Colourimetry The absorption of light by a coloured

Colourimetry �The absorption of light by a coloured solution is directly related to the concentration of the solution. �Colourimetry determines the concentration of a coloured solution by comparing its colour with solutions of known concentration.

How a colourimeter works � Rather than simply relying on the eye to compare the colour of a sample to solutions of known concentration, an instrument called a colourimeter can accurately measure the amount of light that is able to pass through a sample. � If the absorption of light by a number of solutions of known concentration is recorded then graphed, a calibration graph is obtained. � By measuring the absorption of light for the unknown solution and making use of the calibration graph, the concentration of the unknown solution can be determined.

A simple colourimeter is made up of the following components: The sample solution is placed in a special cell, called a cuvette (made of glass or plastic), which allows light to pass through it easily. A reference reading is taken with a cell containing only pure solvent. This is used to compensate for any scattering or absorption of the light by the cell and the solvent. The reference cell is then replaced with a cell containing a solution of the sample.

Choice of light � When a substance appears coloured, it does so because it absorbs some colours while reflecting others for the eye to see. � The colour that we see is not the colour absorbed by the substance; instead it is the complement of the absorbed colour. � Solution appears blue because it absorbs all

UV Visible Spectroscopy �The UV Visible Spectrometer is much the same as the colorimeter but is more sensitive and is able to detect absorption of light across the whole visible spectrum and also UV wavelengths. �A UV Visible Spectrometer is made up of the following components: In a UV visible spectroscope, the cuvette is made of quartz or fused silica which allows both ultraviolet and visible light to pass through easily.

Choice of light � A wavelength is chosen that allows for maximum absorption. � By measuring the absorbance at various wavelengths a graph, or spectrum, for the sample is obtained. � In this example, the wavelength most strongly absorbed by the sample is _________.

Example: Consider the following spectrum for a coloured solution. Absorbance (nm) (a) Wavelength Through what wavelength range does this coloured solution absorb visible light? (b) Hence, what colour is the solution likely to be? (c) What is the wavelength at which absorption is a maximum?

� If we place a known stock solution of a coloured compound into a UV-Visible spectrophotometer and scan the spectrum, we are able to obtain a wavelength at which the absorbance is a maximum i. e. . 450 nm …. . as shown in above exercise. � We can then use this single wavelength (450 nm) to determine the absorbance of a series of standard solutions to obtain a calibration curve. � Our unknown is then determined by measuring its absorbance at this wavelength and determining its concentration from the calibration curve.

�While it can be used for qualitative analysis, UV-visible spectroscopy is mainly used for quantitative analysis, ie determining the concentration of a substance in a sample. �This procedure involves selecting a wavelength at which the substance absorbs strongly but other components in the sample do not. �UV-Visible spectroscopy is a quantitative technique used in: �pathology labs in hospitals for specific chemicals in urine and blood. �determination of amount of coloured dyes in plastics. �identification of the presence of metal ions… if metal ion is not coloured it may possibly be converted into a coloured compound. �determination of hemoglobin and sugar levels in blood.

Advantages of UV-visible spectroscopy: � Can be used for atoms, ions and molecules. � Since the choice of wavelength can be very specific, one particular species can be selected analysed in a complex sample. � UV-visible spectroscopy can be used to detect coloured compounds and some compounds that produce colourless solutions. This is because the compound may be able to absorb “invisible” UV radiation. (ie colourless solutions reflect all wavelengths from the visible region of the electromagnetic spectrum but may absorb UV wavelengths). � Colorimetry is limited to detecting coloured compounds that absorb radiation from the visible region of the electromagnetic spectrum.