Spectrophotometry and Beer's Law: Flashcards
Revise transmittance, absorbance, Beer's law, the blank, calibration curves and replicate measurements.
Spectrophotometry and Beer's Law: Flashcards
- QuestionDefine transmittance.Answer
T = I ÷ I₀, the fraction of light that passes through the sample (no unit). %T = T × 100 %.
- QuestionHow is absorbance related to transmittance?Answer
A = −log T = log(I₀ ÷ I). It has no unit.
- QuestionState Beer's law.Answer
A = εbc: absorbance is proportional to concentration (ε in L mol⁻¹ cm⁻¹, b in cm, c in mol/L).
- QuestionWhat is ε?Answer
The molar absorptivity: a constant for a substance at a given wavelength, in L mol⁻¹ cm⁻¹.
- QuestionWhy measure at λmax?Answer
Absorption is strongest there, so the method is most sensitive, and small wavelength errors matter least.
- QuestionWhat is a blank, and why is it used?Answer
The solvent and reagents without the analyte. It zeroes the instrument so only the analyte absorption is measured.
- QuestionWhat is a calibration curve?Answer
A plot of absorbance against concentration for standards of known concentration; the unknown is read from its best-fit line.
- QuestionWhy measure the unknown in replicate?Answer
To assess precision: report the mean ± standard deviation (at least triplicate) instead of a single reading.
- QuestionWhat is the absorbance when 10 % of the light is transmitted?Answer
A = −log(0.100) = 1.000.
- QuestionWhen does Beer's law break down?Answer
At high concentrations or high absorbances (above about 1), and with non-monochromatic light: the calibration line curves.
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