Activity coefficient

An activity coefficient is the ratio between a substance’s thermodynamic activity and its concentration or composition measure. It quantifies departures from ideal-solution behavior.

Chemical activity: A dimensionless thermodynamic measure of a substance’s effective concentration relative to a specified standard state. The coefficient converts composition into this effective thermodynamic quantity.
Mole fraction: The amount of one component divided by the total amount of all components in a mixture. It is a common composition scale for defining liquid-phase activities.
Debye–Hückel theory: A theory that estimates ionic activity coefficients in dilute electrolyte solutions from electrostatic interactions. It provides a predictive correction for charged solutes at low ionic strength.
Concentration: A quantitative measure of the amount of a component in a specified volume or amount of mixture. Concentration is directly measurable, while the coefficient accounts for its thermodynamic nonideality.
Chemical potential: The change in a system’s Gibbs energy when an infinitesimal amount of a component is added at fixed conditions. Activity enters chemical potential as a logarithmic correction to the standard-state value.
Molality: The amount of solute per kilogram of solvent in a solution. Electrolyte activity coefficients are often reported on a molality scale.
Davies equation: An empirical equation used to estimate ionic activity coefficients in aqueous solutions over a moderate ionic-strength range. It extends practical electrolyte corrections beyond the simplest dilute-limit model.
Osmotic coefficient: A factor relating a solution’s osmotic behavior to that predicted from ideal solute concentration. It describes a related electrolyte nonideality but is not the component activity coefficient.
Fugacity: A pressure-like thermodynamic quantity that represents a real gas’s escaping tendency and replaces pressure in chemical-potential equations. Gas-phase activity coefficients relate fugacity to pressure.
Molar concentration: The amount of solute per unit volume of solution, usually expressed in moles per liter. Concentration-based activity conventions use molarity as their composition measure.

Linked from 23 pages