Onsager limiting law

In an analogous way, for a weak uni-univalent electrolyte the Onsager limiting law has the form... 

Fuoss and Kraus [13] and Shedlovsky [14] improved Eq. (7.6) by taking the effect of ion-ion interactions on molar conductivities into account. Here, Fuoss and Kraus used the Debye-Huckel-Onsager limiting law [Eq. (7.1)] and Shedlovsky used the following semi-empirical equation ... 

For most of the salts the slope of the phoreogram is steeper (i.e., more negative or catabatic) than would be calculated by the Onsager limiting law. Most researchers have rejected the possibility of ionic association as a cause of this deviation - principally because the Bjerrum contact distance in NMA, for the temperature range of 30 to 60 °C, is less than 0.2 nm and thus less than the sum of the crystallographic radii of most possible cation-anion contact pairs. 

The first two terms on the right hand side of the equation give the Debye-Hiickel-Onsager limiting law equation ... 

F. 6. Conductance of propanol solutions of Pt4NI ( o ) and i-AnuNI (O) at 25 C The full lines are calculated with the help of Eqs. (36) using A.f-values from transference numbers, ai and R values from calorimetric measurements. Values ( o) and (o) from conductance measurements were added for comparison. The broken lines reprint the Onsager limiting law of conductance... 

Figure 4 shows the typical concentration and temperature dependence of the conductivity of a moderately associated electrolyte in a nonaqueous solvent. The calculated conductivity curve [Eqs. (65)] reproduces the measured conductivities (open circles) with a precision of better than 0.02%. The broken line of 25°C is the limiting law, Eq. (62). There is no doubt that the Onsager limiting law can be confirmed by highly precise measurements, but its validity range is limited at very low concentrations. 

Figure 1 shows the temperature and concentration dependence of the molar conductivity of potassium iodide in ethanol. It is typical for systems with small or moderate association constants (K ranges from 77 dm /mol at 25 °C to 29 dm /mol at 5 °C for KI in ethanol) and the extrapolation to zero concentration (A ) can easily be done. The Onsager limiting law (only the linear term in Eq. 6) is plotted at 25 °C for comparison. 

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