In: Chemistry
Molar absorptivity data for the cobalt and nickel complexes with
2,3-quinoxalinedithiol are εCo = 36400 and
εNi = 5520 at 510 nm and εCo = 1240 and ε
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.
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