Liquid Junction Potential Calculator

LJPcalc Theory

Table of Contents

Calculation Method
LJPcalc Ion Mobility Library
Influence of Ion Sequence on Calculated LJP
How to Correct for LJP in Electrophysiology Experiments
Example Patch-Clamp LJP Calculation & Correction
Notes about offset voltage, Ag/AgCl pellets, and and half-cell potentials
Measuring LJP Experimentally
Effect of Temperature on LJP
Calculating Ionic Mobility from Charge and Conductivity
References

Calculation Method

LJPcalc calculates the liquid junction potential according to the stationary Nernst-Planck equation which is typically regarded as superior to the simpler Henderson equation used by most commercial LJP calculators. Both equations produce nearly identical LJPs, but the Henderson equation becomes inaccurate as ion concentrations increase, and also when calculating LJP for solutions containing polyvalent ions.

LJPcalc Ion Mobility Library

LJPcalc uses an extensive ion mobility library: IonTable.md.html

The ion mobility table is stored in Markdown format. Not only does Markdown make it easy to display the table nicely in a browser, but it also makes the table easy to edit in any text editor. Users desiring to use their own ion mobilities or add new ions to the table can do so by editing the IonTable.md file adjacent to LJPcalc.exe as needed.

Influence of Ion Sequence on Calculated LJP

💡 LJPcalc automatically sorts the ion table into an ideal sequence prior to solving for LJP. Attention only needs to be paid to the ion sequence if automatic sorting is disabled.

When calculating LJP for a set of ions it is important to consider the sequence in which they are listed. Additional information can be found in Marino et al., 2014 which describes the exact computational methods employed by LJPcalc.

How to Correct for LJP in Electrophysiology Experiments

Electrophysiologists often measure (or clamp) the voltage of cells impaled with glass microelectrodes, but the difference in ionic composition between the intracellular (pipette) and extracellular (bath) solutions produces a LJP which is added to the measurements:

Vmeasured = Vcell + LJP

To compensate for LJP, the electrophysiologist must calculate LJP mathematically (using software like LJPcalc) or estimate it experimentally (see the section on this topic below). Once the LJP is known, recorded data can be adjusted to accurately report cell voltage:

Vcell = Vmeasured - LJP

⚠️ This method assumes that (1) the amplifier voltage was zeroed at the start of the experiment when the pipette was in open-tip configuration with the bath, and (2) the concentration of chloride (if using Ag/AgCl electrodes) in the internal and bath solutions are stable throughout the experiment.

Example Patch-Clamp LJP Calculation & Correction

This ion set came from in Figl et al., 2003 Page 8. They have been loaded into LJPcalc such that the pipette solution is c0 and the bath solution is cL. Note that the order of ions has been adjusted to place the most abundant two ions at the bottom. This is ideal for LJPcalc's analytical method.

Name Charge pipette (mM) bath (mM)
K +1 145 2.8
Na +1 13 145
Mg +2 1 2
Ca +2 0 1
HEPES -1 5 5
Gluconate -1 145 0
Cl -1 10 148.8

Loading this table into LJPcalc produces the following output:

Values for cL were adjusted to achieve electro-neutrality:

 Name               | Charge | Conductivity (E-4) | C0 (mM)      | CL (mM)      
--------------------|--------|--------------------|--------------|--------------
 K                  | +1     | 73.5               | 145          | 2.8098265   
 Na                 | +1     | 50.11              | 13           | 144.9794365 
 Mg                 | +2     | 53.06              | 1            | 1.9998212   
 Ca                 | +2     | 59.5               | 0            | 0.9999109   
 HEPES              | -1     | 22.05              | 5            | 4.9990023   
 Gluconate          | -1     | 24.255             | 145          | 0           
 Cl                 | -1     | 76.31              | 10           | 148.789725

Equations were solved in 88.91 ms
LJP at 20 C (293.15 K) = 16.052319631180264 mV

💡 Figl et al., 2003 Page 8 calculated a LJP of 15.6 mV for this ion set (720 µV lesser magnitude than our calculated LJP). As discussed above, differences in ion mobility table values and use of the Nernst-Planck vs. Henderson equation can cause commercial software to report values slightly different than LJPcalc. Experimentally these small differences are negligible, but values produced by LJPcalc are assumed to be more accurate. See Marino et al., 2014 for discussion.

If we have patch-clamp data that indicates a neuron rests at -48.13 mV, what is its true resting potential? Now that we know the LJP, we can subtract it from our measurement:

Vcell = Vmeasured - LJP

Vcell = -48.13 - 16.05 mV

Vcell = -64.18 mV

We now know our cell rests at -64.18 mV.

Notes about offset voltage, Ag/AgCl pellets, and and half-cell potentials

The patch-clamp amplifier is typically zeroed at the start of every experiment when the patch pipette is in open-tip configuration with the bath solution. An offset voltage (Voffset) is applied such that the Vmeasured is zero. This process nulls several potentials:

When the amplifier is nulled prior to experiments the half-cell potentials can typically be ignored. However, if the [Cl] of the internal or bath solutions change during the course of an experiment (most likely to occur when an Ag/AgCl pellet is immersed in a flowing bath solution), the half-cell potentials become significant and affect Vmeasured as they change. See Figl et al., 2003 for more information about LJPs as they relate to electrophysiological experiments.

Measuring LJP Experimentally

It is possible to measure LJP experimentally. However, this technique is discouraged because issues with KCl reference electrodes make it difficult to accurately measure LJP (Barry and Diamond, 1970). However, this technique can be used in cases when ion mobilities are not known:

To measure LJP of an intracellular vs. extracellular solution for whole-cell patch-clamp experiments:

⚠️ Use of an Ag/AgCl pellet will not produce accurate results. This is because intracellular solution typically has a low [Cl]. Using a KCl reference is ideal because intracellular solution has high [K] and extracellular solution has high [Cl] so there is excellent mobility in all cases.

Effect of Temperature on LJP

The LJP is temperature dependent. There are two sources of temperature-dependent variation: the Einstein relation and the conductivity table. The former can be easily defined at calculation time, while the latter requires modifying conductances in the ion mobility table. These modifications typically have a small effect on the LJP, so standard temperature (25C) can be assumed for most applications.

The Einstein relation defines diffusion as D = µ * k * T where:

The ion conductivity table is temperature-specific. Ion conductivity was measured experimentally and varies with temperature. The ion conductivity table here assumes standard temperature (25C), but ion conductivity values can be found for many ions at nonstandard temperatures. LJPcalc users desiring to perform LJP calculations at nonstandard temperatures are encouraged to build their own temperature-specific ion tables.

Calculating Ionic Mobility from Charge and Conductivity

Ionic mobility is µ = Λ / (N * e² * |z|) where:

References