The spelling sulfur predominates in United States technical usage, while both sulfur and sulphur are common in general usage. British usage tends to favor sulphur for all applications. The same pattern is seen in most of the words derived from sulfur.
Variants of SULFUROUS
sul·fu·rous also sul·phu·rous\ˈsəl-fə-rəs, -fyə- also especially for 1aˌsəl-ˈfyu̇r-əs\