Electric potentials generated at crystal interface

Development of electric potentials has been reported when (i) water containing an electrolyte as an impurity crystallizes [19], (ii) an electrolyte crystallizes from its supersaturated solution [20, 21], (iii) when an electrolyte dissolves in a solvent (water and non-aqueous solvents) [22] and (iv) molten electrolyte crystallizes [23]. With respect to (i) Workman and Reynolds suggested that preferential incorporation of ions in the ice lattice is an important factor responsible for the generation of freezing potentials. This view has been supported by other workers [24]. Alternatively, it has been suggested that these potentials might depend on dipole orientation, proton conductance and ionic entrapment in the ice lattice and ionic diffusion. It may be noted that freezing potentials are not developed when pure water is allowed to freeze. 

Systematic experimental and theoretical studies on generation of precipitation and dissolution potentials have been undertaken by Rastogi and co-workers [22, 25-27]. 

The experimental set-up used for the purpose of recording precipitation and dissolution potentials are schematically recorded (Fig. 5.5a and b). 

It has been shown that precipitation or dissolution potential = observed potential -Nernst potential - diffusion potential - phase potential - thermo-emf. 

Each type of potential has been measured using elaborate experimental set-up. For uni-univalent electrolytes like KCI, KBr and KI, the estimated values of precipitation and dissolution potentials are found to be in the range of 20-100mV with a negative sign. Experimental values of some typical electrolytes are recorded in Table 5.1. 

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