Question

The colorless, water-soluble ligand 1,10-phenanthroline (phen) forms a chelate complex with Fe²⁺. The wavelength maximum is 510 nm. This ligand can be used to determine the concentration of unknown Fe²⁺. The experimental procedure involves treating an iron containing solution with a reducing agent to ensure than all iron is in the +2 oxidation state, then adding excess phen.
An 8.00 ml aliquot of a solution containing 15.00 μg of iron is added to a 100 ml flask and diluted to volume with distilled water. The absorption at 510 nm is recorded as 0.397 using a standard 1 cm cell. In a separate experiment, a 5.00 ml aliquot of an unknown iron solution was also diluted to 100 ml, but the 1 cm cell was being used by some pesky graduate student. Instead, a longer 20 cm cell was used and the absorbance at 510 nm was 0.142. How many μgrams of Fe²⁺ were in the aliquot?

Answer 1

Calculate extinction coeeficient from the first experiment .

8mL aliquot contains 15 micrograms of iron. Concentration = 15/8 microgram/mL = 1.88microgram/mL

= 1.88 *10^-3mg/mL

It is then diluten to 100mL. So, the concentration of the diluted solution is 8 * 1.88 *10^-3mg/mL/100 = 15.04 *10^-5 mg/mL

A = * c *l

= A/c* l = 0.397 / 15.04 *10^-5 mg/mL * 1 cm = 0.026 *10^5

Use the value of in case of the second experiment to calculate the concentration

A = * c *l

c = A/   *l =0.142 / 0.026 *10^5 * 20cm = 0.27 *10^-5 mg/mL

This is the concentration of the 100mL diluted solution.

So, the concentration of the 5mL aliquote is = 100* 0.27 *10^-5/5 = 5.4 *10^-5 mg/mL

5 mL contains 5 * 5.4 *10^-5 mg/mL = 27 *10^-5 mg = 0.27 microgram