bisulphates

Whether the latter be sulphates or bisulphates may be indicated by the aqueous extract of the above residue, since neutral reaction would indicate the absence of bisulphates, acid reaction their presence in addition to possible normal sulphates; the quantitative estimation of the metals would decide this point definitely.

Synthetic Tannins Georg Grasser

I could not decide this by bisulphates of the alkalies, for as long as the second proportion of acid remained, water was retained with it.

Experimental Researches in Electricity, Volume 1 Michael Faraday 1829