Molar absorptivity data for the cobalt and nickel complexes with 2,3-quinoxaline

Question

Molar absorptivity data for the cobalt and nickel complexes with 2,3-quinoxalinedithiol are Co = 36400 and Ni = 5520 at 510 nm and Co = 1240 and Ni = 17500 at 656 nm. A 0.322-g sample was dissolved and diluted to 250.0 mL. A 25.0-mL aliquot was treated to eliminate interferences; after addition of 2,3-quinoxalinedithiol, the volume was adjusted to 50.0 mL. This solution had an absorbance of 0.650 at 510 nm and 0.588 at 656 nm in a 1.00-cm cell.
Calculate the concentration of cobalt (in ppm) in the initial solution prepared by dissolving the sample.

Explanation / Answer

Absorbance of solution = Absorbance due to Co + absorbance due to Ni

Absorbance = molar absorptivity x concentration x path length

with path length = 1 cm

molar absorptivity at 510 nm,

For Co = 36400 M-1.cm-1

For Ni = 5520 M-1.cm-1

molar absorptivity at 656 nm,

For Co = 1240 M-1.cm-1

For Ni = 17500 M-1.cm-1

let [Co] and [Ni] represent the concentration of Co and Ni in diluted solution

then,

0.650 = 36400[Co] + 5520[Ni]  ...... at 510 nm

0.588 = 1240[Co] + 17500[Ni]  ...... at 656 nm

Solving the two equations,

[Co] = 1.3 x 10^-5 M

[Ni] = 3.3 x 10^-5 M

Concentration in 25 ml aliquot,

[Co] = 1.3 x 10^-5 M x 50 ml/25 ml = 2.6 x 10^-5 M

[Ni] = 3.3 x 10^-5 x 50 ml/25 ml = 6.6 x 10^-5 M

concentration (ppm) in original solution prepared by dissolving sample,

[Co] = 2.6 x 10^-5 M x 250 ml x 58.93 g/mol x 1000/25 ml = 15.322 ppm

[Ni] = 6.6 x 10^-5 M x 250 ml x 58.69 g/mol x 1000/25 ml = 38.735 ppm

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