| http://www.w3.org/ns/prov#value | - optionally substituted, for example, by halogen, such as fluorine, or an aromatic sulphonic acid, for example a benzenesulphonic acid optionally substituted by lower alkyl, such as methyl, halogen, such as bromine, and/or by nitro, for example a methanesulphonic acid, trifluoromethanesulphonic acid or p-toluenesulphonic acid, preferably a chloride, bromide or iodide.
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