A pharmaceutical laboratory routinely measures the concentration of an intensely…
CHEMISTRY · Multi-part · AP · Chemistry · Properties of Substances and Mixtures · Properties of Substances and Mixtures
Problem
A pharmaceutical laboratory routinely measures the concentration of an intensely colored drug in quality-control samples. The standard method employs a 1.00cm cuvette, but a new high-throughput protocol proposes switching to a microvolume cuvette with a 0.100cm path length to conserve sample.
During method transfer, the analyst prepares a stock solution of the drug at 8.00×10−4mol L−1. Using the 1.00cm cuvette, the absorbance of the stock registers as 1.28, which is outside the recommended upper limit for reliable measurements. The analyst must adapt the procedure for the 0.100cm microcuvette so that readings fall within the 0.10--1.00 range.
Answer the following questions.
(a) Calculate the absorbance the stock solution would yield in the 0.100cm microcuvette. Show your reasoning.
(b) Determine the minimum dilution factor required (relative to the stock solution) so that the absorbance measured in the microcuvette is as close as possible to 0.75 without exceeding 1.00. Provide all work, including any assumptions.
(c) After implementing the dilution in part (b), the analyst measures an absorbance of 0.72. Compute the percent error relative to the expected value of 0.75, and discuss one likely experimental source of this error that specifically relates to the reduced path length of the microcuvette.
(d) Propose a daily calibration check procedure that accounts for potential variability in the microcuvette path length, and justify how this procedure ensures data reliability over time.