Analysis of mixtures

6.2 Analysis of ions

Analysis of mixtures

  1. Reasoning

    1. Since the compounds in a mixture are not chemically combined, the chemical properties of each substances are unchanged

  2. Physical methods

    1. Distillation extracts the solvent from a solution
    2. Fractional distillation separates substances according to boiling point
    3. Filtration extracts insoluble substances from a suspension
    4. Filtration also separates substances according to particle size
    5. Crystallisation extracts the solute from a solution
    6. Crystallisation also separates substances according to solubility

  3. Paper chromatography

    1. Paper chromatography can be used to analyse the substances in a solution, such as food colouring and inks or dyes
    2. It involves a piece of paper, called the stationary phase, with one end placed in a solvent, called the mobile phase
    3. A base line on the stationary phase is determined and a spot of solution is placed on it
    4. The mobile phase is absorbed by the stationary phase and begins travelling up it, creating a line called the solvent front
    5. As the solvent front travels past the base line, substances in the solution dissolve in the mobile phase and begin moving with the solvent front
    6. At particular distances, different substances, and finally the mobile phase, stop moving
    7. The distance travelled by a substance is divided by the distance travelled by the solvent front to determine the retention factor Rf of the substance in the solvent and can be cross-referenced to an index of known retention factors in that solvent at that temperature to determine the substance
    8. The higher the retention factor, the more soluble the substance is in the solvent

  4. Instrumental methods

    1. Instrumental methods are accurate, sensitive and rapid
    2. They are particularly useful when the amount of a sample is very small

    3. Gas chromatography

      1. Gas chromatography separates a mixture of gases by vaporising it and carrying it through a column packed with a solid by an unreactive gas called the carrier gas
      2. The stronger a substance is attracted to the solid, the slower it travels though the column, separating the substances in the mixture
      3. The retention times, the times taken for each substance to travel through the column, are recorded on a gas chromatograph as peaks which can be used to identify not only the number of substances by the number of peaks, but also the individual substances

    4. Mass spectrometry

      1. A mass spectrometer can identify even small quantities of a substance very quickly and accurately
      2. A mass spectrometer can be attached to the output of a gas chromatography column to identify the substances leaving the column
      3. The molecular mass of a substance is given by the molecular ion peak, which is the peak with the largest mass