Can metabisulphite be substituted for bisulphite?
Simply put, metabisulfite is the solid form you buy when you want to add bisulfite to a solution.
Pure bisulfite
in solid form to the best of my knowledge
does not exist.
why do some formula call for both?
Can you given an example?
I'm aware of formulas that call for sulfite as well as bisulfite. Metabisulfite and bisulfite...nothing comes to mind.
The reason why you sometimes see bisulfite and sulfite mentioned as part of the same solution is because there's an equilibrium between them (and sulfur dioxide) that is dependent on pH (acidity/alkalinity):
From this, you can also glean that if you put sulfite into a solution and then drop pH by adding an acid, you'll essentially get bisulfite in the solution even though you never specifically added any (meta)bisulfite. The sulfite converts to bisulfite (partly, depending on pH). If you drop pH even further, some of the sulfite gases out in the form of sulfur dioxide - a pungently odorous gas. Conversely, if you put (meta)bisulfite into a solution and then raise pH, you'll get sulfite in solution.
When it comes to having both sulfite and bisulfite in the same formula, the example of a Kodak C41 developer posted here years ago by
@Photo Engineer (the late Ron Mowrey) comes to mind. I suspect the sulfite and bisulfite only occur there separately as (supposedly) part of the 'same' mix because in reality, they weren't in the same mix. The bisulfite would be part of the solution together with the CD4 developing agent, while the sulfite would be included in either of the other two concentrates. Upon combination, the resulting pH would be around 10.0 and virtually all that's left would be sulfite. The metabisulfite would be there in the CD4 concentrate to stabilize it - I assume. Someone can put a definitive answer to this, I expect. I'd have to look in my correspondence with Photo Engineer what he said on the topic (if any).
The TLDR is that metabisulfite = bisulfite for all intents and purposes, and if a formula calls for the latter, what you'll actually purchase & weigh out is the former (although it may still be labeled as the latter, confusingly...) And keep in mind that sulfite and bisulfite are different things, although the are related as explained above - which means you can't substitute one for the other just like that, although a substitution is sometimes possible to an extent and under certain conditions.