SED & PREDOM

Ionic Strength

Electrically charged species (ions) affect the properties of aqueous solutions. Because of this electrolyte solutions do not behave ideally, and an activity coefficient describes the relation between concentration, molality, and what the ideal value should be, the activity. In very dilute solutions the Debye-Hückel model shows that activity coefficients depend on the ionic strength.

The ionic strength corresponds to the concentration of electric charge in a solution:

I = ½ ∑ m_i Z_i²

where m_i is the molality of an aqueous species “i” with charge Z_i. For strong 1:1 electrolytes (like NaCl) the ionic strength is equivalent to the total concentration.

See also:

• activity
• activity coefficients
• models to calculate activity coefficients
• enter a value for the ionic strength when making a diagram
• effect of ionic strength on diagrams
• concentration units in diagrams