Qualitative Analysis Identifying Anions and Cations

Qualitative analysis is used to identify and separate  cations and anions  in a sample substance. Unlike quantitative analysis, which seeks to determine the quantity or amount of sample, qualitative analysis is a descriptive form of analysis. In an educational setting, the concentrations of the ions to be identified are approximately 0.01 M in an aqueous solution. The semimicro level of qualitative analysis employs methods used to detect 1-2 mg of an ion in 5 mL of solution. While there are qualitative analysis methods used to identify covalent molecules, most covalent compounds can be identified and distinguished from each other using physical properties, such as  refractive index and melting point. Lab Techniques for Semi-Micro Qualitative Analysis Its easy to contaminate the sample through poor laboratory technique, so its important to adhere to certain rules: Do not use tap water. Rather, use distilled water or deionized water.Glassware must be clean prior to use. Its not essential that it be dried.Dont put a reagent dropper tip into the mouth of a test tube. Dispense reagent from above the test tube lip to avoid contamination.Mix solutions by flicking the test tube. Never cover the test tube with a finger and shake the tube. Avoid exposing yourself to the sample. Steps of Qualitative Analysis If the sample is presented as a solid (salt), its important to note the shape and color of any crystals.  Reagents are used to separate cations into groups of related elements.Ions in a group are separated from each other. After each separation stage, a test is performed to confirm certain ions truly were removed. The test is not performed on the original sample!Separations rely on different characteristics of ions. These may involve redox reactions to change oxidation state, differential solubility in an acid, base, or water, or precipitating certain ions. Sample Qualitative Analysis Protocol First, ions are removed in groups from the initial aqueous solution. After each group has been separated, then testing is conducted for the individual ions in each group. Here is a common grouping of cations: Group I: Ag, Hg22, Pb2Precipitated in 1 M HCl Group II: Bi3, Cd2, Cu2, Hg2, (Pb2), Sb3 and Sb5, Sn2 and Sn4Precipitated in 0.1 M H2S solution at pH 0.5 Group III: Al3, (Cd2), Co2, Cr3, Fe2 and Fe3, Mn2, Ni2, Zn2Precipitated in 0.1 M H2S solution at pH 9 Group IV: Ba2, Ca2, K, Mg2, Na, NH4Ba2, Ca2, and Mg2 are precipitated in 0.2 M (NH4)2CO3 solution at pH 10; the other ions are soluble Many reagents are used in the qualitative analysis, but only a few are involved in nearly every group procedure. The four most commonly used reagents are 6M HCl, 6M HNO3, 6M NaOH, 6M NH3. Understanding the uses of the reagents is helpful when planning an analysis. Common Qualitative Analysis Reagents Reagent Effects 6M HCl Increases [H+]Increases [Cl-]Decreases [OH-]Dissolves insoluble carbonates, chromates, hydroxides, some sulfatesDestroys hydroxo and NH3 complexesPrecipitates insoluble chlorides 6M HNO3 Increases [H+]Decreases [OH-]Dissolves insoluble carbonates, chromates, and hydroxidesDissolves insoluble sulfides by oxidizing sulfide ionDestroys hydroxo and ammonia complexesGood oxidizing agent when hot 6 M NaOH Increases [OH-]Decreases [H+]Forms hydroxo complexesPrecipitates insoluble hydroxides 6M NH3 Increases [NH3]Increases [OH-]Decreases [H+]Precipitates insoluble hydroxidesForms NH3 complexesForms a basic buffer with NH4+