The invention relates to derivatives of 3,3',5,5'-tetraalkylbenzidine bis {mono-carboxylatobenzenepolycarboxylic} (I), wherein: R1, R2, R3 and R4 represent C1-C3 alkyl; R5 is hydrogen or an alkaline or an alkaline earth metal salt from a sulphonic group; and A is the monovalent anion of a benzenepolycarboxylic acid (II). According to the invention, in formula (II), R6, R7, R8, R9 and R10 represent H or a carboxylic group on the condition that at least one of R6, R7, R8, R9 and R10 represents a carboxylic group. The aforementioned derivatives are used as chromogen reagents and can be employed to determine the total usable chlorine in an aqueous medium by means of a colorimetric method consisting in: bringing into contact a composition containing a compound (I) and a solid support with the sample to be tested, and determining the chlorine content by visually comparing the colour that develops with a calibration scale.
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