As salts of the formula (I) which are preferably used for carrying out the process according to the invention, there are mentioned those in which: Ar is a naphthyl group, R1 represents a methyl or ethyl radical and the cationic radicals X are inorganic cations derived from metals such as lithium, sodium, potassium, calcium and barium, ammonium ions NH4 + and quaternary ammonium ions such as tetram