# Ion/Counter ion layers in the colloid precipitate of silver chloride

You use a precipitation method to determine [Ag+] in an unknown sample using Cl- ions. Reaction is conducted in presence of HNO3 in excess (relative to Ag+) Which statement is correct concerning the colloid precipitate at the start of the rxn (rxn not yet complete).

A) The ion in Primary Absorption layer is Cl-
B) The Colloid Particle is (-) charge
C) Ag+ is the major ion in Counter-ion Layer
D) Most Cl- ions added are in the precipitate form.

So I think the correct answer is D through process of Elimination and not chemistry knowledge. I know B is incorrect, B/C the precipitate must be AgCl, or possibly AgNO3 since there's Nitric acid here. Both of these are neutrally charged.

I'm pretty sure that Primary absorption layer is the "opposite" of the counter ion layer. So whichever ion goes into the Primary, the opposite ion goes in the counter. Therefore I eliminated A and C, since one cannot be correct without the other being correct as well. (although my first instinct was to put Ag+ in the primary ion layer).

I still have doubts about D. Since the reaction is not complete, and we are adding Cl- to a solution of Ag+, its safe to assume that any Cl- added will react. But HNO3 is a strong acid which dissociates completely, that means there's an excess of NO3- ions floating around. I'm not given any precipitation constants. Can I safely assume Ag+ will bind with Cl- before making AgNO3?

Also could you please confirm or correct my assumptions about Primary/counter-ion layer? I can't find any info it besides vague definitions.

## 1 Answer

It isn't absolutely clear from the wording, but it seems you're adding $\ce{Cl^{-}}$ to a $\ce{Ag^{+}}$ solution.

As some of the $\ce{Cl^{-}}$ is added, $\ce{Ag^{+}}$ is still in excess in the solution so virtually all of the $\ce{Cl^{-}}$ precipitates. $\ce{Ag^{+}}$ will be the primary adsorbent on the precipitate, and the colloidal particles will have a positive charge.

Since the colloidal particles have a positive charge, the ion in the counter-ion layer around the particle needs to be negatively charged. There isn't any $\ce{Cl^{-}}$ in solution so it has to be $\ce{NO3^{-}}$ as the anion.

Thus the correct answer is (D).

• Thanks for clearing this question up. So what happens if this or similar reaction is run in, for example, water. The Ag+ will be in the primary ion layer. and there is no Cl- in solution because it all precipitates out. Will the counter ion layer simply not exist? or maybe it fills with OH-? – Jess L Mar 14 '16 at 21:27
• (1) I would assume that the original reaction was in water since you didn't specify otherwise. (2) You have to be careful when you say "all" the $\ce{Cl^-}$ precipitates. $\ce{AgCl}$ does have a Ksp. (3) With $\ce{Ag^+}$ in solution there must be some anion. You can't have a solution with just cations. So that anion would be in the counter layer. – MaxW Mar 14 '16 at 21:43