COMMON ION EFFECT

The statement of the common ion effect can be written as follows – in a solution wherein there are several species associating with each other via a chemical equilibrium process, an increase in the concentration of one of the ions dissociated in the solution by the addition of another species containing the same ion will lead to an increase in the degree of association of ions.

An example of the common ion effect can be observed when gaseous hydrogen chloride is passed through a sodium chloride solution, leading to the precipitation of the NaCl due to the excess of chloride ions in the solution (brought on by the dissociation of HCl).
This effect cannot be observed in the compounds of transition metals. This is because the d-block elements tend to form complex ions. This can be observed in the compound cuprous chloride, which is insoluble in water. This compound can be dissolved in water by the addition of chloride ions leading to the formation of the CuCl2– complex ion, which is soluble in water.
Effect on Solubility:
How the solubility of a salt in a solution is affected by the addition of a common ion is discussed in this subsection.

The common ion effect can be used to obtain drinking water from aquifers (underground layers of water mixed with permeable rocks or other unconsolidated materials) containing chalk or limestone. Sodium carbonate (chemical formula Na2CO3) is added to the water to decrease the hardness of the water.
In the treatment of water, the common ion effect is used to precipitate out the calcium carbonate (which is sparingly soluble) from the water via the addition of sodium carbonate, which is highly soluble.
A finely divided calcium carbonate precipitate of a very pure composition is obtained from this addition of sodium carbonate. The CaCO3 precipitate is, therefore, a valuable by-product that can be used in the process of manufacturing toothpaste.
Since soaps are the sodium salts of carboxylic acids containing a long aliphatic chain (fatty acids), the common ion effect can be observed in the salting-out process which is used in the manufacturing of soaps. The soaps are precipitated out by adding sodium chloride to the soap solution to reduce its solubility.
However, it can be noted that water containing a respectable amount of Na+ ions, such as seawater and brackish water, can hinder the action of soaps by reducing their solubility and therefore their effectiveness.

1. Course Title: COMMON ION EFFECT Course Code: CHM-404 Semmester: 4 th E

2. Common ion effect and industrial applications

3. The reduction of the degree of dissociation of a salt by the addition of a common-ion is called the common-ion effect. Example In a saturated solution of silver chloride, we have the equilibrium AgCl (s) Ag + ( aq) + Cl – ( aq) When sodium chloride is added to the solution, the concentration of Cl – ions will increase. The equilibrium shown above will be shifted to the left to form more of solid Ag Cl. Thus the solubility of AgCl, a typical sparingly soluble salt, will decrease. Common ion effect

4. Factors effecting degree of dissociation Nature of solute Nature of solvent Concentration Temperature common ion effect

5. Example Dissociation of hydrogen sulphide in presence of hydrochloric acid H 2 S 2H +  + S 2- By applying the law of mass action, we have K a = [H + ] 2 [S 2- ] / [H 2 S] To the above solution of H 2 S , if we add hydrochloric acid, then it ionizes completely as HCl H +  + Cl - common ion effect

6. Solubility equilibria and solubility product When an ionic solid substance dissolves in water, it dissociates to give separate cations and anions. For example, for a sparingly soluble salt, say Ag Cl, we can write the equilibrium equations as follows : AgCl (S) Ag + + Cl - common ion effect

7. Solubility equilibria and solubility product According to law of mass action K= [Ag + ] [Cl – ] / [AgCl] The amount of Ag Cl in contact with saturated solution does not change with time and the factor [Ag Cl] remains the same. common ion effect

8. Solubility equilibria and solubility product As [AgCl] is constant so equilibrium expression is K sp = [Ag + ] [Cl – ] where [Ag+] and [Cl – ] are expressed in mol/L The equilibrium constant in the new context is called the solubility product constant (or simply the solubility product ) and is denoted by K sp . common ion effect

9. Solubility of barium iodate in presence of barium nitrate Barium iodate, Ba(IO 3 ) 2 , has a solubility product K sp  = [Ba 2+ ][IO 3 - ] 2  = 1.57 x 10 -9 Its solubility in pure water is 7.32 x 10 -4  M. common ion effect

10. C ommon ion effect with weak acids and bases common ion effect

11. C ommon ion effect with weak acids and bases common ion effect

12. Activity Calculate the solubility of silver chromate, Ag 2 CrO 4 , in a 0.100 M solution of AgNO 3 . ( K sp for Ag 2 CrO 4 = 9.0 × 10 – 12 ) Activity Calculate the K sp for Bismuth sulphide (Bi 2 S 3 ), which has a solubility of 1.0 × 10 – 15 mol/L at 25°C. Activity The solubility of BaSO 4 is 2.33 × 10 – 4 g/ml at 20°C. Calculate the solubility product of BaSO 4 assuming that the salt is completely ionised. common ion effect

13. Common ion effect on solubility Adding a common ion decreases solubility, as the reaction shifts toward the left to relieve the stress of the excess product. Adding a common ion to a dissociation reaction causes the equilibrium to shift left, toward the reactants, causing precipitation. Applications of common ion effect

14. Common ion effect on solubility Example AgCl(s ) Ag + + Cl – If S be the solubility of AgCl, we have K sp = [S mol/l Ag + ] [S mol/l Cl – ] Suppose 0.25 mol/L excess of HCl is added to the solution. Then ion product (Q) will be Q = [S mol/l Ag + ] [(S + 0.25) mol/l Cl – ] If Q > K sp Precipitation If Q = K sp Saturated solution If Q < K sp No precipitation Applications of common ion effect

15. Salting out of soap RCOONa (aq.) RCOO - (aq.) + Na + (aq.) NaCl (aq.) Na + (aq.) + Cl - (aq.) Applications of common ion effect

16. Purification of salt Applications of common ion effect NaCl (aq.) Na + (aq.) + Cl - (aq.) H Cl (aq.) H + (aq.) + Cl - (aq.)

17. Lime Softening CO 2 + Ca(OH) 2 CaCO 3 + H 2 O Ca(HCO 3 ) 2 + Ca(OH) 2 2CaCO 3 + 2H 2 O Mg(HCO 3 ) 2 + 2Ca(OH) 2 2CaCO 3 + 2H 2 O Applications of common ion effect

18. Harris, B. C. and W.H. Harris. 2010. Quantitative Chemical Analysis. 8 th Edition. Freeman and Company New York. Christian, G.D. 2006. Analytical Chemistry. Sixth edition, John Wiley and Sons, New York. Skoog, D. A. and D.M. West 2005. Fundamentals of Analytical Chemistry. Hot Reinehart Inc., London. Kealey, D and P.J.Haines, 2002. Analytical Chemistry, Bios Scientific Publishers Limited, Oxford, UK. Sharma, B. K. 2004. Instrumental methods of chemical analysis, In; Introduction to Analytical chemistry: Goel Publishing House Meerut, 23 th Edition. Reilley, C. 1993. Laboratory Manual of Analytical Chemistry. Allyn & Bacon, London. Reference books