| http://www.w3.org/ns/prov#value | - The salts that are useful thus can have the formula: ##STR1## where M is an alkali metal ion such as sodium, potassium or lithium; R1 is as defined above; and R2 represents a hydrocarbon chain, preferably a saturated hydrocarbon chain, having from 7 to 17 carbon atoms, especially 9 to 13 carbon atoms.
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