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A new kinetic-spectrophotometric method is proposed for the determination of copper ( Ⅱ ). The method is based on the catalytic effect of copper ( Ⅱ ) on the oxidation of weak acid brilliant blue dye (RAWL) by hydrogen peroxide. The copper ( Ⅱ ) can be determined spectrophotometrically by measuring the decrease of absorbance of RAWL at λ = 626 nm using the fix-time method. The optimum reaction conditions are as follows: pH 7.20, buffer solution NaOH-KH2PO4, RAWL (200 mgL-1) 5.00 mL, H2O2 (30％) 0.50 mL, reaction temperature 80 ℃ and reaction time 20 min. The linear range of this method is between 0 μg L-1 and 12 μg L-1 and the limit of detection is 0.011 μg L-1, the relative standard deviation (RSD) in five replicate determinations for 2 and 8 μg L-1 copper ( Ⅱ ) are 3.2％ and 2.3％, respectively. Twenty ions do not interfere in the determination of copper ( Ⅱ ). The method has been applied satisfactorily to the determination of copper ( Ⅱ ) in freshwater samples (tap water and Yellow River water from Lijin, Shandong, China) and seawater samples (from the South China Sea), the recovery rates are 98.0％, 102.5％ and 96.0％, respectively.