Qualitative Analysis

Learning Goals

You will learn how to apply equilibrium principles to the differential separation of ions in solution.

Synopsis

The separation of ions into separate solutions, each of which only contains one ion, is called qualitative analysis. [Determining the quantity of each ion present would be quantitative analysis. ] The basic approach is really quite simple. You have one solution of ions {a,b,c,d, ...n} and you wish to separate them into n solutions each only containing the single ions a, b, c, d, ...n. First you would look in a solubility chart to see if there is a salt, such as a carbonate, that forms a soluble salt with only some of them. Then you would use the solubility differences between the carbonate salts, such as temperature differences of solubility, to further separate the ions. You would continue this process until your task was completed. Simple in principle, not necessarily so simple in practice.

Suppose that you have Au⁺ and Ag⁺ ions in solution. You then slowly add portions of a 0.010-M solution of NaCl to the solution containing the gold and silver ions. Which would precipitate first, AgCl or AuCl? We can use the Ksp equation for each to calculate the concentration of Cl^- ions required to first precipitate each. We would then have [Au⁺] = 2.0 x 10^-13/0.010 = 2.0 x 10^-11 M and [Ag⁺] = 1.8 x 10^-8 M. We then see that AuCl will precipitate first. What would be the concentration of Au+ ions remaining in solution when the AgCl begins to precipitate? Again we use the Ksp equation to obtain [Au⁺] = 2.0 x 10^-13/1.8 x 10^-8 = 1.1 x 10^-5 M.

Review Questions

What reagent would you choose to separate the ions Ag+ and Pb2+ from a water solution?
If you began with 0.00100 M of each of the above ions together in a water solution, what percentage of one of the ions could you remove by the chosen reagent before the other began to precipitate?