Jonstyrkan

Jonstyrkan, or ionic strength, is a measure of the total concentration of charge carried by ions in an electrolyte solution. It is defined by the formula I = 1/2 sum_i c_i z_i^2, where c_i is the molar concentration (mol/L) of ion i and z_i is its charge number. The resulting value has units of mol/L. The factor 1/2 accounts for pairwise interactions in an electrolyte. Ionic strength captures how strongly ions interact with each other, influencing activity coefficients and thus solubility, equilibria, and reaction rates. Higher ionic strength generally lowers activity coefficients and changes electrostatic screening in the solution, described by models such as Debye–Hückel and Davies.

Calculation: to compute I, list all ions present, multiply each concentration by the square of its charge, sum these values, and multiply by 1/2. For example, a 0.1 M solution of NaCl yields I = 1/2[(0.1)(+1)^2 + (0.1)(−1)^2] = 0.1 M.

Applications: Ionic strength is widely used in chemistry, biochemistry, and environmental sciences to predict activity coefficients, adjust equilibrium constants, interpret titrations, and model protein stability and solubility in saline media.

History: The concept was introduced by Debye and Hückel in the 1920s to account for non-ideal behavior of electrolytes.

In practice, jonstyrkan is a fundamental parameter linking ion concentrations to their effective interactions in aqueous solutions.