Final answer:
The appropriate units for Absorbance on a calibration curve are unitless since absorbance is calculated using a log ratio of incident to transmitted light intensities.
Step-by-step explanation:
The appropriate units for the Absorbance (y-axis) on a calibration curve are unitless, simply because absorbance (A) is a measure of the amount of light absorbed by a sample. Absorbance is calculated using the logarithm of the ratio of the incident light intensity (Io) to the transmitted light intensity (I), without any specific units attached. However, when using Beer's law, which relates absorbance to concentration (c) and path length (l), the units of the molar absorptivity (ɛ) come into play, which are L·mol⁻¹·cm⁻¹. This is because the absorbance is directly proportional to both the concentration of the absorbing species in the solution and the path length that the light travels through the solution. Therefore, when constructing a calibration curve, you often report the concentration in moles per liter (M) if the path length is kept constant, typically 1 cm, and then calculate absorbance using the given molar absorptivity and Beer's law for a direct relationship.
For instance, if the molar absorptivity of a compound at its maximum absorbance wavelength (Amax) is known, the concentration of the sample can be easily calculated from the absorbance measured at Amax. A practical example includes using a known molar absorptivity of NAD* at 260 nm (ɛ = 18,000 L·mol⁻¹·cm⁻¹) and an observed A260 of 1.0, which would result in a calculated concentration of the sample as 5.6 x 10⁻⁵ M, using the Beer-Lambert law equation: A = ɛcl.