Abstract: A new microwave digestion-inhibition catalytic kinetic spectrophotometric method to determine trace nickel in food was developed based on the inhibition effect of nickel on the oxidation of bromcresol purplet by hydrogen peroxide in borax medium. The optimal microwave digestion condition was using 70% HNO3－30 % H2O2 (4:1, V/V) as digestion solution for 15 min digestion at 1.5 MPa. The fading reaction of bromocresol purple induced by hydrogen peroxide was inhibited in borax medium and PEG-200 could activate the reaction system. Reaction in boiling water for 14 min and detection at 590 nm proved optimal. The linear range for determination of nickel was 1.7 × 10-3－0.14 mg/L and the detection limit was 1.7 × 10-3 mg/L. This method has been satisfactorily applied to the determination of nickel in food with maximum relative standard deviation of 5.2%. The average spike recoveries of nickel from five kinds of botanic materials ranged from 94.4% to 107.1% (n = 5). Compared with the method of CB/T 5009 138－2003, the maximum relative error was lower than 6.3%.

Key words: microwave digestion, inhibitory catalytis, kinetic spectrophometry, nickel, bromcresol purplet, hydrogen peroxide