Shedlovsky’s equation

Another method, also based on the assumption that Onsager s equation is the true limiting relation, depends upon Shedlovsky s equation, which for uni-univalent electrolytes takes the form... 

FIGURE FIO. Equivalent conductance of NaCI solutions at 25°C. Shedlovsky s equation (X +... 

Very accurate values are available for the common ions in water at 298 K, based on the work of Macinnes, Shedlovsky and Longsworth. These authors measured transport numbers and conductivities of HCl, KCl, NaCl and LiCl over a range of concentration, thus obtaining values for the molar conductivity of the chloride ion in these solutions. These were extrapolated according to Shedlovsky s equation (see conductivity at infinite dilution) with the result shown in figure M.I. A very accurate value, A (C1") = 76.3412" cm moP is thus available for the chloride ion, and by subtraction of this from the A values of the four electrolytes, AT values for the four cations are obtained. Other transport number data are now unnecessary, as the molar conductivity of a cation can be found by subtraction from the A value of its chloride, or of an anion from A" for its sodium or potassium salt. Values for the commoner ions are given in table M.l. 

Evans, Zawoyski and Kay analysed data for R4N salts in acetone (AC) " with the Fuoss-Onsager equation. They found Ka decreases with cation size, and for the anions, association decreases in the order Bu4NBr(i = 264) > I-(143) NOg > CIO4 (80) > Pic-(17). This agrees with data for methylethylketone. The fact that association of Bu4NC104 in AC, benzonitrile, and methylethyl-ketone corresponds to = 4.85 A for the three solvents, indicates formation of contact ion pairs. Tetraalkylammonium halides in dimethyl-formamide (DMF) have small association constants when the data are evaluated with Shedlovsky s eqn. 5.4.10. When the data for Me4NPic in is assessed with Fuoss and Hsia s eqn. 5.2.31, a is 6.0 A. 

The apparent acid dissociation constants (p s)Ka) of two water-insoluble drugs, ibuprofen and quinine, were determined pH-metrically in ACN water, dimethyl-formamide water, DMSO water, 1,4-dioxane-water, ethanol water, ethylene glycol-water, methanol water, and tetrahydrofuran water mixtures. A glass electrode calibration procedure based on a four-parameter equation (pH = alpha-i- SpcH -i-jH[H+] -i-jOH[OH ]) was used to obtain pH readings based on the concentration scale (pcH). We have called this four-parameter method the Four-Plus technique. The Yasuda Shedlovsky extrapolation p s)K a + log [H2O] = A/epsllon -1- B) was used to derive acid dissociation constants in aqueous solution (pKa). It has been demonstrated that the pK a values extrapolated from such solvent-water mixtures are consistent with each other and with previously reported measurements. The suggested method has also been applied with success to determine the pKa values of two pyridine derivatives of pharmaceutical Interest. Spectrometric, ultraviolet (UV) ... 

Evers and Frank s value for is about a factor of 7 smaller than Kraus. The reasons for the discrepancy are probably due to (a) the sensitivity of the process of fitting the data to the Shedlovsky equation to small errors in conductance and (b) the neglect of process (34) by Kraus which is not justified for data with solutions of concentration > 10 M. Using their calculated values of Ag, ki, and Evers and Frank find very satisfactory agreement between their predicted conductances and experimental conductances at various concentrations less than 0.04M. Above this concentration, the agreement with experiment is not expected to be too good because of the onset of the process of metallic conduction which is predominant at higher concentrations. 

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