Thermal diffusion potential

The passage of electric current through an aqueous solution in the presence of temperature, potential and concentration gradient involves interesting phenomena where three forces are coupled. This leads to development of (i) diffusion potential and (ii) thermal diffusion potential. This also affects transport numbers. 

Although heat of transport of gases has been given due attention, it is equally interesting to study the characteristic properties of transfer and its dependence on temperature, concentration and presence of other ions [14, 15]. 

Within the range of validity of Debye-Huckel theory, the heat of transfer of all ions should approach the same value in the medium of same ionic strength. For binary electrolytes, we have in the stationary state [16] 

The asterisk denotes that the quantities are at the ideal state. 

For investigating the effect of the nature of anions on the heat of transport of silver ions, the cells Ag/AgN03/Ag Ag/AgCH300/Ag were studied. 

---

In a solution with a cation 1 and an anion 2, one can considered two cases for which the thermal diffusion potential is defined. One case is at the initial state, when the concentration is uniform and a temperature gradient is established and a second case when the steady state is reached, in other words, when the migration of ions by the temperature gradient has ceased. 

The thermal diffusion potential, td> arises if an electrochemical system is nonisothermal. This phenomenon is due to the heat transport of ionic species and can be taken into account if the individual ion entropy of transport, conductivity, and activity coefficients of the species of interest are known. Therefore, the thermal diffusion potential depends on the temperature, pressure, and composition of the electrolyte liquid junction. Also, td is a function of the temperature gradient and can be a substantial value from tens to hundreds of millivolts [19]. 

Chapters 5 and 6 deal with systems where interaction between temperature gradient, concentration gradient and potential gradients without any barrier are involved. In these chapters, theoretical and experimental studies relating to thermal diffusion, Dufour effect, Soret effect, thermal diffusion potential, thermo-cells, precipitation and dissolution potential have been described. Physical implications of the experimental results have also been described. 

Similar phenomena such as diffusion potential and thermal diffusion potential in systems where ion transport is involved are also of considerable interest. Coupling of flow of ions relative to solvent is involved in the development of diffusion potential, while in the case of thermal diffusion potential, coupling of flow of ions and energy flow is involved. In such situations, the effective transference number as compared to Hittorf transference number is affected. Interesting experimental results have been reported in the context of galvanic cells (thermo-cells), in which the two electrodes are not at the same temperature where results have been interpreted in terms of thermodynamics of irreversible processes [3]. 

The thermal diffusion potential arises when there is a temperature gradient within an electrolyte bridge and is due to heat transport by ionic species. The magnitude of Etd can be estimated from the entropy of transport, conductivity and activity coefficients of the individual ions. Therefore, the magnitude of Etd depends on the temperature, pressure and composition of the electrolyte liquid junction. The value of Ejd can be as high as tens to hundreds of mV. 

A reference with a calomel internal should not be used at temperatures of 80°C or higher for a prolonged period. The silver-silver chloride reference, however, is better suited for high temperature measurements. Often the reference electrode is placed in a remote, cooler position. Thermal diffusion potentials are created by the use of the remote salt bridge, but their influence is compensated for by buffer standardization under similar conditions. 

Thermal diffusion potential — Nonisothermal solutions develop an electric - potential gradient, dEjdx), but it is a not measurable quantity, because there is no method to determine electrical potential differences (A ) between two points at different temperatures, even for a uniform composition single phase. A similar situation appears when it is considered a potential difference between two points in phases which are chemically dif-... 

As noted above, an external pressure-balanced electrode suffers from two irreversible thermodynamic potentials the isothermal liquid junction potential and the thermal hquid junction potential (thermal diffusion potential). The external pressure balanced reference electrode (EPBRE) is amenable to formal thermodynamic analysis in terms of Thermocell I ... 

The quantity E-ru in Eq. (45) is the thermal liquid junction potential, or the thermal diffusion potential, which arises from the coupling between heat flow and mass flow (diffusion) in accord-... 

The potential difference for the two different solutions was shown to be independent of the thermal diffusion potential and can be calculated theoretically. The calculated and measured poten-... 

---