I use a silver based disinfectant in my hot tub. It is in the form of silver nitrate beads. There is chlorine in the tub that I presume becomes chloride ion. Why doesn't the silver form a precipitate of silver chloride in the water?

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    $\begingroup$ Ag+ is bioactive, it kills bacteria. There is chlorine in the tub in what form? In the paint? Please be more specific. $\endgroup$ – Alex Jul 31 '13 at 5:00
  • $\begingroup$ Oh, I assume you are tossing the AgNO3 breads in the hot tub full of water with "chlorine", is that it? Cl is mostly in the form of HOCl+ in the tub, which is an disinfectant. If that's the case, I guess you should use either "chlorine" or silver as disinfectant. You're probably just not seeing the precipitate, it's white and hard to see in small amounts (compared to all the water on a hot tub I doubt you'd see anything, and there's probably not much Ag+ on that). $\endgroup$ – Alex Jul 31 '13 at 5:10

If you're referring to chlorination of public water supplies, keep in mind that this is likely not the main source of chloride ion. Elemental chlorine disproportionates via the following reaction:

$\ce{Cl2 + H2O <=> HCl + HOCl}$

Most chloride ion probably derives from various natural and industrial sources, as well as miscellaneous human activity. One source (somewhat outdated) I found indicates an average chloride ion concentration in U.S. public water of $11.5$ mg/L, which equates to a molarity of $3.24 \cdot 10^{-4}$. Using the $K_{sp}$ of $AgCl$ you can calculate the concentration of silver ion necessary to produce a precipitate, but given that it has a value of $1.77 \cdot 10^{-10}$ under standard conditions, you can assume that a fairly minimal amount of silver would be required, even with the large volume of a tub and increasing solubility in hot water. It's likely the case that any precipitate is just finely dispersed in solution and not clearly visible.

  • $\begingroup$ The chloride ion I am referring to is from the addition of dichloro-s-triazenetrione, which generates HOCL and HCL when added to the water, and then, I assumed, further dissociates into Cl-. $\endgroup$ – Richardbernstein Jul 31 '13 at 15:58
  • $\begingroup$ Since most of the Ag is in the form of insoluble AgCl, would it still have anti-microbial properties? $\endgroup$ – Richardbernstein Jul 31 '13 at 16:00
  • $\begingroup$ @Richardbernstein, yes, HCl would dissociate to produce chloride ion, but the quantity of chloride introduced in that manner constitutes a small percentage of the total. As for whether the bactericidal action is inactivated, I frankly don't know; it's not my area of expertise. $\endgroup$ – Greg E. Jul 31 '13 at 16:11

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