PURPOSE: To make direct quantitative analysis of a sample liquid and easy concn. control of benzotriazole BTA in water by determining the concn. of the BTA by measuring the absorbancy at the absorption wavelength in a UV region.

CONSTITUTION: UV rays are transmitted through the water contg. the BTA and the absorbancy for at least either of 254nm or 270nm absorption wavelength is measured. A calibration curve which is preliminarily made from the concn. of the BTA and the absorbancy at the absorption wavelength is applied to the measured absorbancy, by which the concn. of the BTA is determined. More specifically this method makes use of the absorption characteristic that the BTA contained in the water possesses at 254nm and 270nm absorption wavelengths of the UV region and the correlativeness which exists between such absorbancy and the quantity of the BTA.