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According to the text I follow, in peptization the ions of the added electrolyte (peptizing agent) get adsorbed by the particles of the precipitate and turn the precipitate into a sol. An example of this is silver chloride, which can be converted into a sol by adding hydrochloric acid (chloride being the common ion).

What is the mechanism behind this? And why does having a common ion matter?

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What is the mechanism behind this?

From the NCERT textbook:

During peptization, the precipitate adsorbs one of the ions of the electrolyte on its surface. This causes the development of positive or negative charge on precipitates, which ultimately break up into smaller particles of the size of a colloid. Note that the presence of equal and similar charges on colloidal particles is largely responsible in providing stability to the colloidal solution, because the repulsive forces between similarly charged particles prevent them from coalescing or aggregating when they come closer to one another. Thus, they remain in dispersion medium as stable colloidal sols.

And why does having a common ion matter?

The sol particles acquire positive or negative charge by preferential adsorption of positive or negative ions. When two or more ions are present in the dispersion medium, preferential adsorption of the ion common to the colloidal particle usually takes place.

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    $\begingroup$ RE: "This causes the development of positive or negative charge on precipitates, which ultimately break up into smaller particles of the size of a colloid." // Think this sentence needs to be tweaked. It implies that a large single crystal would breakup which really isn't true. Rather the sol size particles of $\ce{AgCl}$ coalesced, but excess $\ce{Cl-}$ disperses the particles. // Ostwald ripening actually favors formation of large particles. $\endgroup$
    – MaxW
    Mar 26, 2020 at 22:59
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    $\begingroup$ Please cite references when quoting. $\endgroup$
    – Buck Thorn
    Aug 26, 2021 at 14:27
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Extra chloride ions adsorb on surface of solid $\ce{AgCl}$ particles.

The charged particles get then more easily hydrated, what stabilizes the colloid.

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