Sulfur trioxide sulfuric acid precursor

Iron(II) sulfate hydrate (and its mineral analogue melanterite) does not appear to have been used directly as a pigment, but found widespread employment historically as a precursor compound calcination of so-called copperas leads to a loss of water and sulfur trioxide - that is, sulfuric acid - to give synthetic iron oxide compounds. 

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No research work about the post-sulfonation of a naphthalenic structure has yet been reported and only few examples of the post-sulfonation of a phthalic polyimide are mentioned in the literature [45,46]. Both mentioned articles are concerned with the sulfonation of the polyimide synthesized from bis[4-(3-aminophenoxy)phenyl]sulfone and PMDA, either in the presence of chlorosulfonic acid (CISO3H) or sulfur trioxide-triethyl phosphate (2SO3 TEP) (Fig. 1). Because of the very low solubihty of the polyimide precursor, the post-sulfonation is performed in heterogeneous conditions. 

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