Among them, particularly preferable examples of the salt include alkali metal salts such as sodium, potassium and lithium, onium salts of mono-, di- or tri(lower alkanol having 1 to 4 carbon atoms)amine such as monoethanolamine, diethanolamine, triethanolamine, monoisopropanolamine, diisopropanolamine and triisopropanolamine, and ammonium salts.