What is Light?
Light is a form of energy called electromagnetic radiation that is visible to the human eye. Other electromagnetic radiation includes gamma rays, UV-light, x-rays and radio waves.
Each of these types of radiation varies by their wavelength, with each wavelength corresponding to a particular amount of energy. It is the wavelength of light that then dictates its colour.
Humans are able to see light with wavelengths of 400nm (blue light and higher energy) to 700nm (red light and lower energy).
Above: Electromagnetic Spectrum
What is light used for in the lab?
Light in the ChemCentre lab is of particular importance to two types of instruments; colourimeters and atomic emission spectrometers (AES). These instruments measure light changes in both the UV and visible spectrum.
These two instruments are capable of identifying and quantifying a huge range of elements and ions (charged particles in solution). Everything from heavy metals in water, nitrate levels in fertilisers to phosphorus levels in soils.
What is colourimetry and how does it work?
Colourimetry measures the concentration of a sample by relating the intensity of colour to the concentration of the sample. Most samples are colourless to the naked eye, so another chemical is added to cause a colour change that the instrument measures.
You can use red cordial as an example of the way this works. In colourimetry you have several standards that you know the concentration of:
The concentration of the unknown sample can then be determined by comparing the intensity of the colour of the unknown to the colour intensity of the known concentrations.
These instruments are sensitive enough to determine differing concentrations even when both samples look colourless to the naked eye.
Want a little more science?
The standards (solutions of known concentrations) are analysed by selecting one specific wavelength of light that the sample absorbs. This wavelength light passes through the sample and the amount of light absorbed is measured by a detector. The absorbance value of each of the standards is plotted as a graph against concentration called a calibration curve. The absorbance of the unknown is measured and plotted on the graph which can be extrapolated to find the concentration.
Stay tuned to ChemCentre’s newsletter to find out about Atomic Emission Spectrometry in part two of Light - Brightening the World of Chemistry!