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23	23	* True visual colorimeter or tristimulus colorimeter.
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25	25	: 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.
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	29	+= Quantitative explanation =
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	31	+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.
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	33	+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.
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27	27	= References =
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29	29	* [[https:~~/~~/nl.wikipedia.org/wiki/Colorimetrie>>https://nl.wikipedia.org/wiki/Colorimetrie]]