Chemistry 1B Experiment 16

77

16
Qualitative Analysis
Introduction
The purpose of qualitative analysis is to determine what substances are present in detectable amounts in a sample. This experiment has two parts. In the first part, you will analyze an unknown solution for the presence of seven common ions. In the second part, you will test an unknown solid to determine which of two possible identities is correct.

Part I. Spot Tests for Some Common Ions
A simple approach to the qualitative analysis of an unknown solution is to test for the presence of each possible ion by adding a reagent which will cause the ion, if it is in the sample, to react in a characteristic way. This method involves a series of “spot” tests, one for each ion, carried out on separate samples of the unknown solution. The difficulty with this way of doing qualitative analysis is that frequently, particularly in complex mixtures, one species may interfere with the analytical test for another. Although interferences are common, there are many ions which can be identified in mixtures by simple spot tests. In this experiment we will use spot tests for the analysis of a mixture which may contain the following commonly encountered ions in solution: CO32– SO42– PO43– SCN Cl
– –

carbonate sulfate phosphate thiocyanate chloride acetate ammonium

C2H3O2– NH4+

78

Chemistry 1B Experiment 16

The procedures we involve simple acid-base, precipitation, complex ion formation or oxidation-reduction reactions. You will carry out each test three times. First, you will test 1 M solutions of each ion, so you can easily observe the expected results. Then you will test 0.1 M solutions of each ion, which will require more careful observation. Finally, you will test an unknown solution, which will be a dilute mixture of 3 or 4 of the ions. SAFETY PRECAUTIONS: Wear your SAFETY GOGGLES. If you