M as a salt-forming cation is e.g. an alkali metal, e.g. sodium or potassium, an alkaline earth metal such as calcium or magnesium, or also ammonium or an ammonium group which is derived e.g. from an aliphatic amine such as diethylamine or triethylamine, or from mono-, di- or triethanolamine, or from a heterocyclic amine such as piperidine, morpholine or pyridine.