Patent ID: 7485436

Claim:
A method of storing N-(carboxymethylamninocarbonyl)-4,4′-bis(dimethylamino)diphenylamine sodium salt stably in a state where the N-(carboxymethylaminocarbonyl)-4,4′-bis(dimethylamino)diphenylamine sodium salt is dissolved in a solution, comprising: preparing a solution by dissolving N-(carboxymethylaminocarbonyl-4,4′-bis(dimethylamino)diphenylamine sodium salt as well as a fructosyl amino acid oxidase and/or a peroxidase and in an aqueous solvent; and storing the solution at a temperature in a range from 0° C. to 40° C. for at least one day; wherein a degree of spontaneous color development of the solution stored for at least one day is suppressed to ½ or smaller relative to a case where neither the fructosyl amino acid oxidase nor the peroxidase is contained, so that the solution stored for at least one day is usable as a reagent of an oxidation color former.