Chemlab: Chemistry 3/5

Qualitative Analysis of Cations


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Qualitative analysis schemes are often based on selective precipitation of different cations. By carefully selecting reagents and conditions for reactions, groups of ions can be separated on the basis of their reactivity and the solubility of their salts. This enables the experimenter to use simple techniques with standard laboratory equipment, as well as to learn some descriptive chemistry.

The Separation Scheme
The possible cations in your unknown sample are given in the following list, organized according to the separation scheme. These ions should all be familiar to you.

Group I: Pb2+, Ag+
Group II: Cu2+
Group III: Fe2+, Ni2+, Mn2+

The first step in any scheme separates the ions into several groups, each of which exhibits a common chemical property. The groups are then examined to identify the components. The unknown solution contains ions from three groups which may be separated as follows:

1. The Group I ions form insoluble chlorides upon addition of dilute HCl.

2. In the absence of Group I ions, the Group II ions are insoluble in acidified H2S solution.

3. After Group II ions have been removed, the Group III ions are insoluble in a basic solution saturated with H2S.

Once the ions are separated into these three groups, additional reactions are used to separate and analyze for individual cations. For example, Pb2+ and Ag+ ions are separated by dissolving the PbC12 precipitate in hot water and removing it from the less-soluble AgCl. Following this separation, a test is performed on the solution to confirm the presence of Pb2+. The individual separation reactions and confirmation tests for each ion are outlined in the procedure section.

These procedures will provide crucial information regarding the presence or absence of particular ions in your mixture. You will obtain an "unknown" solution, which contains a subset of the six ions, and you will use your scheme to determine its composition. Some of the experiments may be unnecessary for your particular combination of unknown cations, but that is for you to decide. In any event, you should convince yourself that you have devised a reliable scheme before you pick up your unknown sample.

In addition to your unknown cation mixture, you will also have in lab a set of 0.1 M solutions of each individual cation. You can carry out parallel experiments with these solutions and your unknown mixture of cations, to see what a positive test looks like. If your unknown does not contain a particular ion, you should do the confirmation test for that cation on the known sample and record your observations in your notebook. Note that it is not appropriate to ask your TA or instructor questions like, "Is this the color of the precipitate that copper gives with thioacetamide?" or "Is this the color of the solution that you get when ammonia is added to cupric ion?" When you have the urge to ask such questions, try the experiment yourself with a single-cation solution and see the answer!

Finally, when you think that you know which of the six cations are present in your unknown (and, just as importantly, which are not) you should carefully repeat your analysis to confirm your results.

A Note on Net Reactions
The balanced net reaction describes the chemical changes of any reaction. It is balanced in the usual sense: as many moles of each element in all the reactants as in all the products, and a net charge for all reactants equal to that for all products. It is a net reaction in the sense that only those species in solution that actually change or participate in new bonding situations are included. For example, in the confirming test for Fe3+, (Section 3D), solid NH4SCN is added to a solution containing Fe3+. The net reaction, however, does not mention NH4SCN, since it dissociates in solution to the ammonium ion, NH4+, and the thiocyanate ion, SCN-. Only the thiocyanate ion is involved in the chemistry; the ammonium ion is a spectator. Thus, we write the balanced net reaction as
Fe3+ + 6 SCN- Fe(SCN)63-
Writing net reactions thus requires you to be able to identify the reacting species in solution as well as the resultant new product, be it solid precipitate or, as in this case, a complex ion that stays in solution.

Some of the net reactions are redox reactions that will include H3O+ or OH- reactants (according to the pH of the solution). These may require more time and effort to balance. Always start with oxidation and reduction half-reactions. Balance these for both mass and charge. Then add the half-reactions together for the overall reaction. See your textbook or TA for help with these reactions, if you have difficulty.
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