Analyte P has a visible absorption maximum at in aqueous solution

CHEMISTRY · Multi-part · AP · Chemistry · Properties of Substances and Mixtures · Properties of Substances and Mixtures

Problem

Analyte P has a visible absorption maximum at

440\,\text{nm}

in aqueous solution. In many real samples, interferent Q is present at a fixed concentration of

1.0 \times 10^{-5}\,\text{mol L}^{-1}

. The table lists molar absorptivities (in

\text{L mol}^{-1}\,\text{cm}^{-1}

) for both species at two wavelengths.

\epsilon_P

(

\times 10^4

)

\epsilon_Q

(

\times 10^4

)

440\,\text{nm}

5.0 2.0

480\,\text{nm}

3.0 1.5 A

1.00\,\text{cm}

cuvette will be used throughout. Address the tasks below concerning determination of the unknown concentration of analyte P. (1) Part 1: (a) Write algebraic expressions for the total absorbance of the sample at

440\,\text{nm}

and at

480\,\text{nm}

in terms of the concentrations of P and Q. (2) Part 2: (b) Because the concentration of Q is known, derive an explicit equation that gives the concentration of P using a single-wavelength measurement. Clearly state any assumptions made. (3) Part 3: (c) Suppose the true concentration of P is

4.0 \times 10^{-6}\,\text{mol L}^{-1}

and the instrumental uncertainty in absorbance is

\pm 0.003

at both wavelengths. Calculate the percent relative uncertainty in the calculated concentration of P when measurements are made at (i)

440\,\text{nm}

and (ii)

480\,\text{nm}

. Based on your calculations, evaluate whether using the absorption maximum is still advantageous in the presence of the interferent, and justify your conclusion.

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