Qualitative Chemical Analysis - Anion Analysis

(revised 4 June 2024)

Introduction

There are two broad categories of problems in analytical chemistry. Quantitative chemical analysis that deals with the determination of the amounts of certain component present in a sample. Whereas, qualitative chemical analysis (also known as qualitative chemical tests or spot tests) deals with the detection, by observation, of a particular component in a sample, but gives little information about the amount present. The particular component tested could be a functional group in a molecule, like the hydroxyl (HO-) group of alcohols, or an anion, like the carbonate or phosphate anion (more information is given below). Qualitative chemical analysis is the concept used in this experiment.

In this experiment, a series of qualitative chemical analyses will be conducted on five (5) different samples known to contain the following ions: CO₃^2- , PO₄^3- , Cl ^- , SO₄^2- , and CrO₄^2-. This is done by adding a specific reagent to each of the five samples, that reacts with the anion in the sample, if it is present at a concentration above the detection limit of the test, to cause a characteristic change that is easily observable with the eye. Characteristic changes may be, for example, a change in color, the formation of a precipitate, or the evolution of a gas. Once the characteristic changes are known for all the different anions, then the same tests are conducted on the unknown sample.

The reagent added to the sample reacts with the anion by the following reactions: acid-base, precipitation, complex-ion formation or oxidation-reduction reactions.

Prelab

Required reading: Chapter 4, sections 4.1, 4.2, 4.3, 4.4.
Define and provide an example of a molecular equation for each of the following types of reactions: Precipitation reaction, neutralization reaction, neutralization reaction with gas formation, oxidation-reduction reaction, and reaction involving the formation of complex ions (see chapter 17, section 17.5 only the section entitled Formation of complex ions, and chapter 23, section 23.2 Transition-metal complexes, only the first 2 paragraphs).

Safety

Wear safety glasses. Acids used in the experiment could cause chemical burns.

Procedure

Conduct the spot tests on known solutions. For each solution, you will test the solution provided and one diluted to 1/10 its concentration (i.e. 1 part stock solution and 9 parts distilled H₂O) for each ion as directed below in the procedure. The diluted solution will enable you to investigate the detectable concentration of the test. Record your observations as you will need them to determine the composition of your unknown solution. When performing a test, if no reaction is apparent stir and warm in a water bath. After completing the known solutions, repeat the test on your unknown.

Test for CO₃^2-

Carefully add 1 mL of 6 M HCl to 1 mL of solution (for the known solution, use 1 M Na₂CO₃) in a small test tube. CO₂ gas will be given off if a carbonate is present.

Test for SO₄^2-

Add 1 mL of 6 M HCl to 1 mL of solution (for the known solution, use 0.5 M Na₂SO₄). Then add a few drops of 1 M BaCl₂. A white precipitate of BaSO₄ indicates the presence of a sulfate.

Test of PO₄^3-

Add 1 mL of 6 M HNO₃ to 1 mL of solution (for the known solution, use 0.5 M Na₂HPO₄). Then add 1 mL of 0.5 M (NH₄)₂MoO₄ and stir thoroughly. A yellow precipitate of (NH₄)₃PO₄×12 MoO₃ indicates the presence of PO₄^3-. In dilute solution, the reaction is slow. Heat the solution if no precipitate forms.

Test for CrO₄^2-

Chromate-containing solutions are yellow when neutral or basic and orange under acidic conditions. The chromate ion under highly acidic conditions is a good oxidizing agent. Add 2 mL of 6 M HNO₃ to 1 mL of solution (use 0.5 M K₂CrO₄ for the known solution). At this point, if reduction of chromate occurs to form dichromate, a color change to pale blue will be observed shortly after addition of the acid and confirms the presence of CrO₄^2-. If no color change is observed, cool the solution under the cold water tap and add 1 mL of 3% H₂O₂. If chromate is present, a rapidly fading blue color caused by the formation of unstable CrO₅ will be observed.

Test for Cl ^-

Add 1 mL of 6 M HNO₃ to 1 mL of solution (for the known solution, use 0.5 M NaCl). Add a few drops of 0.1 M AgNO₃ . A white precipitate of AgCl will form if Cl^- is present.