Study of the Zn | ZnSO4 aq. | Zn electrochemical thermocouple at low temperature

Thermoelectric properties of the non-isothermal cell Zn\ZnSO(4) aq.\Zn have been studied with salt bridges without KCl (A) and containing saturated KCl solution (B). It has been established that the value of the thermal electromotive force (emf), E, is larger in case (A) than in case (B). In both cases, the value of E changes linearly with temperature up to the freezing point of the solution. At temperatures lower than the freezing point of the: solution, however, at concentrations where the frozen phase consists of pure ice, the concentration of the solution increases resulting a rise in the value of E, whereas at compositions where the frozen phase is ZnSO(4). XH(2)O, the value of E decreases due to the decrease in the concentration of the solution. Thus, due to concentration change taking place during the freezing of the solution, the potential difference measured contains, besides the thermal emf, also the electric potential difference resulting from the concentration difference between the half cells. The above experience has been made at the electrochemical thermocouple built of Cu\Cu(++)electrodes as well [1]. According to our studies, the E(h). values measured in case (B) corrected by the temperature dependence of the mean activity coefficient of ZnSO(4) solutions, (partial derivative 1ga(+/-),/partial derivative T), and by plotting the E(h)* values thus obtained vs. 1ga(+/-), the slope of the straight line is epsilon(h) = 0.113 mV/K, which agrees well with the theoretically calculated values of 2.303 R/zF = 0.0992 mV/K. E(h)* is nearly free of thermodiffusion potential, thus it practically coincides with the temperature coefficient of the Galvani potential difference. The phi(TD)(.) value calculated from the difference between E' measured in case (A) and E(h)' obtained for case (B) decreases monotonously with increasing concentration, up to the concentration of 0.6 mol/kg.