Overview Activity Chem Equilibrium Acids Carbonate System Mineral Phases

Activity Coefficients (Activity Models)

Activity Models

The step from molar concentrations (analytical data) to activities (used in mass-action law calculations) requires the calculation of activity coefficients γ_i. For this task several semi-empirical approaches are available, whereas each activity model has its own range of validity defined by the ionic strength I as shown here:¹

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	Model	Equation		Validity
(1)	Debye-Hückel	\(\lg \gamma_{i} = -Az^{2}_{i} \ \sqrt{I}\)		I < 10-2.3 M
(2)	Extended Debye-Hückel	\(\lg \gamma_{i} = -Az^{2}_{i} \ \left ( \dfrac{\sqrt{I}}{1+Ba_i\sqrt{I}} \right )\)		I < 0.1 M
(3)	Davies	\(\lg \gamma_{i} = -Az^{2}_{i} \ \left ( \dfrac{\sqrt{I}}{1+\sqrt{I}} - 0.3 \!\cdot\! I\right )\)		I ≤ 0.5 M
(4)	Truesdell-Jones   (WATEQ Debye-Hückel)	\(\lg \gamma_{i} = -Az^{2}_{i} \ \left ( \dfrac{\sqrt{I}}{1+B a_i^0 \sqrt{I}}\right ) + b_i \!\cdot\! I\)		I < 1 M

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Here, z_i is the valence of ion i. All quantities carrying the subscript i are ion-specific parameters (a_i, a_i⁰ and b_i). On the other hand, the prefactors A and B depend on the temperature T and the dielectric constant ε_r:

(5a)	A = [1.82∙106 K3/2 M-1/2]   ⋅ (εrT)-3/2
(5b)	B = [50.3 nm-1 K1/2 M-1/2] ⋅ (εrT)-1/2

For water at 25 (T = 298 K) and with ε_r = 78.54, we obtain:

(6a)	A = 0.5085 M-1/2
(6b)	B = 3.281 M-1/2 nm-1

Note the length unit in the last equation: 1 nm = 10^-9 m = 10 Ångström.

Pitfalls. The prefactors A and B depend on the units chosen (and other conventions). So be careful when comparing them with other prefactors given in the literature.

Example 1. If the activity corrections are expressed in terms of ln γ_i (instead of lg γ_i), then A should be multiplied by (ln 10):

A \(\quad\Longrightarrow\quad\) A · (ln 10) = 1.172 M-1/2

Example 2. If the prefactor B in 6b is expressed using the non-SI unit Ångström (instead of nm), then B should be divided by 10:

B \(\quad\Longrightarrow\quad\) B/10 = 0.3286 M-1/2 Å-1

Model Hierarchy. The relationship between the various activity models becomes most clear when they are all traced back to the simple Debye-Hückel formula in 1. Denoting the “Debye-Hückel building block” by lg γ_i⁰, the equations above can be rewritten as:

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	Model	Equation		Validity
(1b)	Debye-Hückel	\(\lg \gamma_{i}^{0} \ =\ -Az^{2}_{i} \ \sqrt{I}\)		I < 10-2.3 M
(2b)	Extended Debye-Hückel	\(\lg \gamma_{i} \ =\ \dfrac{\lg \gamma_{i}^{0}}{1+Ba_i\sqrt{I}}\)		I < 0.1 M
(3b)	Davies	\(\lg \gamma_{i} \ =\ \dfrac{\lg \gamma_{i}^{0}}{\ 1+\sqrt{I}\ } \ + \ 0.3 Az^{2}_{i} \!\cdot\! I\)		I ≤ 0.5 M
(4b)	Truesdell-Jones	\(\lg \gamma_{i} \ =\ \dfrac{\lg \gamma_{i}^{0}}{1+B a_i^0 \sqrt{I}} \ + \ b_i \!\cdot\! I\)		I < 1 M

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The activity coefficients decrease steadily when the ionic strength I rises. However, both the Davies and the Truesdell-Jones equations contain an additive term that causes a re-increase when I approaches 1 mol/L – see diagrams below.

