On semilogarithmic paper used in spectrophotometry, what is plotted on the y-axis?

• A. %Transmittance against concentration
• B. Absorbance against concentration
• C. Transmittance against wavelength
• D. Absorbance against wavelength

Answer: (A)

The key idea is that semilogarithmic plotting turns exponential relationships into straight lines. In spectrophotometry, transmittance T is related to concentration c by T = 10^{-εbc}, so transmittance drops exponentially as concentration rises. On a semilog graph, the y-axis is in a logarithmic scale, so plotting percent transmittance against concentration yields a straight line because log(%T) = log(100) + log(T) = 2 - εbc is linear in c. This makes %Transmittance against concentration the quantity that best produces a linear calibration curve on semi-log paper. Absorbance against concentration is already linear on linear paper, and plotting transmittance or absorbance against wavelength addresses spectral data rather than a concentration calibration.