Types of chemical analysis
Chemical analysis finds out what is in a sample (qualitative) and how much (quantitative). Choosing a method depends on the sample, the amount available and the accuracy needed.
| Method | Principle | Example |
|---|---|---|
| Gravimetric | Weigh a pure precipitate of known formula | Barium sulphate for sulphate |
| Volumetric (titration) | Measure the volume of a standard solution that reacts exactly | Acid–base, redox and complexometric titrations |
| Colorimetry and UV–visible spectroscopy | Absorption of light is proportional to concentration | Nitrate or phosphate in water |
| Atomic absorption spectroscopy | Atoms absorb light of characteristic wavelengths | Metals in water and soil |
| Chromatography | Components separate between a mobile and a stationary phase | Dyes, amino acids, drugs |
| Infrared spectroscopy | Bonds absorb characteristic infrared frequencies | Identifying functional groups |
| Mass spectrometry | Ions separate by mass to charge ratio | Relative molecular mass and structure |
| NMR spectroscopy | Hydrogen nuclei in different environments absorb different radio frequencies | Structure of organic compounds |
Chromatography
In paper or thin-layer chromatography the stationary phase is paper or a thin layer of silica on a plate and the mobile phase is a solvent that rises by capillary action. In gas chromatography (GC) the mobile phase is a gas and the stationary phase is a liquid on a solid in a column; in HPLC a liquid is forced through a column at high pressure. Components that stick more strongly to the stationary phase move more slowly.
Measure the distances from the start line. Find the Rf value.
Spectroscopy and the Beer–Lambert law
- molar absorptivity (dm³ mol⁻¹ cm⁻¹)
Choose the absorptivity, the concentration and the width of the cell. Find the absorbance and the percentage of light that passes through.
| Technique | What it tells you |
|---|---|
| Infrared | O–H (broad, 3200–3600 cm⁻¹), C=O (1680–1750 cm⁻¹), C–H (about 2900 cm⁻¹) |
| Mass spectrum | The molecular ion peak gives the relative molecular mass; fragments show the structure |
| ¹H NMR | The number of peaks shows the number of different hydrogen environments; the area shows how many hydrogens |
Practise a volumetric analysis.
Key terms
- Qualitative analysis
- Finding out which substances are present.
- Quantitative analysis
- Finding out how much of a substance is present.
- Rf value
- The distance moved by a spot divided by the distance moved by the solvent front.
- Calibration graph
- A graph from known standards used to find the concentration of an unknown.
Summary
- Chemical analysis can be gravimetric, volumetric or instrumental.
- Chromatography separates mixtures; R_f = distance of spot ÷ distance of solvent.
- Beer–Lambert: A = ε c l; use a calibration graph.
- IR, mass spectrometry and NMR identify the structure of compounds.