Abstract: When 0.10 mol/L KNO₃ blank solution and Cl^- test solution are switched each other, day-to-day reproducibility of the peak height ΔE_p of transient signal responded by Cl^--selective electrode is very good. The detection limit of Cl^--selective electrode and the intercept of ΔE_p～p(Cl) straight line accord with both values calculated by K_sp (AgCl) respectably. At positive activity step, the jump time of transient signal is smaller than 1.5s, the fast and accurate analysis of test solution in small volume may be achieved. The potential variance velocity in different stage of transient signal may be explained by the variance of Cl^- hydration Gibbs free energy, ΔG_h(Cl^-)=ΔG_h⁰ (Cl^-)+ RTln (a Cl(H₂O)_n^- / a⁰ )/(p (Cl_g^-)/ p⁰ ), where a⁰ =1 mol/L, p⁰=1.013×10^-5 Pa, ΔG_h⁰ (Cl^-)=-337 kJ/mol. At the start of positive activity step, p (Cl_g^-) is very small, a Cl(H₂O)_n^- larger, then ｜ΔG_h(Cl^-)｜ the smallest. Because the smaller the ｜ΔG_h(Cl^-)｜, the larger the hydration velocity of Cl(H₂O)_n^-, the jump of transient potential is the fastest. At the approach of peak potential, both p (Cl_g^-) and ｜ΔG_h(Cl^-)｜ are almost the largest, the dehydration velocity of Cl(H₂O)_n^- the smallest, then the jump of transient potential very slow. At the start of negative acitivity step, a Cl(H₂O)_n^- is almost zero, p (Cl_g^-) the largest, then ｜ΔG_h(Cl^-)｜ the largest. Contrary to the dehydration of Cl(H₂O)_n^-, the hydration velocity of Cl_g^- is the largest, then the jump of transient potential from peak potential to positive direction the fastest. With the decrease of p (Cl_g^-) and the increase of a Cl(H₂O)_n^-, both the ｜ΔG_h(Cl^-)｜ and hydration volecity of Cl_g^- are getting smaller and smaller, so the transient potential tends slowly to the base line.The automatically switching setup of activity step used in this experiment is hom made, the setup is provided with both injectors for blank and test solutions.

Key words: Chloride ion-selective electrode, Transient potential signal, Activity step method, Transient potentiometry, Ion hydration free energy, Electrode response mechanism