A salt is any species that is held together by electrostatic forces, e.g., CaSO4 consists of Ca2+ and SO42-. The concentration and charge of salts in a solution determines its ionic strength (m).
where ci is the concentration of the ith species and zi is its charge. In calculating ionic strength, it is important to remember that all charged species contribute to the ionic strength, including the buffer. For example, the ionic strength of 50 mM sodium acetate, pH 4.76 (where the acetate is 50% ionized) containing 0.5 M KCl is
Changes in the salt concentration in the mobile phase for ion exchange chromatography are frequently described simply in terms of the molarity of the KCl or NaCl added, but it is more accurate to describe the mobile phase conditions in terms of ionic strength. The salts most commonly used to modify the ionic strength of the mobile phase for ion exchange chromatography of proteins are sodium chloride (NaCl) or potassium chloride (KCl). By gradually increasing the ionic strength of the mobile phase (see gradient) proteins with increasing charge attraction to the stationary phase can be displaced.