Determination of free calcium at a carbon paste electrode adsorptive stripping voltammetric method

A cathodic adsorptive stripping voltammetric method for the determination of free calcium is described, based on the reduction of calcium-alizarin red S (ARS) complex at a carbon paste electrode (CPE). In 1.5 x 10(-2) mol/L potassium hydroxide solution containing 2.0 x 10(-5) mol/L ARS, the complex adsorbed on the CPE surface yields a reduction wave with peak potential 0.89 V. When accumulation potential is -0.1 V, accumulation time is 90 s and scan rate is 100 mV/s, the second-order derivative cathodic stripping peak current of the adsorbed complex is proportional to free calcium concentration in the range of 3.0 X 10(-8) similar to 2.0 X 10(-6) mol/L by single sweep voltammetry. The detection limit is 9.4 x 10(-9) mol/L. With rinsing the CPE in 0.2 mol/L HCl solution for 2 min, it shows good reproducibility. The proposed method has been applied to the rapid determination of free calcium in serum, milk and tap water.