Development of highly sensitive cadmium ion-selective electrodes by titration method and its application to cadmium ion determination in industrial waste water

摘要

Characteristics of cadmium ion-seiective electrode made cadmium sulphide (CdS)-silver sulphide (Ag_2S) mixture were studied. CdS-Ag_2S mixtures were obtained by gas/solid-phase reaction between silver-cadmium mixed powder and hydrogen sulphide gas (dry method) and by ionic reaction between cadmium-silver mixed ions and sulphide ion (wet method). As a result, it was found that the CdS-Ag_2S mixture had to be made in the condition of excess existence of sulfur and had better regulate the excess sulfurquantity minimum, for the CdS-Ag_2S pressed membrane gave a good Nernstian response against the cadmium ion concentration change. As the best way, CdS-Ag_2S mixture was obtained by adding sulphide ion solution to 5 mol% cadmium ion and 95 mol% silver ionmixed solution while measuring silver sulphide (Ag_2S) electrode potential as an indicator electrode. According to the reaction was stopped when the potential variation from the initial potential in the sulphide ion solution reached at 87-116 mV which the sulphide ion concentration became 10~(-3)-10~(-4) of the initial concentration, the cadmium ion membrane pressed diameter of 8 mm and thickness of 2 mm showed a Nernstian response from 10~(-8) to 10~(-1) M of cadmium ion concentration. Furthermore, aiming to its application for industrial waste water, masking buffer for interfering metal ions such as lead ion (Pb~(2+)) and copper ion (Cu~(2+)), which were possibly coexisted and to adjust total ionic strength and pH of sample was developed. The present Cd~(2+) ion-selective electrode was applied to the determination of Cd~(2+) in the industrial waste water. The good regression line with correlation factor of 0.984 was obtained compared with the conventional atomic absorption spectroscopy.