as lithium sodium potassium rubidium cesium beryllium magnesium calcium strontium barium zinc cadmium aluminum manganese nickel gallium germanium tin organotins such as mono or dimethyltin mono or dibutyltin or mono or dioctyltin lead antimony and bismuth ammonium hypophosphite hypophosphites of an aliphatic or aromatic amine such as mono di or trimethylamine mono di or triethylamine mono di or tributylamine mono di or trioctylamine mono di or triethanolamine mono di or triisopropanolamine methyldiethanolamine stearyldiethanolamine hexamethylenediamine ethylenebis diethanolamine diethylenetriamine triethylenetetramine hexamethylenetetramine benzylamine aniline diethylaniline and diethanolaniline hypophosphites of a heterocyclic amine such as pyridine lutidine toluidine pyrimidine pyrazine piperidine n methylpiperidine piperazine hexahydrotriazine morpholine pyrrole pyrroline pyrrolidine imidazole imidazoline imidazolidine pyrazole pyrazolidine and indole hypophosphites of a polymeric amine such as polyvinylpyridine polydiallylamine and polyethyleneimine quaternary ammonium hypophosphites such as tetramethyl trimethylethyl triethylmethyl tributylmethyl tetrabutyl octyldimethylhydroxyethyl triphenylmethyl and tribenzylmethylammonium hypophosphites and sulfonium hypophosphites such as triethylsulfonium hypophosphite among these compounds hypophosphites of the group ia or iia metals and organic amine hypophosphites are preferable since they are readily available and hardly toxic the amount of the hypophosphorous acid or its salt to be added or applied is not particularly limited it may be determined depending on the desired level of the prevention of the discoloration or the paper to be treated generally speaking the hypophosphorous acid or its salt may be added in an amount of 0 1 to 20 by weight preferably 1 to 10 by weight based on the paper in terms of dry matter the method and time for adding or applying the hypophosphorous acid or its salt are not particularly limited namely the hypophosphorous acid or its salt may be added in the.