Qualitative Analysis of Cations

Experiment (10) Qualitative Analysis of Cations Objectives: Separation and identification of some soluble and insoluble chlorides: Mg2+ , Ca2+ , Ba2+ , Ag+ , , Pb2+ Qualitative analysis is a method of finding out what kind of chemicals (compounds, elements or ions) are presented in a sample regardless of their quantities or concentrations. Based on the physical and / or chemical behavior of a certain chemical we will be able to determine whether this chemical is present or not in an unknown sample. We usually use the expression positive test: test which indicates that the substance is present, and negative test: test which shows that the substance is not present in the sample or its concentration is too low to be detected by this test. The evidence for a positive test is one or more of the following observations: a- Formation, disappearance or changing of a color. b- Formation or disappearance of precipitate. c- Formation of a gas. d- Change in the temperature. e- Change in the reading of an instrument. In this experiment, you are going to identify some cations: Mg2+ , Ca2+, Ba2+ , Ag+ , and Pb2+ and detect them in unknown solutions. According to the solubility of their chloride salts, these cations can be classified into two groups: Soluble (Mg2+ , Ca2+ and Ba2+) and insoluble (Ag+ , and Pb2+) Chlorides. Thus by adding a Cl- ion the insoluble salts (AgCl, Hg2Cl2 and PbCl2) precipitate and the other cations stay in solution. The precipitate and the solution (supernatant) can be separated after centrifugation, then the cations in each side can be identified using certain chemical test: I- Analysis of known solution: This known solution contains all cations (soluble and insoluble chlorides) which you are going to separate and analyze. Performing this analysis in a systematic was (following the procedure step by step as shown below) is very important to obtain correct results: 1- Separation and identification of soluble chlorides: Obtain about 1 mL of the known solution in a small test tube (that can fit in a centrifuge). Add 2-3 drops of 6.0 M HCl to this solution and then centrifuge the solution. Decant the supernatant solution and use it for testing (Mg2+ , Ca2+ and Ba2+). Keep the precipitate (AgCl, Hg2Cl2 and PbCl2) for part 2. A- Test for Mg2+: You need first to isolate Mg2+ from Ca2+ and Ba2+. To the supernatant add 2 drops of 8 M NH3, 2 drops of 2 M ammonium carbonate (NH4)2CO3 (see the reaction scheme next page). Centrifuge, decant the supernatant (which contains Mg2+ and keep the precipitate for the next step. Add 4 drops of 1M (NH4)2HPO4 to the supernatant solution. Formation of a white precipitate indicates the presence of Mg2+ in the solution (eq.1). B- Test for Ba2+: Add 10 drops of 6 M HC2H3O2 and 5 to 10 drops of 1 M K2CrO4 to the supernatant of part A above. A yellow precipitate confirms the presence of barium ion (eq.2). Centrifuge and save the supernatant for part C below. C- Test for Ca2+: Make the supernatant from part B basic to litmus with 6 M NH3. Add 5 to 10 drops of 1 M K2C2O4 and stir. A white precipitate indicates calcium (eq.3). if no precipitate appears immediately, warm the solution in water bath. 2- Separation and identification of insoluble chlorides: A- Test for Pb2+: Since PbCl2 is soluble in hot water but AgCl and Hg2Cl2 are not, Pb2+ can be separated from Ag+ and by heating the solution. Wash the precipitate obtained from part 1 by 1 mL of water then centrifuge and decant. Add 1 mL of distilled water to the precipitate and heat the test tube in water bath for about 3 minutes. While the solution is hot centrifuge the mixture and transfer the supernatant solution to another test tube and save the precipitate for next step. To the supernatant solution and few drops of 1 M K2CrO4. Formation of a yellow precipitate indicates the presence of Pb2+. B- Test of : Ammonia, NH3, reacts with both Hg2Cl2 and AgCl. With Hg2Cl2 the reaction produces a grey precipitate due to the formation of black Hg(l) and white NgNH2Cl (eq.4). Reaction of AgCl with NH3 forms the soluble complex ion, (eq.5). To the precipitate (from step A above) add 10 drops of 6 M NH3, stir and centrifuge the solution. Formation of a grey to black precipitate indicates the presence of (eq.4). Transfer the supernatant solution to clean test tube and keep it for the next step. C- Test for Ag2+: Acidify the supernatant solution by adding few drops of 6 M HNO3, check the acidity with blue litmus paper. Formation of a white precipitate indicates the presence of Ag2+ in the solution (eq.6). II- Analysis of unknown solution: 1- Ask your instructor for a sample of unknown solution of cations. Your unknown solution may contain 1, 2, or 3 cations. Record your unknown number on the report sheet. 2- Repeat the qualitative analysis tests described above and the reaction scheme below to identify all cations (among Mg2+ , Ca2+ , Ba2+ , Ag+ , present in your unknown solution. , Pb2+) Unknown Number: ………………………… Mark each action below to indicate if it is present (P) or absent (A) in your unknown. a- Mg2+ ………………………… b- Ba2+ ………………………… c- Ca2+ ………………………… d- Pb2+ ………………………… e- ………………………… f- Ag+ …………………………