This online Ionic Strength Calculator finds the ionic strength of a solution using the concentration of ions and their charge values (charge numbers). Ion concentration can be specified as molar concentration (mol/L) or as molality (mol/kg). It is only important that all the concentrations be expressed in the same unit. The ionic strength will also be expressed in the same unit. To add or delete an ion species you may click the “+” symbol or the “–” symbol respectively.
Understanding the Thermodynamic Activity
It is known that one of the most important laws in chemical thermodynamics, the law of mass action in its classical form, applies only to non-electrolytes or weak electrolytes in dilute aqueous solutions. All strong electrolytes (alkalis, strong acids, salts) and weak electrolytes in concentrated aqueous solutions do not obey this law.
The discrepancy between the properties of strong electrolytes and the classical form of the law of mass action can be explained using the theory of strong electrolytes proposed by Debye and Hückel. The main idea of this theory is that in solutions between ions of strong electrolytes there are forces of mutual attraction and repulsion, which cannot be neglected. These inter-ionic forces cause the behavior of strong electrolytes to deviate from the laws of ideal solutions. The presence of these interactions leads to the mutual deceleration of cations and anions.
For this reason, for more accurate calculations based on the law of mass action, instead of equilibrium concentrations, so-called ion activities are used.
This value was introduced to take into account the mutual attraction of ions, the interaction of a solute with a solvent, and other phenomena that change the mobility of ions and are not taken into account by the theory of electrolytic dissociation.
The activity for infinitely dilute solutions is equal to the concentration. For real solutions, due to the strong manifestation of interionic forces, the activity is less than the concentration.
Activity can be considered as a quantity characterizing the degree of binding of electrolyte particles. Thus, activity is an effective (acting) concentration, which manifests itself in chemical processes as a really acting mass, in contrast to the total concentration of a substance in a solution.
Numerically, the activity is equal to the concentration multiplied by the coefficient, called the activity coefficient:
$$a= \gamma \ c,$$
where
• \(a\) is the activity,
• \(\gamma\) is the activity coefficient,
• \(c\) is the concentration.
What is Ionic Strength?
According to Debye–Hückel limiting law the activity coefficient of an ion in a dilute solution can be found by the formula:
$$ln(\gamma_{i}) = \ – A z_{i}^2 \sqrt{I} \ ,$$
where
• \(\gamma_{i}\) is the activity of ion species \(i\),
• \(A\) is a constant that depends on temperature,
• \(z_{i}\) is the charge value (charge number) of ion species \(i\),
• \(I\) is the ionic strength of the solution.
Ionic strength is a measure of the concentration of ions in a solution and represents the overall electrostatic effect of these ions. The ionic strength of a solution is calculated using the following formula:
$$I = \frac{1}{2} \sum_{i} c_{i} z_{i}^2,$$
where
• \(c_{i}\) is the molar concentration of ion species \(i\),
• \(z_{i}\) is the charge value (charge number) of ion species \(i\).
Note that the sum in the above ionic strength formula is taken over all ions in the solution. In addition, in the case of dilute aqueous electrolyte solutions, the difference between the concentration expressed in mol/L and mol/kg is not significant.
Therefore, using our Ionic Strength Calculator, you can use concentrations expressed in any of these units. The ionic strength will be expressed in the same unit.
Example of Ionic Strength Calculation
Let us calculate the ionic strength of the solution containing in 1 L of aqueous solution 0.001 mole of ferric ammonium sulfate (ferric alum): (NH4)2SO4 · Fe2(SO4)3.
It is easy to see that the following ions are present in the solution: NH4+, SO42-, Fe3+. These ions have the following concentrations and charges:
c(NH4+) = 2 ions x 0.001 mol/L = 0.002 mol/L, z = +1;
c(SO42-) = 4 ions x 0.001 mol/L = 0.004 mol/L, z = -2;
c(Fe3+) = 2 ions x 0.001 mol/L = 0.002 mol/L, z = +3.
Using the above formula, it is not difficult to obtain the result. However, it is much easier to use our Ionic Strength Calculator. Plugging the initial data into it, we immediately get: I = 0.018 mol/L.
Application of Ionic Strength
Ionic strength finds many applications in various areas of chemistry.
• Ionic strength affects the equilibrium position of chemical reactions involving ions. It influences the solubility of salts and the precipitation of ions.
• In electrochemical processes, such as redox reactions and electrolysis, ionic strength plays a crucial role. It affects the conductivity of electrolytes and determines the rate of electron transfer.
• Ionic strength is essential in the preparation and maintenance of buffer solutions. The ionic strength of a buffer affects its capacity to resist changes in pH, ensuring accurate and stable experimental conditions.
• Colloidal systems, such as suspensions and emulsions, are stabilized by electrostatic interactions between charged particles. Ionic strength influences the stability of these systems by altering the electrostatic forces.
• Proteins and enzymes are highly sensitive to ionic strength. Changes in ionic strength can alter their structure, stability, and activity.
Related calculators
Check out our other chemistry calculators such as Buffer pH Calculator or pH Calculator.