Ionic Mobility Tables

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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. 

LISTING OF SUPPLIED IONIC MOBILITIES WITH FULL ION NAMES FOR THE PROGRAM JPCalc/JPCalcW

 The following table of relative (generalised) mobility values (relative to K⁺; see Appendix below for more information and relationship to limiting equivalent conductivities) was extracted from Table 1 of Barry & Lynch¹, with a slightly amended value for choline following later direct measurements (Ng & Barry⁴).  A supplementary list of other ionic mobilities is given in Table 2.

Note that a number of values in the tables of Lange (2) and CRC (7) have been updated in their most recent editions,  currently listed in the references.  Where these differ from the values previously listed and incorporated in JPCalc, the new updated values are now listed below in blue. These differences are invariably small.

TABLE 1.  THESE IONS ARE CURRENTLY INCLUDED IN THE JPCalc/JPCalcW AND JUNCTION POTENTIAL CALCULATOR (in AXON'S pCLAMP) PROGRAMS

For values of some other ions, see Table 1 of Barry & Lynch¹ and Table 2 following and Refs. 2, 6 and 7.

 TABLE 2: SUPPLEMENTARY LISTING OF MOBILITIES WITH FULL ION NAMES FOR THE PROGRAM JPCalc/JPCalcW

 The following table of relative mobility values was extracted from Ng and Barry⁴ and Keramidas et al.³. 

where the following standard abbreviations apply: NMDG, N-methyl-D-glucamine; Tris, tris[hydroxymethyl]aminomethane; HEPES, N-[2-hydroxyethyl]piperazine-N’-[2-ethanesulfonic acid]; MOPS, 3-[N-morpholino]propanesulfonic acid; MES, 2-[N-morpholino]ethanesulfonic acid.  The estimated error in the measurements from Ng and Barry⁴ was considered to be less than about 0.005.  EGTA(2-) and EGTA(3-) are from Keramidas et al.³

TABLE 3.  ADDITIONAL LISTING OF MOBILITIES WITH FULL ION NAMES FOR THE PROGRAM JPCalc/JPCalcW

 The following table of relative mobility values was calculated from limiting equivalent conductivities in the references below. 

TABLE 4.  FURTHER LISTING OF RELATIVE ION MOBILITIES  (ADDED IN OCTOBER 2003)

*Note that both  methylsulfate ( Ref. 7) and methylsulfonate (Ref. 2) had identical limiting equivalent conductances. 

 The same was also true of ethylsulfate ( Ref. 7) and ethylsulfonate (Ref. 2).  This may mean that, in each case, one of

 the values was incorrectly copied from the other and is wrong.

**The relative proportions of each valency species depends on pH and the ionic composition of the solution.

***u_ATP/u_K was calculated from the value of 3.0x10^-6 cm².s^-1 for the diffusion coefficient of ATP in free solution (Ref. 9). 

 TABLE 5  LISTING OF RELATIVE ION MOBILITIES OF AMINOPYRIDINES (ADDED MAY 2012)

*Calculated from the relative limiting equivalent conductivities of 2-AP and 3-AP to that of K⁺ in both the paper (Ref 10) and  PhD thesis (Ref 11), where both are in agreement.

**Calculated only from the relative limiting equivalent conductivity value in the thesis (Ref. 11), which the author considered to be more correct, in contrast to multiple errors in the paper (R. Foley,  personal communication), which in this case a simple transposition error would have reduced the relative mobility of 4-AP right down to 0.17.

REFERENCES FOR MOBILITY AND LIMITING EQUIVALENT CONDUCTIVITY DATA

1. Barry, P.H. and Lynch, J.W. (1991).  Topical Review.  Liquid junction potentials and small cell effects in patch clamp analysis.  J. Membrane Biol.  121: 101-117.

2.  Dean, J.A.. (1999). Lange’s Handbook of Chemistry, 15^th Edition, McGraw-Hill, New York.

3.  Keramidas, A., Kuhlmann, L., Moorhouse, A.J. and Barry, P.H. (1999). Measurement of the limiting equivalent conductivities and mobilities of the most prevalent ionic species of EGTA (EGTA^2- and EGTA^3-) for use in electrophysiological experiments.  J. Neurosci. Method., 89: 41-47.

4. Ng, B. and Barry, P.H. (1995).  The measurement of ionic conductivities and mobilities of certain less common organic anions needed for junction potential corrections in electrophysiology. J. Neurosci. Method., 56: 37-41.

5. Robinson, R.A. and Stokes, R.H. (1965).  Electrolyte Solutions. (2nd ed.revised), Butterworth's, London.

6. Zuidema, T., Dekker, K. and Siegenbeek van Heukelom, J. (1985).  The influence of organic counterions on junction potentials and measured membrane potentials. Bioelectrochem. Bioenerget., 14: 479-494.

7. Vanysek, P. (2002).  Ionic conductivity and diffusion at infinite dilution.  In: CRC Handbook of Chemistry and Physics (83rd  Edn; ed. D.R. Lide), CRC Press, Boca Raton.

8. Shapovalov, G. and Lester, H. (Division of Biology, Caltech, Pasadena, CA, USA). Personal communication (2001). Average of 4 measurements at pH 7.0.  Ion information: MW 75.11, Molecular Formula: C3H9NO, Structural Formula:  HOCH2CH2NHCH3, CAS: 109-83-1, MDL Number: MFCD00002839, pKa = 9.40.

9. Diehl, H., Ihlefeld, H., and Schwegler, H. (1991).  Physik fur Biologen.  Springer-Verlag, Berlin, p. 391, quoted by Rostovtseva, T.K. and Bezrukov, S.M. (1998), Biophys. J., 74: 2365-2373.

10. Haddad, P.R. and Foley R.C. (1989).  Aromatic bases as eluent components for conductivity and indirect ultraviolet absorption detection of inorganic cations in nonsuppressed ion chromatography.  Anal. Chem. 61: 1435-1441.

11. Foley, R. (1990).  Studies on detection and retention characteristics of ionorganic cations in non-suppressed ion chromatography.  PhD Thesis, 1990, The University of New South Wales, Sydney, Australia (Chapter 6, P. 126).

 Application of Junction Potential Corrections before an experiment.  (PDF)

Acknowledgement The assistance of Jennifer Anderson in sourcing the new reference editions and in compiling the new mobility data for the 2003 update has been greatly appreciated.

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