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

Question

Molar absorptivity data for the cobalt and nickel complexes with 2, 3-quinoxalinedithiol are e_Co = 36400 and e_Ni = 5520 at 510 nm and e_Co = 1240 and e_Ni = 17500 at 656 nm. A 0.451-g sample was dissolved and diluted to 50.0 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 0.455 at 51 nm and 0.297 at 656 nm in a 1.00-cm cell. Calculate the concentration in parts per million of cobalt and nickel in the sample. Concentration of cobalt = ppm Concentration of nickel = ppm

Explanation / Answer

A510 = 510(Co) x c(Co) x l + 510(Ni) x c(Ni) x l

A656 = 656(Co) x c(Co) x l + 656(Ni) x c(Ni) x l

0.455 = 36400c(Co) + 5520c(Ni)

0.297 = 1240c(Co) + 17500c(Ni)

C(Co) = 1.00 x 10-5 M,

C(Ni) = 1.63 x 10-5 M

Note 25mL taken from 50mL original solution and then diluted to 50mL

Moles of Co in sample = 2 x 50/1000 x 1.00 x 10-5  = 1.0 x 10-6 mol

Moles of Ni in sample = 2 x 50/1000 x 1.63 x 10-5 = 1.63 x 10-6 mol

Mass of Co in sample = 1.0 x 10-6 x 58.93 = 5.89 x 10-5 g = 0.0589 mg

Mass of Ni in sample = 1.63 x 10-6 x 58.69 = 9.57 x 10-5 g = 0.0957 mg

Mass of sample = 0.451 g = 0.000451 kg

Concentration of Co = 0.0589 /0.000451 = 131 ppm

Concentration of Ni = 0.0957 /0.000451 = 212 ppm

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