Spectrophotometry determines the concentration of a solute in a solution.
Spectrophotometry is a common lab technique for determining solute concentration in a solution. A spectrophotometer uses a prism or diffraction grating to select light of a desired wavelength and pass it through the test sample. A detector on the other side measures the intensity of light striking it. The measurement gives the percent of transmittance and absorbance when the instrument is properly calibrated. After you've measured the absorbance, finding concentration is a straightforward calculation that relies on a formula called Beer's Law.
Instructions
1. Write down the following equation: Absorbance = (absorptivity) * (length) * (concentration). This formula is called Beer's Law. You know the absorbance from your experiment, while the length component is the distance through the "cuvette" -- the container used to hold the sample in the spectrophotometer. For most cuvettes the distance is 1 cm, although you should verify it. Finally, molar absorptivity is a constant specific to a given substance, solvent and wavelength. Calculate molar absorptivity using the absorbance of the standard solutions you measured in your experiment.