The empirical model of Truesdell-Jones in 4 and 4b, with its two ion-specific parameters a_i⁰ and b_i, represents the most general approach. By specifying and/or ignoring these two parameters we obtain the other three activity models.

Extended Debye-Hückel Equation

Due to the narrow validity range of the Debye-Hückel formula in 1, this approach was extended in 2 by an additional term in the denominator containing the parameters B and a_i. The extended formula takes into account the fact that the central ion has a finite radius (instead of a point charge). The parameter a_i represents the effective size of the corresponding ion, for example:²

a = 0.9 nm	for	H+, Fe+3, Al+3
a = 0.8 nm	for	Mg+2
a = 0.6 nm	for	Ca+2, Fe+2
a = 0.4 nm	for	Na+, HCO3-, SO4-2
a = 0.3 nm	for	K+, NH4+, OH-, Cl-, NO3-

Note. The ion size parameter a_i is an empirical fitting parameter; it is larger than the ionic radius because it contains a part of the hydrate shell.

Davies Equation

The Davies formula in 3 is an empirical approach which differs in two respects from the extended Debye-Hückel 2:

• it get rid of the ion-size parameter a_i (which is not well known for complex ions)
• the additional term 0.3⋅I, which is linear in the ionic strength I

Irrespective of the fact that there is no strict theoretical justification for the additional term, it improves the empirical fit to higher ionic strengths up to I ≈ 0.5 M. Due to its mathematical simplicity and the absence of free parameters, the Davies equation is a preferred choice in hydrochemistry modeling (see below).

For neutral species (z_i = 0) the Davies formula collapses to the Setchenow equation: lg γ_i = const⋅I.

Truesdell-Jones (WATEQ Debye-Hückel)

The empirical approach of Truesdell and Jones³ was proposed for the hydrochemistry program WATEQ in 1974 with the aim of describing NaCl-containing solutions. The additional fit parameter b_i in 4 extended the scope to seawater (i.e. I = 0.72 M and above).

Equation (4) is based on two fit parameters: a_i⁰ and b_i, whereas the effective ionic radius differs from the extended Debye-Hückel model (a_i⁰ ≠ a_i). Typical values for b_i are in the order of 0.1.

Example: Activity Coefficient for Mg

The subsequent two diagrams plot the activity coefficient γ_i of the ion Mg⁺² as a function of the ionic strength:

• upper diagram:	I = 0 … 2 M	(linear)
• bottom diagram:	I = 0.0001 … 5 M	(logarithmic)

The calculations are based on the following ion-specific parameters:

	zi = 2
	ai  = 0.80 nm	for Extended Debye-Hückel
	ai0 = 0.55 nm	for Truesdell-Jones 4
	bi = 0.2	for Truesdell-Jones 4

[Please note: The diagrams display γ_i on the y-axis (and not lg γ_i).]

At I=0 (ideal solution) the activity coefficient is 1. It decreases with increasing ionic strength I. At high ionic strength (I ≈ 1 M) there is again an increase of γ_i, but only for Davis and Truesdell-Jones caused by the additive terms in 3 and (4).

What Activity Model is used in the Program?

The program aqion, which is based on PhreeqC, uses

• either the Davies equation (3) – as default setup
• or the Truesdell-Jones equation (4) – if the parameters a_i⁰ and b_i are provided⁵

Model	Parameters	Name	Used in aqion?
Davies	0	–	yes (default)
Extended DH	1	ai	no
Truesdell-Jones	2	ai0, bi	yes

The type of the activation model is set in the thermodynamic database (e.g. wateq4f.dat) for each aqueous species separately.

High-Saline Solutions (I > 1 M)

The Pitzer approach⁶ is a sophisticated ion-interaction model used for high-saline solutions up to I = 20 M. Unfortunately, a lot of additional parameters (virial coefficients) are required for this. The Pitzer model is not implemented in aqion.

References & Remarks