Method for preserving leuco chromogen-containing aqueous solution

A method for preserving a leuco chromogen-containing aqueous solution comprising:

• adding at least one acid compound selected from the group consisting of a first compound, a second compound, a third compound, and a fourth compound to the leuco chromogen-containing aqueous solution,
• wherein the first compound is selected from the group consisting of alkyl phosphoric acid, polyoxyethylene alkyl ether phosphoric acid, polyoxyethylene polycyclic phenyl ether phosphoric acid, and salts thereof,
• wherein the second compound is selected from the group consisting of polyoxyethylene alkyl ether fatty acid, N-acylamino acid in which a hydrogen atom of the amino group may be substituted with a substituent, and salts thereof,
• wherein the third compound is selected from the group consisting of N-acyl taurine in which a hydrogen atom of the amino group may be substituted with a substituent, α-olefin sulfonic acid, alkyl sulfocarboxylic acid, polyoxymethylene alkyl sulfocarboxylic acid, and salts thereof,
• wherein the fourth compound is selected from the group consisting of alkyl sulfuric acid, polyoxyethylene alkyl ether sulfuric acid, and salts thereof, and
• wherein the leuco chromogen comprises a phenothiazine chromogen.