Selective Determination of Chloride Ion and Sulfide Ion Using Flow Coulometry at Ag Electrode
Authors: Hasebe Maki , Sawada Manami , Fukutake Hiroaki , Fukuyama Mao , Yoshida Yumi , Maeda Kohji 
Published: February 2020

[Introduction] Chloride ion and sulfide ion cause not only the pollution of air and water, but also the corrosion of metal construction. Therefore, it is necessary to establish the analytical method to determine chloride ion and sulfide ion quickly and precisely. If we can use the coulometry for the determination of these anions, it allows for the absolute determination method without the standard solution. Heretofore, we proposed the simplified flow cell, where a silver electrode, a reference electrode and a counter electrode are inserted in one tube of Teflon. The coulometric determination of chloride ion was accomplished successfully [1], but selective determination of the chloride ion and sulfide ion by connecting two cells was unsuccessful because of the pollution of chloride ion in the second step [2]. In this research, by using the tube made of cation exchange membrane, in which only a silver wire is inserted as a working electrode in tube, the electrolysis cell for chloride ion and that for sulfide ion were connected without interference of each other. The selective determination of two kinds of anion was performed by injection of the mixed sample and successive electrolysis in each cell.

[Experimental] In the electrolytic cell, a silver wire was inserted into the tube made of cation exchange membrane, using 1 M (= mol dm-3) NaClO4 as a supporting electrolyte in the inside and outside solutions. In the measurement of cyclic voltammetry, the tube was filled with aqueous solution containing 10-4 ~ 10-3 M NaCl or Na2S. In the flow coulometry, while passing the supporting electrolyte solution in 0.01 ml/min, 20 μL of the aqueous solution containing NaCl or Na2S was injected from upstream. For the selective determination of mixed samples, the tubes of two cells were connected to attain one consistent flow.

[Results and Discussion] In the determination of chloride ion, after the deposition of chloride ion as AgCl by applying 0.25 V vs. Ag/AgCl, chloride ion was determined from the electrical quantity of dissolution of AgCl at 0.05 V. The efficiency of electrolysis was 98 % in the concentration range of 10-4 ~ 10-3 M. In the determination of sulfide ion, the deposition of Ag2S is utilized, which occurred at more negative potentials than the deposition of AgCl and dissolution of Ag. The linear relationship between the electric quantity of Ag2S deposition and the concentration of sulfide ion was obtained, but the efficiency of electrolysis was about 35 % in the concentration range of sulfide ion was 10-4 ~ 10-3 M. We tried the successive determination of 5×10-4 M chloride ion and sulfide ion in the mixed solution. Chloride ion was determined from coulometry and sulfide ion was determined from the calibration curve, which indicated the possibility of the selective and successive determination of chloride ion and sulfide ion.
[1] H. Fukutake, N. Okumura, Y. Tatsuno, Y. Yoshida, K. Maeda, Bunseki Kagaku, 61 (2012) 1055.
[2] M. Sawada, H. Fukutake, Y. Yoshida, K. Maeda, Rev. Polarogr., 59 (2013) 195.