Qualitative Analysis in Chemistry

Introduction to Qualitative Analysis - Identifying Anions and Cations

Test tubes
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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 index of refraction and melting point.

Lab Techniques for Semi-Micro Qualitative Analysis

It's easy to contaminate the sample through poor laboratory technique, so it's 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. It's not essential that it be dried.
  • Don't 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), it's important to not 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+, Pb2+
    Precipitated in 1 M HCl

    Group II: Bi3+, Cd2+, Cu2+, Hg2+, (Pb2+), Sb3+ and Sb5+, Sn2+ and Sn4+
    Precipitated in 0.1 M H2S solution at pH 0.5

    Group III: Al3+, (Cd2+), Co2+, Cr3+, Fe2+ and Fe3+, Mn2+, Ni2+, Zn2+
    Precipitated in 0.1 M H2S solution at pH 9

    Group IV: Ba2+, Ca2+, K+, Mg2+, Na+, NH4+
    Ba2+, 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.

    Next: Complex Ions and Precipitation Reactions

    Common Qualitative Analysis Reagents

    ReagentEffects
    6M HClIncreases [H+]
    Increases [Cl-]
    Decreases [OH-]
    Dissolves insoluble carbonates, chromates, hydroxides, some sulfates
    Destroys hydroxo and NH3 complexes
    Precipitates insoluble chlorides
    6M HNO3Increases [H+]
    Decreases [OH-]
    Dissolves insoluble carbonates, chromates, and hydroxides
    Dissolves insoluble sulfides by oxidizing sulfide ion
    Destroys hydroxo and ammonia complexes
    Good oxidizing agent when hot
    6 M NaOHIncreases [OH-]
    Decreases [H+]
    Forms hydroxo complexes
    Precipitates insoluble hydroxides
    6M NH3Increases [NH3]
    Increases [OH-]
    Decreases [H+]
    Precipitates insoluble hydroxides
    Forms NH3 complexes
    Forms a basic buffer with NH4+