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Colorimetry is the field of determining the concentration of a coloured compound in a solution. A colorimeter, also known as a filter photometer, is an analytical machine that acts as the tool quantify a solutions concentration by measuring the absorbance of a specific wavelength of light.
Colorimeters are used for a wide range of applications across the chemical and biological fields including, but not limited to, the analysis of blood, water, nutrients in soil and foodstuffs, determining the concentration of a solution, determining the rates of reaction, determining the growth of bacterial cultures and laboratory quality control.
Colorimeter Principles
Colorimeters are used to detect colour and determine the solutions concentration, i.e. when a wavelength is passed through a sample, some of the light is absorbed and some passes through. It is the wavelengths of light that pass through that are detected.
By knowing which wavelengths have passed through, the detector can also work out which coloured wavelengths were absorbed. If the solution to be tested is colourless, a common procedure is to introduce a reagent that reacts with the solution to produce a coloured solution. The results are compared against known standards.
The colorimeter uses the Beer-Lambert law to detect the absorbance of the wavelength. Beer-Lamberts law is commonly written as:
A= Ɛcl
Where, A is the absorbance, Ɛ (epsilon) is the molar absorptivity, c is the concentration of the solution and l is the length that the light passes through (also known as the mean free path). Aside from this, if there is a continual changing of the solution, i.e. it is a reaction, then % of transmittance against time is generally used.
To measure concentrations, the amount of light absorbed is dependent upon the amount of solute (also known as the analyte as it is the species being measured) in the solution- a higher concentration of dissolved solute means that more light will be absorbed, and vice versa, hence, the concentration can be backed out from the absorption of specific wavelengths.
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The Colorimeter Itself
A colorimeter is composed of many parts. Aside from using a known standard solution, alongside either known concentrations and unknown concentrations, there are many vital components to a colorimeter.
As the principles are based around light, a light source is required and usually takes the form of a filament lamp. Other components include an adjustable aperture to let the light through, coloured filters to filter specific wavelengths of light, a cuvette to hold the solution (commonly made of quartz), a photodetector to measure the transmitted light and a meter to quantify the values into a readable output.
The coloured filters are chosen to select the wavelength in which the dissolved solute will absorb the most. For most experiments the common wavelength range is between 400 and 700 nm, but when some analytes absorb in the ultraviolet range (less than 400 nm) then modification of the colorimeter is generally required. This normally takes the form of removing the filament lamp and replacing it with light-emitting diode(s) of a specific colour.
The output can be either analogue or digital in nature and, depending of the principle used, will give either an absorbance (0-infinity logarithmic output) or a %transmittance (0-100%) readout. The ideal output for an absorbance measurement is between 0 and 2, but it is desirable to have a reading between 0 and 1, as above 1 the results can become unreliable due to the scattering of light. The readout is usually in the form of a spectrum.
Most calorimeters will require calibration, which is the solvent alone and not the measurable contents with the solvent- i.e. a standard or ‘blank’ solution. The calibration allows the absorbance of the solvent to be measured, also known across many instruments as the background noise. Once measured, the solvent absorption values are removed from any future readings, allowing the absorbance (or %transmittance) to be calculated (and plotted on a spectrum) for the desired analyte(s) without noise interference.
There are a wide variety of colorimeters out there, where some colorimeters are large machines and generally used for a wide-range of laboratory analyses, but some colorimeters are now hand-held and can be used for on-site analyses such as the determination of in-situ water and soil samples. In the case of handheld colorimeters, a numerical readout is the common procedure as opposed to a spectrum found on the larger laboratory machines.
Learn More About Companies Referenced
Sources:
http://sciencing.com/use-colorimeter-5382170.html
Seton Hall University: http://pirate.shu.edu/~rawncarr/colorimetry/colorimetry.htm
AZoSensors: http://www.azosensors.com/article.aspx?ArticleID=324
University of Michigan: https://encyclopedia.che.engin.umich.edu/
https://www.logitworld.com/
Humboldt State University: https://sites.google.com/humboldt.edu/paselkr1/home
Sherwood Scientific: https://www.sherwood-scientific.com/
“Absorbance Measurement by Colorimeter”- Mukesh J. Z. and Shinde A. A., International Journal of Advanced Research in Computer Science and Software Engineering, 2013,
HACH- https://www.hach.com/pockets
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