There seems to be some confusion, and even controversy, over the spelling of the element formerly known as sulphur, as well as the -SO4 radical in chemical names. Some refer to sulphur/sulphate, others to sulfur/sulfate and even the the best brains of the old Content Rescue Team and britnoders were unable to throw too much light on it in the time available. The general consensus was, however, that the difference reflected the old American vs. British spelling issue.
For some years now, the IUPAC (International Union of Pure and Applied Chemistry) have used the spellings "sulfur" and "sulfate" (and presumably, "sulfite" and so on), as confirmed by the following email, quoted verbatim with permission:
Dear Kevin,
Official IUPAC spelling for the SO4 radical has been sulfate for many years.
Best regards,
Alan
Alan J. Senzel, Ph.D.
Publications Manager, IUPAC
As far as I am able to tell from my research, the spelling has been standardised since about
1970, and also applies to compounds such as
sulphuric acid (aka
sulfuric acid). Despite my reservations (being a British spelling type), I vow that henceforth, I shall use the 'f' spelling, though possibly with gritted teeth.
Of course, common usage may still include the 'ph' spelling, and I would suggest that any write-ups relating to these compounds be firm linked to the alternative spelling. Perhaps an editorial decision may yet be made to move 'ph' write-ups into 'f' nodes.
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