Chapter 1. Experiment 11a

Qualitative Inorganic Analysis Preview

Purpose of the Experiment

Separate and identify representative cations from each of three traditional inorganic qualitative analysis groups. Determine the presence or absence of each cation in unknown mixtures.

Background Information

An important aspect of chemistry is the analysis of substances for their chemical composition. If you want to know WHAT is present in a substance, this is qualitative analysis. However, if you want to know HOW MUCH of a substance is present in a compound, this is quantitative analysis. In the next set of experiments, you will be performing qualitative inorganic analysis on groups of metal cations. There are five groups of cations in the traditional inorganic qualitative analysis scheme. The cations are grouped together on the basis of a common characteristic solubility. In the traditional qualitative analysis scheme, each of the groups is successively separated from a mixture and then analyzed to determine individual composition. However, you will analyze Group 1 and 2 together and then Group 3. Only a few representative cations of each group will be analyzed. Each group’s background and procedure will be discussed separately.

In These Experiments

For each group, you will first run through the procedure with a solution of known composition. In the following experiment, you will repeat the procedure with an unknown solution. By comparing the observations of the known and unknown solutions, you will determine which cations are present and write a rationale. A rationale is a logical discussion outlining the reasons why you think certain cations are present and others are absent.

Background Vocabulary

A solid compound that forms as a result of mixing reagents together in a solution is a precipitate. The liquid above the solid in a mixture of solid and solution is the supernate. To physically separate the precipitate from the supernate, you would centrifuge (spin rapidly) the sample to compact the solid. After centrifuging, the liquid is very carefully poured into a new test tube, leaving the compacted solid behind in the original test tube. To decant a liquid is to carefully pour the supernate off a solid. The liquid that was poured off or decanted into a separate container is the decantate.

Background Techniques

1. Mixing Solutions or Mixtures: Stir the mixture with a clean glass stirring rod. Stir in an up-and-down circular pattern. This will efficiently mix solids and solution together. Never mix by holding your thumb over the top of the test tube and inverting or shaking.

2. Centrifuging Mixtures: Place your test tube containing your test solution in a metal sleeve in the centrifuge. Next place a second test tube with approximately the same volume in the sleeve directly opposite your test tube. Note which sleeve has your test tube. Spin the sample for 1–2 minutes. You must always balance the centrifuge!

3. Decanting Mixtures: Centrifuge the sample, then very carefully pour the liquid into a new test tube, leaving the compacted solid behind in the original test tube.

4. Heating a Solution: Place a 250 mL beaker about half filled with distilled water on hot plate. Heat the beaker using a setting of about 7 to maintain a constant low boil. Use a test tube clamp to hold the test tube in the hot water bath.

5. Washing a Precipitate: Add a small amount of the specified solution to the solid and mix with a stirring rod. Centrifuge the sample and decant the supernate. Generally the decantate is discarded, but check your specific instructions. This procedure literally washes the old solution or contaminants off the surface of the solid.

6. Testing pH: Place a piece of litmus paper on a clean, dry, watch glass. Dip a clean glass-stirring rod into the solution to be tested. Touch the end of the stirring rod to the litmus paper. Never dip the paper into the solution to be tested! A solution is acidic if it turns blue litmus paper red. A solution is basic if it turns red litmus paper blue.

In These Experiments - Continued

Recording of Data

In these experiments, your observations will be recorded in a different format, as shown in a sample data sheet on the next page. This table is designed to record the Procedure step, Reagent added, Observation, Formulas of the chemical species present, and the Inferences or conclusion made from the observation. Writing the formulas and inferences helps you keep track of what is chemically going on in your test tubes and to understand the cation chemistry.

You will also need to include a graphical representation of your work in the form of a flowchart. The flowchart gives you the formulas needed to fill in the data sheet and helps you keep track during your experiment. A flowchart for the known solution is given with each group. You will need to copy the known flowchart into your notebook and add your observations to it. For the unknown solution, you will have to modify the flowchart, since some steps may be omitted.

Sample Data Sheet

•In your observations, always describe the precipitate AND decantate. Each will have their own chemical formulas.

•Notice that you do not have to write an observation for steps that are for washing precipitates.

For example, if a solid did NOT appear in 1–A1, then you would know that Group 1 cations, Ag⁺ and Pb²⁺ ions were ABSENT. This would lead you to write a different conclusion in the Inference column.

•You do not have to write an Inference or deduction for each step. This is mainly for forming solids, separating solids, and confirmation steps.

•Writing Inferences as you go will help you later with your unknowns. The Inferences pinpoint the steps to compare the known with the unknown to determine the presence/absence of the cations. It will also help you write your rationales at the end of the unknown experiment.

Discussion

Essentially the Inferences Section of the data sheet is the discussion for the Known solution. The Inferences Section of the data sheet of the Unknown solution is the “outline” for their rationale.

The Rationale for the Unknown is a discussion of the logic used to determine the identity of the unknown. It MUST explain stepwise how the observations prove the presence (or absence) of a cation. It is NOT just the results of the confirmation step.

Rationale Example

Ag⁺ is present because a white precipitate formed when HCl was added in step 1–A, and only Ag⁺ and Pb²⁺ form insoluble salts. Then the white precipitate did NOT dissolve in hot water. Only PbCl₂, not AgCl, dissolves in hot water. The presence of Ag⁺ was confirmed by the white precipitate dissolving in NH₃, step 1–D and then reprecipitating in step 1–E. Only the AgCl salt will dissolve in NH₃ and precipitate again, when the solution is acidified.

The information about the solubility of the compounds, what cations will form solids and what cations will stay in solution, is all given in the Group Background section of the experiment. You must READ and APPLY the information given in the Background section.

Note that the example rationale is not just the inference from the confirmation step for Ag ⁺ , but all the steps that have inferences about Ag ⁺ in the data sheet.

Activity Completed!