CHEMISTRY 31

Summer, 2016 – Dixon

Additional Problem 2.1 - SOLUTIONS

It is desired to determine the cyanide (CN^-) concentration of a solution that also contains CO₃^2-. The method being used to measure CN^- also responds to CO₃^2-, so it is critical to at least partially separate the ions so that CO₃^2- is removed from CN^-. A test solution, which is expected to be like actual samples, contains CN^- and CO₃^2- at concentrations 2.0 x 10^-5 M and 8.5 x 10^-4 M, respectively. The goal in the separation is to be able to retain at least 99% of the CN^- in the original solution while decreasing the concentration of CO₃^2- to no more than 10% of the CN^- concentration. Using K_sp values listed in Appendix F of the text for Ag containing solids, determine if this could be done. Which anion would be precipitated first upon addition of Ag⁺? How much of that anion would be left at the Ag⁺ concentration where the other anion would start precipitating? Does this allow compliance with the requirements listed above? Assume that you could re-dissolve a precipitated anion by adding acid. Ignore activity for this problem.

1. From Appendix F, K_sp values: K_sp(AgCN) = 2.2 x 10^-16 K_sp(Ag₂CO₃) = 8.1 x 10^-12

We can determine which ion precipitates out first by calculating [Ag⁺] in equilibrium with the initial CN^- and CO₃^2- concentration:

For CN^-: [Ag⁺] = K_sp/[CN^-] = 2.2 x 10^-16/2.0 x 10^-5 = 1.1 x 10^-11 M

For CO₃^2-: [Ag⁺] = (K_sp/[CO₃^2-])^0.5 = [8.1 x 10^-12/(8.5 x 10^-4)]^0.5 = 9.76 x 10^-5 M

Since the equilibrium concentration of Ag⁺ is lower with CN^-, CN^- precipitates out first. (1 pt)

2. We can calculate the concentration of CN^- left when CO₃^2- just starts precipitating by determining [Ag⁺] required to begin CO₃^2- precipitation (actually, already done in step 1), and then finding [Ag⁺] in equilibrium with that concentration.

[Ag⁺] to start Ag₂CO₃ precipitation = 9.76 x 10^-5 M

[CN^-] in equilibrium with that [Ag⁺]: [CN^-] = K_sp/[Ag⁺] = 2.2 x 10^-16/9.76 x 10^-5 = 2.3 x 10^-12 M

(2 pts)

3. Yes. Since AgCN(s) is precipitated first, it should be free of CO₃^2- interference as long as [Ag⁺] never reaches a concentration where Ag₂CO₃ would also precipitate. This allows decreasing [CN^-] from 2.0 x 10^-5 M to 2.3 x 10^-11 M. This solid (once filtered from the solution) will be 100% pure and results in a recovery of 100 - 2.3 x 10^-12M*100/2.0 x 10^-5 M = 100 – 0.00001% = 99.99999% of the CN^-. (1 pt)