Ion Concentrations and Partially Dissociated Salts and Chelating Agents

It should be noted that only the free ionic concentration (or activity) values should be used in the program to calculate liquid junction potentials. Of course, non-electrolytes do not directly contribute to the liquid junction potential and can be ignored.

The former note is especially important to appreciate for ions which are not fully ionised in the solution.  For example, this may be because they are derived from weak acids like HEPES or are polyvalent chelating agents like EGTA.

For example, if titrating 10 mM HEPES with NaOH requires about 5 mM NaOH to bring the pH to about 7.4, then at this pH the free HEPES^- concentration will be about 5 mM along with an additional 5 mM of Na⁺ (plus any other Na⁺ contributions), so it would be these values which should be entered into the program. 

Similarly, if 5 mM EGTA and 2 mM CaCl₂ are added to the solution, then contributions from EGTA^2- and Ca-EGTA^2- will be close to about 3 mM and 2 mM at pH 7.4, since virtually all the Ca²⁺ will be chelated mostly in the form Ca-EGTA^2-, and the remaining  EGTA will mostly be in the EGTA^2- form.

It should also be noted that strictly ion activities rather than concentrations should be used in situations like dilution potential experiments, where the test solutions can be of very different overall ionic strength. See JPCalcWManual-web-2009.pdf.  For a recent example of this effect, see also S. Sugiharto et al. (2008), Biophys. J. 95: 4698-4715. 

To return to Liquid Junction Potential Calculator and LJP Calculations, or for other information about such calculations and mobility values, click here. 

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P. H. Barry, Oct 7, 2009