The aqueous medium for use in the present invention may be water by itself or a mixture of water with a water-miscible solvent such as an alcohol, e.g. methanol, isopropanol or ethylene glycol; dimethylformamide; tetrahydrofuran; cellosolve, e.g. methyl cellosolve; or a lower ketone, e.g. acetone or methyl ethyl ketone.