Concentration dependence of the molar conductivity and association constants

3 Concentration dependence of the molar conductivity and association constants 

The dependence of the molar conductivity with concentration is primarily dominated by ion-ion interactions which, in the Umit of low density, could lead to extensive clustering of the ions. 

The precision of the experimental data is a key issue in choosing a conductivity equation to fit the concentration dependence of the molar conductivity and, in the case of associated electrolytes, the association constant. Old meas-mements of conductivity, particularly those by Franck and co-workers in Germany and by Marshall and co-workers in ORNL (USA), having imcertainties aroimd 1% were fitted using the Shedlovsky or the Fuoss-Kraus equations, which allows the simultaneous determination of A° and K,. 

During the last decade more precise conductivity data ( 0.1 /o) could be obtained in the dilute region, especially in the low-density (supercritical) region. Thus, the more complete conductivity equations, like FHFP or TBBK, could been tested for a number of binary electrolytes. 

It is recognized that FHFP equation accounts for the concentration dependence of electrolyte solutions up to moderate concentrations and yields more reliable association constants than the Shedlovsky or Fuoss-Kraus equations. However, it was observed that the FHFP Equation (4.18), or the more simple Shedlovsky Equation (4.16), give similar fitting results, for some supercritical electrolyte solutions at low density (p 0.3 g cm ). The contribution of the electrophoretic effect to the concentration dependence of the molar conductivity is expected to be lower in supercritical water than in ambient water because of the much smaller viscosity and dielectric constant. Moreover, the higher-order terms in Equation (4.18) nearly cancel each other at moderate concentration in supercritical water (Ibuki et al., 2000). This could be the reason why differences among several conductivity equations vanish at supercritical conditions. 

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