In a calibration context, which statement best represents the relationship between unknown and standard samples?

• A. Abs unknown / Abs standard = Conc unknown / Conc standard
• B. Abs standard / Abs unknown = Conc standard / Concentration unknown
• C. A = εbc
• D. I = V/R

Answer: (A)

In calibration with spectrophotometry, absorbance is proportional to concentration when the path length and the molar absorptivity at a given wavelength are the same for both samples. This means the absorbance ratio of two samples equals the concentration ratio: absorbance of the unknown divided by absorbance of the standard equals the concentration of the unknown divided by the concentration of the standard. This proportionality allows you to determine the unknown concentration from the standard by comparing their absorbances.

The general Beer's law form A = εbc underpins this, but the specific two-sample relation used in calibration is the ratio of absorbances equaling the ratio of concentrations. The other statements either invert the ratio, cite the general law without the two-sample comparison, or reference an unrelated principle.