Quick questions on Colorimetry, UV-visible spectroscopy and AAS: VCE Chemistry Unit 2

Volumetric methods are accurate but limited to analytes that react in a known stoichiometry and at concentrations high enough to titrate. Instrumental methods extend the toolkit to:

Coloured solutions absorb visible light because their electrons are promoted between energy levels separated by a visible-light photon. The colour of a solution is the complement of the colour it absorbs (a blue solution absorbs orange/red; a red solution absorbs green).

1. Prepare a series of standards of known concentration spanning the expected range of the unknown. 2. Measure absorbance for each at a chosen wavelength (usually the wavelength of maximum absorbance, lambda_max).

AAS is used for metal ions in water at trace concentrations (ppm or ppb). The principle: free, gaseous, ground-state metal atoms absorb light at the same wavelengths their atoms emit. Each element has a unique line spectrum, so a specific lamp gives a specific element's lines.

Always quote the absorbance peak and pick the wavelength there.

A is only reliably linear up to about A ~ 1.0. Above that, dilute the sample and re-measure.

Colorimetry/UV-visible measures species in solution (often coloured). AAS measures gaseous ground-state atoms in a flame; the solution is atomised.

Always subtract the absorbance of a blank (pure solvent or matrix) from each measurement.

It is best for metal ions. For trace organic compounds, use a different method (HPLC with detector, GC-MS).