In UV–Vis spectrophotometry, what is the correct unit for absorbance (A), where $A=-{\log }_{10}\left(\frac{I}{I_0}\right)$?

• A. Absorbance is reported in meters ($m$).
• B. Absorbance is reported in moles per liter ($\mathrm{mol}·L^{\mathrm{-1}}$).
• C. Absorbance is reported in percent transmittance ($\%$).
• D. Absorbance is unitless (dimensionless).

Answer: (D)

Verified step by step guidance

1. Recall the definition of absorbance (A) in UV–Vis spectrophotometry: \(A = -\log_{10}\left(\frac{I}{I_0}\right)\), where \(I\) is the intensity of transmitted light and \(I_0\) is the intensity of incident light.
2. Understand that absorbance is calculated as the logarithm of a ratio of two intensities, \(I\) and \(I_0\), which have the same units (e.g., watts per square meter).
3. Since absorbance is the logarithm of a ratio of two quantities with identical units, the units cancel out, making absorbance a pure number without any physical units.
4. Recognize that absorbance is therefore dimensionless (unitless), meaning it does not have units like meters, moles per liter, or percent.
5. Note that while absorbance is unitless, related quantities like transmittance can be expressed as a percentage, but absorbance itself remains dimensionless.