Electrochemical studies at low temperatures,: XIII.: Study of the Zn/ZnSO4/Zn electrochemical thermocouple

The thermo- electrochemical properties of non-isothermal Zn\ZnSO(4)aq\Zn cells were investigated in cases (A) without salt bridge and in (B) with salt bridge consisting of saturated KCI solution. We have found that the values of the thermo emf(E) in case (A) are greater than in case (B). At the freezing points of the solutions in both cases the (E) values show a linear dependence on temperature. At temperatures below the freezing points in the interval of concentrations of the ZnSO4 solutions, when the frozen out solid phase consists of pure ice, the values of E increase due to the increase in the concentration of the solution. On the other hand, at compositions where the frozen phase is ZnSO4. XH2O the values of E decrease. At such temperatures the potential difference measured between the poles of the cell includes the potential difference caused by the concentration inequalities between the two half-cells too. The above mentioned phenomena were observed also in the case of the Cu\Cu++ thermocouple(1). According to our results the measured values of E-h in case (B) if corrected by the corresponding values of (partial derivative 1ga(+/-)/partial derivative T) - the temperature coefficients of the mean activity of ZnSO4 solutions - i. e. E-h*- 1ga(+/-) plots result in straight line with a slope epsilon(h) = 0,1 13 mV K-1 that is in good agreement with the theoretical value of The 2,303 R/zF = 0,0992 mV K-1. The E-h* values do not include the temperature coefficients of the thermal liquid junction potential phi(TD'), therefore E-h* is approximately equal the temperature coefficient of the Galvani potential phi(M)'. The values phi(TD)' which, up to a concentration of 0.6 mol kg(-1) decreases with the ZnSO4 content of the solutions, can be calculated from the differences between the values of E' and E-h', measured in case (A) and (B), respectively.