British variant spelling of bisulfate; a chemical salt with two sulfate groups or one hydrogen sulfate group.
From bi- + sulphate (British spelling of sulfate, using 'ph' instead of 'f'). British and American chemistry terminology diverged in the 19th century.
British and American chemists literally spell chemistry differently—'sulfate' vs 'sulphate'—and these variant spellings can make research papers look alien to speakers of the other variety.
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