Ion-selective Electrode Instantaneous Potential Analysis

The transient signals responded by halide ion-selective electrodes (LaF3 AgCl AgBr AgI) to primary ion were studied with activity step method, and a transient potentiometry based on measuring peak height ΔEp of transient signal responded by ion-selective electrodes is proposed. There are many advantages in the transient potentiometry, firstly, the reproducibility of calibration curves is very good. For example, for the calibration curves of AgCl and AgBr electrodes, the average deviation of ΔEp and slopes S are small than 2.0 mV and 1.0 mV respectively. Secondly, the detection limits p(X-) and the lower limits of Nernst response are higher than that of conventional potentiometry, and the detection limits approach to the values calculated by Ksp(AgX) respectably. For example, in conventional potentiometry, the sensitivities of AgI and LaF3 electrodes are low because of the absorption of ion on electrodes surface. When concentration of F- and I- in test solution is lower than 10-5mol/L, the calibration curves have deviated Nernst response. However in the transient potentiometry, the lower limits of Nernst response can reach 10-6mol/L. Thirdly, the jump velocity of transient potential from equilibrium potential E1 of blank solution to peak potential Ep is much faster than the response velocity of conventional potentiometry. For instance, the response of LaF3 electrode to low concentration F- in conventional potentiometry is very slow, and the response time even need one hour, but in the transient potentiometry the response time is about 30 seconds, and for high concentration F- response time is from 0.1 to some seconds. The jump time of other electrodes is all from 1 to some seconds. Therefore, the fast and accurate analysis of test solution in small volume may be achieved. Theoretically, the negative value of Gibbs free energy of ion-hydration, -ΔGh(X-), is considered to be equal to the activation energy in reaction, and the velocity of potential variance in different stage of a transient signal explained by it. The smaller the |ΔGh(X-)| is, the faster the jump of potential in positive activity step is and the slower the jump of potential in negative activity step is. It proved that the potential of ion-selective electrode is not determined by ion transporting across the electrode membrane but by concentration of dehydration ion resulted from the electrode surface reaction. By the transient potentiometry we analyzed the contents of Vitamin B6 and F- of tap water and mineral water . The automatically switching setup of activity step used in this experiment is home-made. The setup is provided with blank and test solution injectors whose volume ratio is 4 to 1.