Colorimeter
Colorimetry is a simple form of concentration determination using light. The method works only for substances with their own color. A number of equal petri dishes are used and different solutions whose concentration is known. The same volume of a solution of the substance whose concentration is to be determined is put into each dish.
Colorimeter
A colorimeter can measure the absorbency of light waves. During colour measurement the change in the intensity of electromagnetic radiation in the visible wavelength region of the spectrum after transmitting or reflecting by an object or solution is measured. Such a measurement can help to find the concentration of substances, since the amount and colour of the light absorbed or transmitted depends on the properties of the solution, including the concentration of particles in it. A colorimeter is an instrument that compares the amount of light getting through a solution with the amount that can get through a sample of pure solvent. A colorimeter contains a photocell which is able to detect the amount of light passing through the solution under investigation. The current produced by the photocell depends on the quantity of light hitting it after passing through the coloured solution. The higher the concentration of the colorant in the solution, the higher is the absorption of light; less light passing through the solution means less current created by the photocell. A colorimeter takes three wideband readings along the visible spectrum to obtain a rough estimate of a colour sample. Traditionally, the word ‘colorimeter’ is used for a device, having three filters, that simulates human vision. Colorimeters can be classified into two types:
- Visual
- Photoelectric.
Visual colorimeters are of two types:
- Visual absorption meters/colour comparators
- True visual colorimeter or tristimulus colorimeter.
- The former type compares the colour of the test sample, usually liquid, with that of standard and finds a match between the two. Such instruments are employed for chemical analysis, concentration determination, and grading on the basis of colour.
Quantitative explanation
For a colorimetric concentration determination, one looks for the wavelength (within the visible spectrum) at which the strongest absorption takes place. At this wavelength one performs the rest of the analysis. In the concentration determination light of the found wavelengths is incident on a solution of the substance to be examined, the sample. The incident light is absorbed by the substance but also by other substances that may be present. To measure the influence of these (other) substances on the absorption spectrum, a blank is used. A blank contains all other substances that are also present in the sample, except the substance to be examined. One then measures the intensity of the transmitted light at the sample and at the blank. The intensity of the light that passes through the sample is indicated by I. The intensity of the light that passes through the blank is identified by the symbol Io The ratio of I to Io is called transmission T. T = I / I0. T is expressed in percent. If the concentration of the solute is 0 then I = I0. In practice, in colorimetry one does not usually use the transmittance, but the extinction (= extinction) this can be read on the spectrophotometer just like the transmittance. The extinction E is the -log(T). One uses the extinction because it is directly proportional to the concentration of the substance being measured and is therefore more practical to use.
To determine the quantity of substance in solution, a calibration line is first made. This requires measuring the absorbance of several standard solutions. Standard solutions are solutions with a known concentration of the substance under investigation. The series of these solutions is also called the calibration series. In a two-dimensional graph, the known concentrations are plotted against the measured absorbances. Since there is a straight line relationship between the absorbance and the concentration, a straight line may be drawn between the calibration points. Then, after measuring the extinction of the sample, the corresponding concentration can be read off the graph and thus one knows the concentration of the sample.
References
- https://nl.wikipedia.org/wiki/Colorimetrie
- Asim Kumar Roy Choudhury, Principles of Colour and Appearance Measurement, Woodhead Publishing, 2014, Pages 221-269, ISBN 9780857092298, https://doi.org/10.1533/9780857099242.221.
