On linear graph paper in spectrophotometry, what is plotted on the y-axis?

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

Answer: (A)

In spectrophotometry, for a fixed path length and a specific wavelength, absorbance varies linearly with concentration according to Beer-Lambert law (A = εlc). That makes the calibration curve a straight line when plotting absorbance against concentration on linear graph paper, which is why the y-axis is absorbance. Transmittance is related to absorbance by A = -log10(T), so plotting transmittance versus concentration would produce a curved relationship on linear paper, not a straight line. Plotting absorbance against wavelength would give the absorption spectrum, not a concentration calibration. Therefore, the y-axis is absorbance.