What is common ion effect?

The common ion effect, also known as the salt effect, is a phenomenon that affects the solubility of a salt in a solution that already contains one of the ions present in the salt.

When a salt dissolves in water, it dissociates into its constituent ions. For example, when sodium chloride (NaCl) dissolves in water, it dissociates into sodium ions (Na+) and chloride ions (Cl-). These ions can form new ionic interactions with other ions present in the solution.

The common ion effect occurs when a solution already contains one of the ions present in the salt being dissolved. In this case, the equilibrium between the dissolved salt and its ions is disturbed, resulting in a decrease in the solubility of the salt.

The common ion effect can be explained by Le Chatelier's principle, which states that a system at equilibrium will shift to counteract any imposed changes. When a solution already contains one of the ions of a salt, the concentration of that ion is increased and therefore shifts the equilibrium towards the formation of more undissolved salt. This leads to a decrease in the solubility of the salt.

For example, let's consider the solubility of silver chloride (AgCl) in water. When AgCl dissolves in water, it dissociates into silver ions (Ag+) and chloride ions (Cl-). If we add sodium chloride (NaCl) to the solution, which also dissociates into sodium ions (Na+) and chloride ions (Cl-), the concentration of chloride ions in the solution increases. According to the common ion effect, this increase in chloride ions will shift the equilibrium towards the formation of more undissolved AgCl. As a result, the solubility of AgCl decreases.

The common ion effect has various applications in chemistry, including in the field of quantitative analysis and separation techniques. It is also important to consider in the understanding of pH buffers and the precipitation of salts from solution.