| http://www.w3.org/ns/prov#value | - ps, and X1 and X2 both represent --O(CH2)n SO3 M in which M is a hydrogen atom, an alkali metal ion or NH4 +, and n is an integer of 2-4, or either of X1 and X2 represents --O(CH2)n SO3 M in which M and n are the same meanings as given above and the other represents a hydrogen atom.
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