SED & PREDOM / Ionic strength

Activity

• Aqueous species
• Water (H₂O)
• Solids
• Gases

Activities of Aqueous Species

Activity corresponds to the concentration for a solution behaving ideally. It has the same units as concentration.

The activity coefficient (γ_i) gives the relationship between concentration and activity:

γ_i = a_i / m_i

where a_i and and m_i are the activity and molality, respectively, of the aqueous species “i”. In aqueous solutions the activity coefficients depend on the ionic strength. Several models may be used to calculate activty coefficients.

When writing equations, concentrations are usually written with brackets, for example “[H⁺]”, while activities are often written with braces, “{H⁺}”.

Activity of Water (H₂O)

In dilute aqueous solutions the concentration of water (and its activity) is approximately constant and may be set = 1 (log activity = 0).

In concentrated salt solutions the activity of water is given by

log aH2O =

− Φ ∑mk / (ln(10) 55.508)

where “k” are all solute species in the aqeuous solution, and “Φ” is the osmotic coefficient which depends on the ionic strength. Several models may be used to calculate the osmotic coefficient.

The Activity of Solid Phases

The activity of a solid present at equilibrium may be assumed constant, independent of the composition of the aqueous phase. This requires that the solid phase has a constant chemical composition, etc. The activity of any solid phase present at equilibrium may then be set = 1 (log activity = 0). This may be used to make solubility diagrams for a specific solid phase.

The saturation index becomes = 0 for a solid at equilibrium, and therefore it may be seen as equivalent to the activity of a solid.

The Fugacity of Gases

In a gas phase the term fugacity is used instead of activity.

Fugacity corresponds to the partial pressure of a gas in a gas phase behaving ideally. It has the same units as pressure (bar). In this software it is assumed that all gases behave ideally.

See also reactions with gases.