Electrolytic dissociation mode

The sorption of a weak electrolyte by a charged polymer membrane is another case where Nernst + Langmuir-like dual mode sorption, involving the undissociated and dissociated species respectively, may be expected. The concentration of each species in solution follows, of course, from the dissociation constant of the electrolyte. The sorption isotherms of acetic acid and its fluoroderivatives have been analysed in this manner, and the concentration dependence of the diffusion coefficient of acetic acid interpreted resonably successfully, using Nylon 6 as the polymer substrate 87). In this case the major contribution to the overall diffusion coefficient is that of the Nernst species consequently DT2 could not be determined with any precision. By contrast, in the case of HC1, which was also investigated 87 no Nernst sorption or diffusion component could be discerned down to pH = 2 and the overall diffusion coefficient obeyed the relation D = DT2/( 1 — >1D), which is the limiting form of Eq. (25) when p — 00. 

We now proceed to take up in a httle more detail the two lines of investigation, or modes of treatment referred to above In doing so, it is to be clearly understood that the sections headed—Degree of Dissociation and the numerical value of the equivalent conductivity at infinite dilution, Osmotic Pressure and concentration in solutions of electrolytes and the calculation of the degree of ionisation, Reaction Velocity m relation to the osmotic pressure of ions and molecules, Ionisation in formic acid solutions—-these sections belong to the first mode of treatment of strong electrolytes, whilst the remaining sections are illustrative of the second method... 

Detector response in the nitrogen mode is due to the formation of NHj by the catalytic reduction of organic nitrogen in the nickel tube. When ammonia is dissolved in the 50% -propanol/water solvent, it forms ammonium hydroxide, a weak base. The ammonium hydroxide dissociates to the electrolytically conducting species, NH/and OH". In order for the proper detector response to be obtained, the conductivity solvent must be slightly basic so that neutralization of the base does not occur. 

In this experimental mode, the error is independent of the size of the disk, but of course steady-state currents are generally achievable only at UMEs. With n = I, Dq = 10 cm /s, and Cq = 1 niM, the conductivity must exceed only 3 X 10 H cm This minimum would characterize 10 M HCl and would be met by all aqueous electrolytes having concentrations above that of the electroactive species, as well as by most common solvent systems of lower polarity containing weakly dissociated electrolytes. 

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