Question

The standard addition method for he trace spectrophotometric analysis of iron is based on dilution of the sample to 500 ml in volumetric flask. two 25 ml samples of this solution are taken. A known volume of ferrous iron is spiked into one of these two 25 ml samples. The final concentration of the ferrous iron standard in the 25 m, sample is 5 mM. A Fe(phen),2+ complex is developed and sufficient water is added to both solution to give each solution a otal volume of 50.0ml the absorbance of each is measured in a 1 cm cuvette at 508nm and is found to be 0.10 for the sample containing only the unknown and 0.60 fr the ferrous iron spiked unknown sample. If a 5.000 g sample is analyzed using this protocol, calculate the % iron (atom wt 55.84g/mol) in the original sample.

Answer 1

Method of standard addition : Analysis of iron in sample

concentration of standard in 50 ml solution (Cs) = 5 mM x 25 ml/50 ml = 2.5 mM

concentration of Fe in unknown sample = Cx

absorbance of sample without standard (Ax) = 0.10

absorbance of sample with standard (Ax + As) = 0.60

So using,

Cx/(Cx + Cs) = Ax/(Ax + As)

Cx/(Cx + 2.5) = 0.1/0.6

0.6Cx - 0.1Cx = 0.25

Cx = 0.25/0.5 = 0.5 mM

concentration of iron in 25 ml solution = 0.5 mM x 50 ml/25 ml = 1.0 mM

concentration of iron in 500 ml solution = 1.0 mM x 500 ml/25 ml = 20 mM = 0.02 M

mass of iron in sample = 0.02 mM x 0.5 L x 55.84 g/mol = 0.5584 g

% iron in the original sample = 0.5584 g x 100/5 g = 11.168%