Question

The following information is to be used for the next two questions. Molar absorptivity data for the cobalt and nickel complexes with 2,3-quinoxalinedithiol are ε_Co = 36400 and ε_Ni = 5520 at 510 nm and ε_Co = 1240 and ε _Ni = 17500 at 656 nm. A 0.514-g sample was dissolved and diluted to 100.0 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 1.246 at 510 nm and 0.367 at 656 nm in a 1.00-cm cell.

1. Calculate the concentration of cobalt (in ppm) in the initial solution prepared by dissolving the sample.

2. Calculate the concentration of nickel (in ppm) in the initial solution prepared by dissolving the sample.

Answer 1

Expression for total absorbance at 510nm can be written as follows:

A510 = A510 due to Co + A510 due to Ni

According to Beer Lambert’s law A = εc for 1cm pathlength.

Where, ε is molar absorptivity and c is the concentration.

Thus, A510 = εCo [Co] + εNi [Ni]

1.246 = 36400 [Co] + 5520 [Ni]   ------------------------------------(1)

Similar expression can be written for absorbance at 656nm as follows:

0.367 = 1240 [Co] + 17500 [Ni] -----------------------------------(2)

Expression (1) can be rearranged as follows:

[Co] = (1.246 – 5520[Ni]) / 36400 --------------------------------(3)

Substitute (3) in eq. (2)

0.367 = 1240 {(1.246 – 5520[Ni]) / 36400} + 17500 [Ni]

0.367 = (1240 x 1.246 / 36400) – (1240 x 5520 [Ni]/36400) + 17500[Ni]

0.367 = 0.04245 – 188[Ni] +17500[Ni]

0.32455 = 17312 [Ni]

[Ni] = 1.875 x 10^-5

Substitute value of [Ni] in expression (3)

[Co] = (1.246 – 5520(1.875 x 10^-5)) / 36400

[Co] = 3.139 x 10^-5

Thus, concentration of final solutions of Co is 3.139 x 10^-5 mol/L and that of Ni is 1.875 x 10^-5mol/L

Concentration of Co in initial solution of 100mL containing 0.514 x 10^-3kg of sample can be calculated as follows:

Concentration of Co = (3.139 x 10^-5 mol/L) ( 0.050 L) (58.9932 x 10³ mg/mol) x (100mL/25mL) x (1/{0.514 x 10^-3kg})

Concentration of Co = 720mg/kg = 720ppm

[Note: molar mass of Co is 58.9932 x 10³ mg/mol and that of Ni is 58.69 x 10³ mg/mol]

Similarly concentration of Ni in initial solution of 100mL containing 0.514 x 10^-3kg of sample can be calculated as follows:

Concentration of Ni = (1.875 x 10^-5 mol/L) ( 0.050 L) (58.69 x 10³ mg/mol) x (100mL/25mL) x (1/{0.514 x 10^-3kg})

Concentration of Ni = 428mg/kg = 428ppm

Therefore, concentration of Co in initial solution is 720ppm and that of Ni is 428ppm