfite and sodium metabisulfite are different chemicals. When metabisulfite is dissolved in water at a reasonably low concentration, one mole of metabisulfite and one mole of water form two moles of bisulfite. Therefore, they are functionally very similar. The difference is the weight they should be substituted at. The difference is small, and is usually nothing to worry about in photographic chemistry. However, in some sensitive developers, this may make sizable difference in pH.
Pure sodium bisulfite is rarely traded for industrial or photographic purposes in dry form, because pure dry form is unstable. However, there is a supplier who sells it in expensive analytical reagent grade in dry form. There are also many suppliers who sell mixture of sodium bisulfite and metabisulfite. The ratio varies across suppliers, but usually about half-half. Sodium bisulfite can be kept in dry form if blended with metabisulfite. Both sodium bisulfite and sodium metabisulfite are most commonly available in anhydrous forms.
Michael Gudzinowicz pointed out that Kodak sells sodium metabisulfite as sodium bisulfite under their label. Kodak's MSDS confirms this. Therefore, when Eastman Kodak Company calls for sodium bisulfite, they mean sodium metabisulfite. If you are buying chemicals from Photographer's Formulary, Johnny Deiure, Digitaltruth or other chemical suppliers, I suggest not to bother with higher priced sodium bisulfite, and purchase sodium metabisulfite. If you publish your formula, it is my opinion that correct name for the chemical you employed, sodium metabisulfite or Na2S2O5, is much preferred to avoid further confusion. (at the expense of four more letters)
Richard Knoppow took his effort and found what we think is Kodak researcher's reasoning for this substitution. Crabtree and Mathews (1938) stated as follows: "Ordinary sodium bisulphite has been shown by analysis to consist chiefly of metabisulphite which is converted into tbisulphite when dissolved in water. Sodium bisulphite may be substituted weight for weight for potasssium metabisulphite. Either chemical is equally satisfactory but the former is usually prefered since it costs from one-third to one-half the prince of teh altter and is available on the American market, at least, in a very pure form." (Ch VII, p.117) They further proceeded to say as follows. "The Preservative. -- It is now customary to substitute sodium bisulphite for potassium metabisulphite weight for weight in either developer of fixing bath formulas. According to Harrison and Carrol(1) the stability of commercial sodium bisulphite is a function of its crystaline structure and also of its lack of moisture content. Furthermore, these authors have proved by analysis that there is very little actual sodium hydrogen sulphite present in commercial sodium bisulphite; the major portion of the sample consisting of sodium metabisulphite. Commercially dry sodium bisulphite of satisfactory quality is now obtainaboe and may be used in mixing formulas calling for either the metabisulphite or the bisulphite. A very intersting discussion of the comparitive properties of the sulphites, maetbisulphites, and acid sulphites has been published by Wall (2)." (Ch. X, p.215)
Crabtree, J. I. and Mathews, G. E. 1938. Photographic Chemicals and Solutions, Boston: American Photographic Book Publishing Co.
Harrison, J. B. P. and Carrol, M. F. 1925. "The Composition and Evaluation of Commercial Sodium Bisulphite," J. Soc. Chem. Ind., 44, 127.
Wall, E. J. 1927. "Sulphites, Metabisulphites, and Acid Sulphite," American Photography, 16, 127
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