Potentiometric EDTA titration of zinc with cadmium ion-selective electrode

The cadmium ion-selective electrodes used were a model 94-48A, Orion Co., and a model CD 125, TOA Electronics Ltd. As the reference electrode a double junction type model HC 305 DS, TOA Electronic Ltd. was used. A 1 M ammonium nitrate solution was filled as the outer bridge solution in the reference electrode. The procedure for the determination of zinc with EDTA is as follows: Take an aliquot of the sample solution containing less than 0.2 m mol of zinc into a 100-ml beaker. Adjust the pH of the solution to 5.5 by adding the acetic acid-sodium acetate buffer solution, add 1.0 ml of 0.01 M cadmium-EDTA solution (add 5 ml in the case of coexistence of 100 times Ca2+), and then dilute to about 50 ml with water. The cadmium ion-selective electrode and the reference electrode were immersed into the solution. The sample solution was stirred and titrated with 0.01 M EDTA solution in the speed of 0.01 ml s-1 In the coexistence of about 100 times Na+, K+, Ca2+, Mg2+, Ba2+, Cl-, NO3- and 10 times SO42-, those ions did not interfere with zinc determination. The coefficient of variation by the present method was 0.07%.