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How to Derive the Debye-Huckel-Onsager Equation for Electrolyte Solutions in PDF Format

The Debye-Huckel-Onsager equation is a theoretical expression that relates the mean ionic activity coefficient of an electrolyte solution to its ionic strength and temperature. The equation is based on the Debye-Huckel theory, which considers the electrostatic interactions between ions in a solution and their effect on the chemical potential and activity of each ion. The equation is useful for extrapolating thermodynamic quantities to low solute molality or infinite dilution, where the electrostatic interactions are dominant.


In this article, we will show you how to derive the Debye-Huckel-Onsager equation for electrolyte solutions in PDF format. PDF stands for Portable Document Format, which is a file format that preserves the layout and appearance of a document across different platforms and devices. PDF files can be created and viewed using various software tools, such as Adobe Acrobat Reader or Microsoft Word. To derive the Debye-Huckel-Onsager equation in PDF format, we will use Microsoft Word as an example.

Step 1: Write the basic form of the Debye-Huckel-Onsager equation

The basic form of the Debye-Huckel-Onsager equation for the logarithm of the mean ionic activity coefficient of an electrolyte solute is given by:

$$\ln \gamma_\pm = -A_DHz_+z_-I_m^1/2\left(1 + B_DHaI_m^1/2\right)^-1$$

where $\gamma_\pm$ is the mean ionic activity coefficient, $z_+$ and $z_-$ are the charge numbers of the cation and anion of the solute, $I_m$ is the ionic strength on a molality basis defined by $I_m = \frac12\sum_i m_i z_i^2$, where $m_i$ and $z_i$ are the molality and charge number of ion $i$, $a$ is the distance of closest approach of two ions, and $A_DH$ and $B_DH$ are constants that depend on the solvent properties and temperature.

To write this equation in Microsoft Word, you can use the equation editor tool, which can be accessed from the Insert tab or by pressing Alt + =. You can type the equation using LaTeX syntax or use the symbols and templates from the equation toolbar. You can also adjust the font size, style, and alignment of the equation as you wish.

Step 2: Write the expressions for the constants ADH and BDH

The constants $A_DH$ and $B_DH$ appearing in the Debye-Huckel-Onsager equation are defined by:

$$A_DH = \fracN_A^2 e^38\pi\left(\frac2\rho_A^*\epsilon_r \epsilon_0 R T\right)^3/2$$

$$B_DH = N_A e \left(\frac2\rho_A^*\epsilon_r \epsilon_0 R T\right)^1/2$$

where $N_A$ is the Avogadro constant, $e$ is the elementary charge, $\rho_A^*$ and $\epsilon_r$ are the density and relative permittivity (dielectric constant) of the solvent, $\epsilon_0$ is the electric constant (or permittivity of vacuum), $R$ is the gas constant, and $T$ is the absolute temperature.

To write these equations in Microsoft Word, you can use the same method as in Step 1.

Step 3: Write the derivation steps for the Debye-Huckel-Onsager equation

The derivation of the Debye-Huckel-Onsager equation involves several steps that combine electrostatic theory, statistical mechanical theory, and thermodynamics. A brief outline of these steps is given below:

Consider an individual ion of species $i$ as it moves through the solution; call it the central ion. Around this central ion, the time-average spatial distribution of any ion species $j$ is not random, on account of the interaction of these ions of species $j$ with the central ion. The distribution must be spherically symmetric about

the central ion; that is,

a function only


the distance



the center


  • the ion.

Assume that

the local concentration,



the ions





a given value



depends on

the ion charge

$z_j e$


the electric potential


at that position.

The time-average electric potential in turn depends on

the distribution

of all ions


is symmetric about

the central ion,

so expressions must be found for

$c_j'$ and $\phi$

as functions of


  • that are mutually consistent.

Assume that $c_j'$ is given by

  • the Boltzmann distribution:

$$c_j' = c_j e^-z_j e \phi / k T$$


the exponential function

in powers



and retain only

  • the first two terms:

$$c_j' \approx c_j (1 - z_j e \phi / k T)$$


the electric potential function

consistent with this distribution

and with

the electroneutrality