# Activity Coefficient

Last updated: August 1, 2017

## What Does Activity Coefficient Mean?

Activity coefficient measures the degree of deviation an actual system has compared to a point of reference. Thus, it measures activity that is derived from the interaction effects between the environment and molecules or ions. This concept is often applied in various industrial settings to measure equilibrium situations, especially in localized corrosion cells.

Noting the difference of the existing system to the point of reference can be extremely helpful in identifying the possibility of damage or corrosion, especially in industrial settings.

## Corrosionpedia Explains Activity Coefficient

Activity can be quite difficult to define as it is typically determined in reference to model conditions. Activity coefficient is a major factor in the area of thermodynamics, as it can be used to account for any deviations from the reference, such as in the case of substances or chemical mixture. Any variation from the ideal system is considered harmful.

Activity coefficient is computed through mole fractions both for solid and liquid mixtures. At times, it can also be measured using molalities. The activity can be calculated through the identification of partial vapor pressure (PVP) within the state or equilibrium with various solutions with unique strength.

Measuring activity coefficients of various species is beneficial in industrial pipelines, nuclear, marine as well as electrochemical plants. By identifying the deviation levels of a certain system to an ideal solution, helpful measurements can be applied to obtain equilibrium and arrest corrosion.

Moreover, activity coefficient has a vital role when it comes to stress corrosion cracking (SCC), particularly in stainless steels, pipelines and other types of metals. Among the factors that influence SCC is the variation of pH in a given concentration. Activity coefficient is broadly applied in calculating the saturation of pH as well as in determining the concentration required to attain equilibrium.

In measuring activity coefficient, comparative chemical potentials are subjected to measurement and any model state is described as the ideal is considered the standard. Therefore, any deviation from the standard is harmful for the system and must be corrected.

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