The pH of a 5.00 ml of a 100-ppm iron (II) is adjusted to pH 4.0 with an appropr

Question

The pH of a 5.00 ml of a 100-ppm iron (II) is adjusted to pH 4.0 with an appropriate buffer and an excess of 1, 10-phenanthroline was added. An orange-colored complex forms, in which each iron (II) ion is complexed with 1, 10-phenanthroline. The volume of the reaction is made up to 100.00 mL with deionized water. The transmittance of the resulting solution was 37.6% T at 540 nm in a 1.2 cm path-length cuvette. Calculate the molar absorptivity of the complex. (Assume that formation constant of the complex is sufficiently large the reaction is complete).

Explanation / Answer

Absorbance of the solution is :

A = log (100/T) = 0.425

Concentration of the complex in 100mL = 5mL * 100ppm/100mL = 5ppm

Molar mass of Fe(o-phen)3 ^2+ = 596.5 g/mol

5ppm = 5mg/L

          = 0.005gm/L

          = (0.005 gm/596.5 g/mol)/L

          = 8.38*10^-6 M

A = ec.l

or, e = A/c.l

         = 0.425 / (8.38*10^-6* 1.2 cm)

          = 42252.8

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