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How can spectroscopy measure the concentration of a substance in a solution?

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In spectroscopy, the concentration of a substance in a solution is measured using spectrophotometry, which relies on the relationship between the absorption of radiant energy and the medium through which it passes.1

This quantitative analysis is governed by the Beer-Lambert law, which states that the amount of radiation absorbed is related to the concentration of the substance and the length of the path the light travels through the solution. Specifically, Beer's law establishes that the absorptive capacity of a dissolved substance is directly proportional to its concentration.21

When a substance does not absorb light strongly on its own, spectrochemical analysis is used. In this process, a chemical reagent is added to the solution to form a product that absorbs light strongly; the absorption of this product is then measured to determine the concentration of the original analyte. Additionally, spectrophotometric titrations can be used, where the absorption is measured as a reagent is added, allowing the concentration to be calculated from the resulting titration curve.3