A study on the dynamic response of precipitate-based ion-selective electrodes to interfering ions

The dynamic response of precipitate-based ion-selective electrodes to a sud den step change in the activity of interfering ions (Y+/-) is studied in th is paper. In this case the responses of AgI electrode to Cl-, Br- and SCN-, AgBr electrode to Cl- and SCN-, CuS electrode to Pb2+, Zn2+, Cd2+, Co2(+), Ni2+ and Mn2+, PbS electrode to Cd2+ and CdS electrode to Pb2+ are all non monotonic overshoot-type transient signals in the presence of certain conce ntration of primary ion (X+/-) in the solution, while all the other transie nt signals in the absence of primary ion in the solution are monotonic-type , except the response of AgBr electrode to Cl-. For the responses of a cert ain electrode to different interfering ions, the experiment discovers that both the smaller solubility product K-SP(MY) of ion-exchange product (MY) a nd difference [Delta H degrees(h(x+/-)) - Delta H degrees(h(Y+/-))] of hydr ation enthalpies of ions in ion-exchange process, the larger the peak heigh t Delta E-p Of transient signal responded to sudden increase in the activit y of interfering ion by electrode I and the effect of [Delta H degrees b(X/-)-Delta H degrees(h(Y+/-))] on Delta Ep is larger than the K-SP(MY). For the responses of AgI electrode to Br- and SCN-, although pK(sp(AgSCN)) appr oximate to pK(sp(AgBr)), Delta Ep(SCN-) Delta Ep(Br-). This is probably rel ated to [Delta H degrees(h(I-)) - Delta H degrees(h(SCN-))] < [Delta H degr ees(h(I-)) - Delta H degrees(h(Br-))]. For CuS electrode, pK(sp(PbS)) < pK( sp(CdS)), pK(sp(MnS)) < pK(sp(CoS)) and pK(sp(NiS)), but Delta Ep(Pb2+) > D elta Ep(Cd2+), Delta Ep(Mn2+) > Delta Ep(CO2+) and Delta Ep(Ni2+). These ar e the same due to [Delta H degrees(h(Cu2+)) - Delta H degrees(h(Pb2+))] < [ Delta H degrees(b(Cu2+)) - Delta H degrees(h(Cd2+))], [Delta H degrees(h(Ca 2+)) - Delta H degrees(h(Mn2+))] <[Delta H degrees(h(Cu2+)) - Delta H degre es(h(Co2+))] and [Delta H degrees(h(Cu2+)) - Delta H degrees(h(Ni2+))]. The relative magnitudes of differences of hydration enthalpies in positive and reverse reactions in ion-exchange can also be used to explain the relative magnitudes of peak heights Delta E-p and Delta E of transient signals in t he positive and negative activity steps. Therefore, on the basis of the abo ve facts, we believe that the peak potential of the transient signer respon ded to the interfering ion by an electrode is determined not by the velocit y of ion diffusion but by that of ion-exchange because the thickness of dif fusion layer is infinitely thin when the solution is sprayed on the surface of electrode at a high velocity. The smaller the difference in hydration e nthalpies, the larger the velocity of ion-exchange, then the larger the pea k height of transient signal may be. Besides, for other electrodes, except CuS electrode, the selectivity coefficients K-xy(c) determined on the basis of the equilibrium potentials in nonmonotonic overshoot-type transient sig nals all conform to the K-ap(MX)/K-ap(MY) values.