Name: ionic strength in salt and acid solutions
Uploaded: 2024-04-04T10:51:15.000Z
Duration: 1 min 12 s
Channel: JoVE

When a salt dissolves in water, the cation and anion dissociate. Each ion attracts ions of the opposite charge due to electrostatic attraction.
This ionic atmosphere around the ions weakly shields the charge of the central ion.
The effectiveness of the ionic atmosphere shielding is related to the ionic strength, which is determined by the concentration and the charge of the ions.
Formally symbolized as I, the ionic strength of a solution incorporates the total concentration of all the ions in the solution and is proportional to the molar concentration of the salt.
As the charge of the ion is squared in the calculation, the greater the charge on an ion, the more significant its contribution to the ionic strength value.
For example, the ionic strength of a potassium nitrate solution is the same as its concentration.
However, the ionic strengths of solutions of potassium sulfate, a 2:1 electrolyte, and calcium sulfate, a 2:2 electrolyte, are three and four times their concentrations, respectively.

ionic strength: overview

The ionic strength of a solution is a quantitative way of expressing the total electrolyte concentration of a solution. This concept was first introduced in 1921 by two American physical chemists, Gilbert N. Lewis and Merle Randall, while describing the activity coefficient of strong electrolytes. During the calculation of ionic strength (I or &#956;), all the cations and anions are considered. However, the concentration (c) of an ion with a greater charge number (z) has a greater contribution to the total ionic strength because the charge of the ion is squared.
While calculating ionic strength for a salt that will produce multiple equivalents of the same ion on dissociation, we need to account for the contribution from each ion. For example, the ionic strength of 0.1 mol/L Na2SO4 can be calculated as follows:
The concentration of Na+ is 0.2 mol/L because one molecule of Na2SO4 will dissociate to give two Na+ ions in solution. The ionic strength of dilute solutions can be calculated easily. However, in a more concentrated solution, the calculation becomes more complex and less accurate, as the salts do not dissociate completely. For example, in an aqueous solution of 0.025 mol/L MgSO4, 25% to 35% of MgSO4 exists as the ion pair MgSO4(aq).
The concept of ionic strength can be further extended to strong and weak acids. Because strong acids dissociate completely in solution, their ionic strengths can be calculated in the same way as those of dissociated salts. For weak acids, the concentration of ionized species can be calculated from the ionization constant value and then used for ionic strength determination. If the acid is very weak and mostly remains non-ionized, its contribution to the total ionic strength of the solution is negligible.

Ionic Strength: Overview

When a salt dissolves in water, the cation and anion dissociate. Each ion attracts ions of the opposite charge due to electrostatic attraction.
This ionic ...

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ionic strength in salt and acid solutions

Ionic strength describes the total electrolyte concentration in a solution. It is a quantitative measure of how many ions are present and how strongly they contribute to the solution. The concept was introduced in 1921 by Gilbert N. Lewis and Merle Randall while they were describing the activity coefficient of strong electrolytes.

To calculate ionic strength, written as I or μ, both cations and anions are included. The concentration of each ion matters, but the charge number z matters even more because it is squared in the calculation. That means ions with a higher charge contribute more to the total ionic strength than ions with a lower charge at the same concentration.

When a salt produces more than one of the same ion after dissolving, each ion must be counted. For example, 0.1 mol/L Na2SO4 gives a sodium ion concentration of 0.2 mol/L because each formula unit forms two Na+ ions in solution. This makes the sodium contribution part of the final ionic strength value.

Ionic strength is easier to calculate for dilute solutions. In more concentrated solutions, the calculation becomes less exact because salts do not dissociate completely. For instance, in 0.025 mol/L MgSO4(aq), about 25% to 35% of the salt can remain as the ion pair MgSO4(aq).

The idea also applies to strong and weak acids. Strong acids dissociate completely in solution, so their ionic strength can be calculated in the same way as fully dissociated salts. Weak acids require the concentration of ionized species, which can be found from the ionization constant and then used for ionic strength. If an acid is very weak and stays mostly non-ionized, its contribution to the total ionic strength is negligible.

Ionic Strength in Salt and Acid Solutions

ionic-strength-in-salt-and-acid-solutions

Ionic strength describes the total electrolyte concentration in a solution. It is a quantitative measure of how many ions are present and how strongly they contribute to the solution. The concept was introduced in 1921 by Gilbert N. Lewis and Merle Randall while they were describing the activity coefficient of strong electrolytes. To calculate ionic strength, written as I or μ, both cations and anions are included. The concentration of each ion matters, but the charge number z matters even...

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Salt Effect on Salt Solubility

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Equilibrium Calculations with Mass and Charge

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How Ionic Strength Changes pH

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Four Reaction Types in Titrimetric Analysis

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key terms and concepts

Learn how ionic strength depends on ion charge, ion concentration, and dissociation in salt and acid solutions, with examples from Na2SO4, MgSO4, and acids.

Ionic Strength in Salt and Acid Solutions | JoVE K12

analytical chemistry

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Ionic Strength in Salt and Acid Solutions

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