| http://www.w3.org/ns/prov#value | - enotes a hydrogen atom or an alkali metal, such as sodium, potassium or lithium, or the equivalent of an alkaline earth metal, such as of calcium; preferably M is a hydrogen atom and in particular an alkali metal.The azo compounds according to the invention can be present in the form of their free acid and preferably in the form of their salts, in particular neutral salts; salts to be mentioned in
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