Using the Spectrophotometer - The Bausch & Lomb Spectronic 20 Colorimeter is an extremely versatile instrument that is useful for the spectrophotometric, or colorimetric, determinations of solutions. Within the optical system of the spectrophotometer, rotation of a prism (diffraction grating) allows the investigator to select specific wavelengths of light in a range from 375 to 625 nm. Light of a selected wavelength is passed through the sample and is picked up by a measuring phototube, where the light energy is converted to a reading on the meter of the spectrophotometer.
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Most spectrophotometers have two scales—one is a linear scale (the transmittance scale) given as percent transmittance, the other is a logarithmic scale with the same gradations as the percent transmittance scale (the absorbance scale). (Since transmittance is related to absorbance as the log of the inverse, values of 0.0 absorbance occur at 100% transmittance and values of infinite absorbanceoccur at 0% transmittance.) Examine the readout meter on the spectrophotometer you will be using. Note that the scales run in opposite directions.
An example of an experiment in which spectrophotometry is used is the determination of the equilibrium constant of a solution. A certain chemical reaction within a solution may occur in a forward and reverse direction where reactants form products and products break down into reactants. At some point, this chemical reaction will reach a point of balance called an equilibrium point. In order to determine the respective concentrations of reactants and products at this point, the light transmittance of the solution can be tested using spectrophotometry. The amount of light that passes through the solution is indicative of the concentration of certain chemicals that do not allow light to pass through. (source: en.wikipedia.com)
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