Electrochemical studies at low temperatures, XVII - Study of the Zn vertical bar Zn(NO3)(2)vertical bar Zn thermocouple

The thermo-electrochemical properties of nonisothermal Zn\Zn(NO3)(2)\ Zn ce lls have been investigated in cases (A) without, and (B) with salt bridge c ontaining saturated KCI solution. We have found that the values of the thermo emf (E) in both (A) and (B) cas es -similarly to the results obtained in previous experiments(1-4) - show a linear dependence on the temperature above the freezing point of the solut ions. In these cases an increase in the concentrations of the solutions was also observed - due to the freezing out of the solvent causing an increase in the values off. The values of E-h in case (B), if corrected by the corr esponding value of partial derivative lga(+/-)/partial derivativeT (the tem perature coefficients of the mean activity of Zn(NO3)(2) solutions) i.e. E- h* - lga(+/-) plots result in a straight line with a slope of epsilon (h) = 0,138 mV K-1. This is in good agreement with the values of epsilon (h) obtained for the Z n\ZnSO4; Zn\ZnCl2 and Zn\Zn(ClO4)(2) thermocouples(1-4). The intercepts (ph i (h)'*) of the E-h* - lga(+/-) lines in the case of solutions of zinc sulp hate (2) and zinc chloride (3-4) have values of 0,947, 0,843 and 0,841 mV K -1, respectively, while that of the zinc perchlorate (1) shows 0,598 mV K-1 . In the solution of zinc nitrate phi (h)'* = 0,647 mV K-1, which is close to value of the intercept obtained in the case of zinc perchlorate. While i n the solutions of zinc sulphate 2, and zinc chloride 3-4 the partial molar entropy of zinc ions calculated from the corresponding values of phi (h)'* are -141,20, and -120,04 J K-1 mol(-1), respectively, in the case of zinc perchlorate and zinc nitrate they are -73.76 JK(-1) mol(-1) and -83.37 J K- 1 mol(-1), respectively. The above results suggest that in the cases of sol utions containing complexing species (chloride or sulphate) the values of S -Zn++(-0) are smaller than in the presence of non-complexing or less comple xing anions (CIO,,NO;). These results support our previous assumptions that the differences between the phi (h)'* values may be explained with the dif ference between the values of the S-Zn++(-0) depending on rite composition of the solutions investigated. The concentration dependence of phi ('*)(TD) calculated from the difference of the values of E-'* and E-h* measured in cases (A) and (B) is similar to that observed in previous cases (1-4).