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This effect is caused by the addition to a solution of a soluble compound that contains an ion that shares an affinity with the precipitate, which results in a decrease in the solubility of an ionic precipitate. As a result of Le Chatelier’s principle for the equilibrium reaction of ionic association/dissociation, this behaviour has been observed in some cases. The effect is most commonly perceived as a reduction in the solubility of salts and other weak electrolytes. Increasing the concentration of one of the salt’s ions by a small amount generally results in increased precipitation of the salt, which reduces the concentration of both salt’s ions until the solubility equilibrium is achieved. Essentially, the effect is created by the fact that both the original salt and the other chemical that has been added have one ion in common with one another.
The Ion Effect is a common phenomenon
The common ion effect is an effect that occurs when an electrolyte is mixed with another electrolyte that contains an ion that is also present in the first electrolyte, i.e. a common ion, and the ionisation of the first electrolyte is suppressed. As a result of Le Chatelier’s principle, it is considered to be a natural consequence (or the Equilibrium Law).
What is the Common Ion Effect, and how does it work
The common ion effect can be stated as follows: In a solution containing several species that are associating with one another through a chemical equilibrium process, an increase in the concentration of one of the ions dissociated in the solution caused by the addition of another species containing the same ion will result in an increase in the degree of association of ions between the species.
In the presence of gaseous hydrogen chloride, a sodium chloride solution will precipitate due to an excess of chloride ions present in the solution. This is an example of the common ion effect in action (brought on by the dissociation of HCl).
The Influence on Solubility
This subsection discusses the effect that the presence of a common ion has on the solubility of a salt in a solution.
In aquifers (underground layers of water mixed with permeable rocks or other unconsolidated materials), such as those containing chalk or limestone, the common ion effect can be used to obtain drinking water. The addition of sodium carbonate (chemical formula Na2CO3) to water has the effect of lowering the hardness of the water’s composition.
By adding sodium carbonate to water, which is highly soluble, the common ion effect can be used to precipitate out the calcium carbonate (which is only sparingly soluble) from the water, which is then filtered out.
Sodium carbonate is used to precipitate calcium carbonate, which results in a finely divided calcium carbonate precipitate with a very pure composition. In this way, the CaCO3 precipitate is a valuable by-product of the toothpaste manufacturing process, and it can be used to make other products.
The common ion effect can be observed in the salting-out process, which is used in the manufacture of soaps, because soaps are the sodium salts of carboxylic acids that contain a long aliphatic chain (fatty acids). It is necessary to reduce the solubility of the soaps before they can be precipitated out. Sodium chloride is added to the soap solution to accomplish this.
The presence of significant amounts of sodium ions in water, such as seawater and brackish water, can, however, impair the action of soaps by decreasing their solubility and, consequently, their effectiveness. Seawater and brackish water are examples of such water.
In addition to the Common-Ion Effect
The common ion effect causes the pH of a buffer solution to change when the conjugate ion of a buffer solution (solution containing a base and its conjugate acid, or an acid and its conjugate base) is added to it. When the conjugate ion of a buffer solution is added to it, the pH of the buffer solution changes.
In one instance, when acetic acid and sodium acetate are both dissolved in a given solution, the formation of acetate ions can be observed as a result of the reaction. Sodium acetate, on the other hand, completely dissociates, whereas acetic acid only partially ionises. This is due to the fact that acetic acid is a weak acid and sodium acetate is a strong electrolyte, respectively.
Sodium acetate, according to Le Chatelier’s principle, produces new acetate ions that aid in the suppression of the ionisation of acetic acid, thereby shifting the equilibrium to the left. Due to a decrease in the rate of acetic acid dissociation, a rise in the pH of the solution is observed.
Consequently, the pH of the common ion solution containing acetic acid and sodium acetate will be higher and, as a result, less acidic when compared to the pH of acetic acid solution.
Conclusion
The common ion effect describes the effect of adding a second substance containing an ion common to the equilibrium on the existing equilibrium. Several salts in a system ionise in solution. Common cation or anion salts contribute to the concentration of common ions. All salt contributions must be included in the common ion concentration calculation.
