Chemical Analysis

Chemical Analysis

1. Chromatography

   • Various chemical analysis techniques can be used to help identify the chemicals present in reaction mixtures or give important information about the products of a chemical reaction.
   • Chromatography is an important analytical technique because it allows chemists to separate substances in complex mixtures. There are a variety of types of chromatography, which can be used in different contexts.
     • In chromatography, substances are separated as they travel in a mobile phase which passes through a stationary phase. Different substances travel at different speeds, so some move further than others in a given time.
   • In paper chromatography, the stationary phase is a sheet of chromatography paper. The mobile phase may either be an aqueous (water-based) liquid or a non-aqueous (carbon-based) organic solvent.
     • For each chemical in the sample, there is a dynamic equilibrium between the stationary phase and the mobile phase. The overall separation depends upon how strongly attracted the chemicals are to the mobile and the stationary phases. This produces a chromatogram where different samples can be compared to a reference material.
   • Thin layer chromatography (TLC) is similar to paper chromatography but instead of paper, the stationary phase is a thin layer of an inert substance (eg silica) supported on a flat, unreactive surface (eg a glass plate).
     • TLC has some advantages over paper chromatography. For example: the mobile phase moves more quickly through the stationary phase the mobile phase moves more evenly through the stationary phase there is a range of absorbencies for the stationary phase
     • TLC tends to produce more useful chromatograms than paper chromatography, which show greater separation of the components in the mixture - and are therefore easier to analyse.
     • The distance a sample travels can depend on the size or the polarity of the molecules involved. Larger molecules take longer to move up the chromatography paper or TLC plate, whereas smaller molecules are more mobile. Likewise, the polarity of the molecules can affect how far the spots travel, depending on the type of solvent used. Polar molecules will be more strongly attracted to polar solvents, and so would move further if a polar solvent was used as opposed to a non-polar solvent. The distance that spots move can be compared to the overall distance the solvent has moved and comparisons and measurements made.
   • In gas chromatography (GC), the mobile phase is an inert gas (eg helium).
     • The stationary phase is a very thin layer of an inert liquid on an inert solid support - such as beads of silica packed into a long thin tube (this flexible tube is coiled many times inside a thermostatically-controlled oven to keep it at a constant temperature).
     • GC is used to separate complex mixtures. It is much better at this than thin-layer or paper chromatography. This is because it is more sensitive - allowing the determination not only of what chemicals are in the mixture, but also how much of each chemical there is.
     • The mixture to be analysed is injected into the stream of carrier gas. As it passes along the column (long thin tube) it separates into the different substances. Substances with a greater affinity (attraction) for the mobile phase reach the detector at the end of the column more quickly. Substances with a greater affinity for the stationary phase move more slowly through the column.
     • Gas chromatography can be used to detect banned substances in urine samples from athletes, or by forensic investigators to detect the presence of fuels that may have been used to deliberately start fires. A gas chromatogram might show the time along the x-axis and the strength of response along the y-axis. The amount of time that a substance takes to pass through the column is called it retention time. The retention time of an unknown substance can be compared with standard reference data to help to identify it.

2. Titration

   • A titration experiment can be carried out to accurately measure the volume of substances that react in chemical reactions.
   • This technique utilises a standard solution (a solution of an accurately known concentration) which is titrated against portions of an unknown concentration until the reaction is just complete. This end-point can be shown by using an indicator. Volumetric titrations are popular in acid-base reactions.
   • The quantity of oxidising or reducing agent present in a redox reaction can be calculated from the results of a titration by using the balanced redox equation