## Electrical Conductivity based on Diffusion Coefficients

Nernst-Einstein Equation

The Nernst-Einstein equation establishes the relationship between the molar limiting conductivity $\Lambda_{m,i}^{0}$ and the diffusion coefficient Di for any given ion i:

 (1) $D_i = \dfrac{RT}{z_i^2 F^2} \, \Lambda_{m,i}^0$ or $\Lambda_{m,i}^0 = z_i^2 D_i \, \left( \dfrac{F^2}{RT} \right)$

with

 zi charge number of ion i T in K absolute temperature F = 9.6485·104 Coulomb/mol Faraday’s constant R = 8.31446 J/(K mol) gas constant F2/(RT) = 3.7554·106 s · S mol-1 proportionality constant at 25°C Di in m2/s diffusion coefficient of ion i $\Lambda_{m,i}^{0}$ in S m2/mol (= 104 S cm2/mol) molar limiting conductivity of ion i

Example. If we enter some well-known values of Di (taken from literature) into Eq.(1) the following molar limiting conductivities are obtained:

ion Di [m2 s-1] $\Lambda_{m,i}^0$ [S cm2 mol-1]
H+ 9.31·10-9 349.6
Na+ 1.33·10-9 50.0
K+ 1.96·10-9 73.6
OH- 5.27·10-9 197.9
Cl- 2.03·10-9 76.2
Br- 2.01·10-9 75.5

The goal is now to exploit this method for the calculation of electrical conductivities (EC) of aqueous solutions of arbitrary composition.

EC of Ideal Aqueous Solutions

In the limit of infinite dilution (non-interacting ions) we obtain from Eq.(7a) a simple formula that relies on diffusion coefficients:

 (2) $EC^{(0)} \ = \ \sum\limits_i \, \Lambda_{m,i}^0 \, c_i \ = \ \left( \dfrac {F^2}{RT} \right) \ \sum\limits_i \, D_i z_i^2 \, c_i$

In the very realistic case of non-ideal solutions, however, the method becomes somewhat more elaborate.

EC of Real (Non-Ideal) Aqueous Solutions

The Nernst-Einstein equation is restricted and valid only for molar limiting conductivities $\Lambda_{m,i}^{0}$. In contrast, the EC of real aqueous solutions rest upon molar conductivities $\Lambda_{m,i}$:

 (3) $EC \ = \ \sum\limits_i \, \Lambda_{m,i} \, c_i \$

Both quantities are related by Kohlrausch’s Square-Root Law, but this law requires the knowledge of an extra parameter K which depends non-trivially on the type of electrolyte (and which is hardly available in tables or literature). An alternative was proposed by Appelo1 who rearranged Eq.(3) into

 (4) $EC \ = \ \sum\limits_i \, \Lambda_{m,i}^0 \, \gamma_{corr} \, c_i\$

where all aspects regarding the ion-ion interaction are put into the correction factor

 (5) $\ln \gamma_{corr} \ \simeq \ - (K /\Lambda_{m,i}^0) \, \mid\! z_i\!\mid^{1.5} \! \sqrt{I}$

with I as ionic strength. It is no surprise that this ion-ion correction factor closely resembles the activity model of Debye-Hückel:

 (6) $\ln \gamma_i \ =\, - (\ln 10) \ Az^{2}_{i} \ \sqrt{I}$ (Debye-Hückel)

with A = 0.5085 M-1/2. This strategy looks promising because we replaced the non-trivial Kohlrausch parameter K by the activity constant γ – a quantity that belongs to the standard repertoire of hydrochemical models anyway (available for each ion and aqueous species).

Formally, Eq.(4) can be converted into

 (7) $EC \ = \ \sum\limits_i \, \Lambda_{m,i}^0 \, (\gamma_i)^{\alpha} \, c_i\$ with
 (8) $\alpha \ = \ \dfrac{\ln \, \gamma_{corr}}{\ln \, \gamma_i} \ = \ \dfrac{K} {\Lambda_{m,i}^0 \, (\ln 10) \ A \, \mid\! z_i\mid^{0.5}}$

The only thing we need is a clever parametrization of α as a fairly constant quantity.

Parametrization à la Appelo (PhreeqC 3)

Appelo1 proposed the following parametrization for Eq.(8):

 (9) $% $

Just this parametrization is used in PhreeqC and aqion. The equation behind the EC calculation is:

 (10) $EC \ = \ \left( \dfrac {F^2}{RT} \right) \ \sum\limits_i \, D_i z_i^2 \, (\gamma_i)^{\alpha} \, c_i$

This equation is used as the default method in aqion. The diffusion coefficients Di are taken from here.

[Remark to γi: While Eq.(6) represents the most simplest form of an activity model (in order to demonstrate the main ideas in the present article), in the program, however, more sophisticated approaches for γi are used.]

Appendix: Rearrangement of the EC Equation

The aim is to convert Eq.(3) into a form similar to the ideal-solution formula:

 (A1) $EC \ = \ \sum\limits_i \, \Lambda_{m,i} \, c_i \$ $\Large \Rightarrow$ $EC \ = \ \sum\limits_i \, \Lambda_{m,i}^0 \, \gamma_{corr} \, c_i$

We start from Eq.(7b) and get from Kohlrausch’s Square-Root Law:

 (A2) %

Using $\Lambda_{eq}^{0} = \Lambda_{m}^{0} \, / \!\mid\! z_i\mid$, the last line yields:

 (A3) $EC \ = \ \sum\limits_i \ \Lambda_{m,i}^0 \ \left\{1 - (K /\Lambda_{m,i}^0) \, \mid\! z_i\!\mid^{1.5} \! \sqrt{c} \, \right\} \, c_i$

Note that c, and not ci, enters the square root. This is because c refers to the ‘medium effect’ of the electrolyte (as a composition of several ion types). In other words, c appears as a sibling of the ionic strength I = ½ Σ zi2 ci.2 Thus, replacing c by I in Eq.(A3) yields the desired equation:

 (A4) $EC \ = \ \sum\limits_i \, \Lambda_{m,i}^0 \, \gamma_{corr} \, c_i\$ with $\gamma_{corr} \ = \ 1 - (K /\Lambda_{m,i}^0) \, \mid\! z_i\!\mid^{1.5} \! \sqrt{I}$

The value of $\gamma_{corr}$ is near to 1, which permits the expansion exp(-a) = 1–a + … :

 (A5) $\gamma_{corr} \ \simeq \ \exp \,\left\{ - (K /\Lambda_{m,i}^0) \, \mid\! z_i\!\mid^{1.5} \! \sqrt{I} \,\right\}$

References & Remarks

1. C.A.J. Appelo: Specific conductance – how to calculate the specific conductance with PHREEQC (2010), http://www.hydrochemistry.eu/exmpls/sc.html  2

2. The following example for NaCl illustrates the close relationship between the electrolyte concentration c and ionic strength: I = ½ (cNa + cCl) = cNaCl