Question

a student uses a spectrophotometer to analyze a solution of blue food dye. the student first rinses a cuvette with distilled water. then the student adds the blue dye solution to the cuvette, forgetting to rinse the cuvette with the blue dye solution first. the student places the cuvette in the spectrophotometer and measures the absorbance of the solution. assuming that some distilled water droplets were still in the cuvette when the blue dye solution was added, how would the measured absorbance be affected?
a the measured absorbance would be too low, because the distilled water left in the cuvette would slightly dilute the solution.
b the measured absorbance would be too low, because the distilled water would change the optimum wavelength of absorbance.
c the measured absorbance would be too high, because the water would allow more light to pass through the cuvette.
d the measured absorbance would be too high, because the distilled water would also absorb some light.

Explanation:

According to the Beer - Lambert Law \(A = \varepsilon cl\) (where \(A\) is absorbance, \(\varepsilon\) is the molar absorptivity, \(c\) is the concentration, and \(l\) is the path length). When there is distilled water left in the cuvette, it dilutes the blue dye solution. Since absorbance is directly proportional to the concentration of the absorbing species (assuming \(\varepsilon\) and \(l\) are constant), a lower concentration (due to dilution) will result in a lower absorbance. Distilled water does not change the optimum wavelength of absorbance (that is a property of the absorbing molecule, the blue dye here), and distilled water does not absorb light in the visible region (where the blue dye would be analyzed) and allowing more light to pass through (if that was the case) would mean lower absorbance (since \(A=\log(\frac{I_0}{I})\), more \(I\) (transmitted light) means lower \(A\)).

Answer:

A. The measured absorbance would be too low, because the distilled water left in the cuvette would slightly dilute the solution.