In a standard curve used for quantitative analysis, what is plotted on the X axis and Y axis?

• A. Concentration on X axis; Absorbance on Y axis
• B. Absorbance on X axis; Concentration on Y axis
• C. Concentration on both axes
• D. Absorbance on both axes

Answer: (A)

In quantitative analysis using a standard curve, the key relation comes from Beer's law: absorbance is proportional to concentration for a fixed path length and molar absorptivity. You prepare standards with known concentrations, measure their absorbance, and plot absorbance (vertical axis) against concentration (horizontal axis). This setup makes it straightforward to determine an unknown concentration from a measured absorbance by using the calibration line (slope and intercept) to solve for concentration. The other layouts—plotting absorbance on the x-axis, plotting concentration on both axes, or plotting absorbance on both axes—aren’t practical for turning a measured absorbance into a concentration, and they don’t reflect the direct, invertible relationship described by Beer's law.