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Electrokinetic potential (or zeta potential) is theA. potential difference between the fixed layer and the diffused layer of opposite chargesB. potential energy of colloidal particlesC. potential required to bring about coagulation of a colloidal sol.D. potential required to give the particles a speed of `1 cm//sec` in the sol

Answer» Correct Answer - A
Consider the formation of silver iodide sol from the double decomposition reaction:
`AgNO_(3)(aq.)+KI(aq.)rarrAgI(s)darr+KNO_(3)`
Suppose that the electrolyte used in excess is potassium iodide. This would result in preferential adsorpition of `I^(-)` ions giving a negatively charged sol of silver iodide. The ions preferentially adsorbed on the surface of colloidal particles are called potential-determining ions.
Having acquired a position or negative charge by selective adsorption on the surface of colloidal particles, this layer attracts counter ions from the dispersion medium forming a second layer:
`AgI//I^(-)K^(+) AgI//Ag^(+)I^(-)`
The combination of the two layers of opposite charges around the colloidal particle is called Helmholtz electrical double layer. According to modern views, the first layer of ions is firmly held and is termed fixed layer while the second layer is mobile which is termed diffused layer. Since separation of charge is a seat of potential, the charges of opposite signs on the fixed and diffused parts result in a difference in potential between these layers. This potential difference between the fixed layer and diffused layer of opposite charges is called the electrokinetic potential or zeta potential. The presence of equal and similar charges on colloidal particles is largely responsible in providing stability to the colloidal solution be cause the repulsive forces between chaged particles having same charge prevent them from coalecing or aggregating when they come closer to one another.


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