An analytical chemist investigates a pigment that is strongly absorbing at 525 nm
CHEMISTRY · Multi-part · AP · Chemistry · Properties of Substances and Mixtures · Properties of Substances and Mixtures
Problem
An analytical chemist investigates a pigment that is strongly absorbing at 525 nm. A series of standard solutions (0.010 M to 0.070 M) is analyzed in a 1.00 cm glass cuvette, and the resulting plot of absorbance versus concentration is linear up to approximately 0.050 M. Beyond that point, the data begin to curve downward, approaching a maximum absorbance of about 1.20 regardless of further increases in concentration.
(a) Identify the concentration region on the plot where quantitative deviations from linearity first become significant and describe the graphical evidence that supports your identification.
(b) Provide a mechanistic explanation, grounded in the operation of the spectrophotometer, for why the absorbance values level off rather than continue to increase proportionally at higher concentrations.
(c) Propose TWO practical adjustments to the experimental setup or procedure that would allow accurate determination of pigment concentrations above 0.050 M, and explain how each adjustment would restore linear response.
(d) Discuss how failing to recognize the non-linear behavior at high concentration would bias the calculated value of the molar absorptivity if all data points—including the non-linear region—were included in the regression.