Spectrophotometers determine the composition of a solution (or the concentration of certain substances within a solution) by measuring how the solution absorbs a particular wavelength of light. By varying the wavelengths in each measurement, one can take a spectrum of a solution and ascertain the composition. More advanced spectrophotometers can measure beyond the visible spectrum, providing readings for ultraviolet and infrared wavelengths.
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Merck litmus paper, 1934
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