Question

2) The ferrous ion-orthophenanthroline complex can be used to analyze for Fe(II) by using the absorption of this complex at 508 nm. A group of samples is known to contain between 0.1 and 0.2 g of Fe. A known sample containing 100 mg of Fe per liter was treated according to protocol, and the absorbance on a filter photometer was set to 0. The absorbance of a second standard containing 200 mg Fe per liter was then determined to be 0.795. What is the concentration of the unknown, which was then found to have an absorbance of 0.650?

Answer 1

Ans. Beer-Lambert’s Law, A = e C l             - equation 1,               where,

                                                A = Absorbance

                                                e = molar absorptivity at specified wavelength

                                                l = path length (in cm)

                                                C = Molar concentration of the solute

For, the second standard-

            0.795 = e (200 mg/ L) x l

            Or, e = 0.795 / [(200 mg/ L) x l]   - equation 2

For the unknown sample,

            0.650 = e (C) x l

            Or, e = 0.650 / [(C) x l]    - equation 3

Now, e is a constant for given wavelength. So, e in equation 2 is equal to e in equation 3.

Or, Comparing value of e in equation 2 and 3

0.795 / [(200 mg/ L) x l] = 0.650 / [(C) x l]

Or, C = (0.650 / 0.795) x (200 mg/ L)

Or, C = 163.52 mg/L

Thus, concentration of Fe in unknown sample = 163.52 mg/L