Advanced Topic 597 of 1102 Acids, Bases & Electrochemistry
Ionic Strength
A measure of total ion concentration weighted by charge squared.
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In-depth explanation of Ionic Strength
Ionic strength I is a measure of the total electrostatic environment in a solution, defined as one half of the sum over all ions of their concentration multiplied by the square of their charge. Ions with higher charge contribute much more strongly, so a solution of divalent ions has a higher ionic strength than a monovalent solution of the same molarity. Ionic strength governs the activity coefficients used in Debye-Huckel theory and affects solubility and equilibrium constants. It is essential for accurate calculations in concentrated solutions. Precise pH and buffer calculations can also require correction for it.
Going deeper: Ionic Strength
Key ideas: I = 0.5 x sum of (c_i z_i^2) over all ions. For 0.10 M NaCl, I = 0.10; for 0.10 M Na2SO4, I = 0.5(0.2 x 1 + 0.1 x 4) = 0.30; for 0.10 M MgSO4, I = 0.40. Higher charges dominate the sum. Ionic strength sets activity coefficients through Debye-Huckel.
Common mistakes and connections: Mistakes: forgetting to include all ions (including spectator ions and buffer salts), forgetting z squared, and omitting the one-half. The ionic strength of seawater is about 0.7 M. Higher ionic strength raises the solubility of sparingly soluble salts without a common ion (diverse ion effect), and shifts pKa values.
Visualization of Ionic Strength
Ionic strength I = half the sum of c z^2: for the same 0.10 M, charges count squared, so CaCl2 gives 0.30 and MgSO4 gives 0.40, versus 0.10 for NaCl. The interactive lesson shows this as a live animation or chart.
Worked example and use case
For 0.10 M CaCl2, I = 1/2 (0.10 × 2^2 + 0.20 × 1^2) = 0.30.
Frequently asked questions about Ionic Strength
What is Ionic Strength?
A measure of total ion concentration weighted by charge squared.
Is Ionic Strength a beginner, intermediate or advanced topic?
Ionic Strength is a advanced topic in Acids, Bases & Electrochemistry. It builds on beginner and intermediate topics, so review those first if needed.
What should I learn before Ionic Strength?
Study these earlier topics in Acids, Bases & Electrochemistry first: Ionic Mobility, Transport Number, Debye-Huckel Theory.
What comes after Ionic Strength?
The next topics in Acids, Bases & Electrochemistry are Activity and Activity Coefficients and Overpotential.
Can I practise Ionic Strength with quizzes?
Yes. The interactive lesson includes a self-check question on Ionic Strength, and the Acids, Bases & Electrochemistry quiz draws on every topic in this area.
Where Ionic Strength fits in Acids, Bases & Electrochemistry
Ionic Strength is topic 87 of 105 in Acids, Bases & Electrochemistry.