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I heard that:

Silver chloride is more soluble in very concentrated NaCl solution (brine) than in pure water.

Is it true? I mean, won't the common ion effect operate? Or is this maybe due to some kind of complex formation as the NaCl solution is highly concentrated?

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I mean won't the common ion effect operate?

The common ion effect always applies for pure substances. With the added sodium chloride, you have changed the effective solvent and changed the ions.

Or may be this is due to some kind of complex formation as NaCl solution is highly concentrated?

Exactly, before your ionic equation was: $$\ce{Ag+ (aq) + Cl- (aq) <=> AgCl (s)}$$ but with brine, the ionic equation is: $$\ce{AgCl (s) + Cl- (aq) <=> [AgCl2]- (aq)}$$ Which has a higher solubility constant, and where as before more chloride made the $\ce{AgCl}$ less soluble, if you note that you have switched regimes and $\ce{Cl-}$ has switched from precipitating a silver ion to dissolving a silver chloride molecule, which means now with increased chloride concentration, silver chloride becomes more soluble.

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The answer by A.K. is eminently reasonable, but I wondered if stability constants were available. I quote from the summary of Ref. 1 :

The solubility of silver chloride in concentrated solutions of various chlorides was determined mainly at 25°. The solubility is nearly doubled in going from o° to 25°, the rate of increase above and below 25° being nearly logarithmic. The solubility is sharply proportional to integral powers of the chloride concentration throughout considerable ranges, a fact explained by assuming the existence of the complex anions AgCl3$^{^-2}$, AgCl4$^{-3}$ and possibly AgCl5$^{-4}$. No evidence of the ion AgCl2$^{^-}$is found.

It seems that the simplest explanation is only the beginning of understanding the solubility increase.

References

  1. The Solubility of silver chloride in chloride solutions and the existence of complex argentichloride ions, J. Am. Chem. Soc. 1911, 33, 12, 1937– 1946 Publ. Date: December 1, 1911 https://doi.org/10.1021/ja02225a008
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  • $\begingroup$ Interesting, I have found earlier today AgCl2- formation constant 1.8e5 on libretexts somewhere. OTOH [AgCl5]4- seems to me suspicious. chem.libretexts.org/Ancillary_Materials/Reference/… $\endgroup$
    – Poutnik
    Jul 7, 2021 at 17:59
  • $\begingroup$ @Poutnik: Yes, indeed, interesting on both issues! Is the 1911 account accurate about AgCl2-, and, sure, AgCl5(4-) is unlikely, but AgCl3(2-) or AgCl4(3-) don't seem so impossible. But it could be easier to just call it AgCl2- and let it go because the solubility of the complex(es) is still so small. $\endgroup$
    – James Gaidis
    Jul 8, 2021 at 13:32

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