Disclosed herein is a high sensitivity method of colorimetrically measuring zinc by using a 2-pyridylazoaminophenol derivative represented by the following general formula (I) or a salt thereof as a coloring reagent, wherein a masking agent excellent in stability and acting specifically upon specific metals is used. As the masking agent is used a surface active agent.
wherein X and Y represent a halogen atom or a hydrogen atom, R 1 represents a hydrogen atom or a lower alkyl group, and R 2 and R 3 represent a hydrogen atom, a lower alkyl group, -(CH 2 ) n SO 3 H in which n is an integer of 1-4, or
in which ℓ is an integer of 0-4 and m is an integer of 0-4. The masking agent may be surface active agent in combination , with dimethylglyoxime in case of masking nickel and/or cobalt.