Electrochemical studies at low temperatures, XIV. Study of the Zn verticalbar ZnCl2 vertical bar Zn electrochemical thermocouple

The thermo-electrochemical properties of non-isothermal Zn\ZnCl2 aq\Zn cell s were investigated in cases (A) without the salt bridge and (B) with a sal t bridge containing of saturated KCl solution. We have found that the values of the thermo emf (E) in case (A) are smaller than in case (B), which is in opposition to the result obtained in the cas e of Zn\ZnSO4\Zn cells(2). Above the freezing points of the solutions in both cases the (E) values sho w a linear dependence on the temperature. At temperatures below the freezin g points in the investigated interval of concentrations of the ZnSO4 soluti ons, when the frozen out solid phase consists of pure ice the values off in crease due to the increase in the concentration of the solution. According to our results the measured values of E-h(.) in case (B), if-corr ected by the corresponding values of (partial derivative lga(+/-)/partial d erivative T) the temperature coefficients of the mean activity of ZnCl2 sol utions - i. e. E-h* - lga(+/-) plots result in a straight line with a slope of Eh = 0,126 mV K-1. This is in good agreement with the value of epsilon( h) = 0,113 mV K-1 found in the case of Zn/ZnSO4/Zn thermocouple and both ar e in good agreement with the theoretical value of 2,303 R/zF = 0,0992 mV K- 1. The intercepts (phi(h)*) of E-h* - lga(+/-) lines measured in the case o f Zn/ZnCl2 (phi(h)* = 0,843 mV K-1) and for the Zn/ZnSO4 (phi(h)* = 0,947 m V K-1) are also in good agreement with this. The phi(h)* values is approxim ately equal to the temperature coefficient of the Galvani potential phi(M)( *.) of the Zn/Zn++ electrodes at a(+/-) = 1. These close values of epsilon( h) and phi(h)* show that they are characteristic of the Zn/Zn++ couple, ind ependently of the chemical identity of the anions. The values of phi(TD)(.) up to a concentration of 0.3 mol kg(-1), decrease with the ZnCl2 content of the solutions, can be calculated from the differe nces between the values of E* and E-h*, measured in case (A) and (B), respe ctively.