EDTA as an analytical reagent in classical volumetric analysis Calamine, which i

Question

EDTA as an analytical reagent in classical volumetric analysis Calamine, which is used for the relief of skin burns and irritation, is a mixture of Zno, Fe203 and other ingredients. A 1.022 gram sample of calamine is dissolved in aqueous acid and diluted to the mark in a 250 mLClass A volumetric flask. Aqueous KF is added to form a very stable fluoride complex with Fe aq that is non-reactive with EDTA; under these conditions, when suitably buffered, zn (aq is titrated quantitatively with EDTA. A 10.00 mLaliquor of the de-treated calamine solution is titrated with Y solution, requiring fluori 38.78 mL M Zn. A second 50.00 mL aliquot is used to determine Fe. This is done via a displacement titration; under these conditions, Fes (aq) reacts quantitatively with znY Under these conditions, the Fe consumes 2.40 mL of a 0.002727 MEDTA solution. A. (5 pts) Write a balanced net ionic equation for the reaction representing the displacement titration of Fe3+ (aq) reacts quantitatively with znY2

Explanation / Answer

Calamine

A. Net ionic equation,

Fe3+ + ZnY2- --> FeY- + Zn2+

B. Equilibrium expression

Keq = [FeY-][Zn2+]/[Fe3+][ZnY2-]

Keq = [ZnY2-]/[Zn2+][EDTA]

C. 10 ml aliquot

moles of Zn2+ present = 0.01294 M x 38.78 ml = 0.502 mmol

moles of Zn2+ in 250 ml = 0.502 x 250/10 = 12.55 mmol

mass of ZnO = 12.55 x 81.41/1000 = 1.021 g

percentage of ZnO = 1.021 x 100/1.022 = 99.92%

50 ml aliquot

moles of Fe3+ present = 0.002727 M x 2.40 ml = 6.545 mmol

mass of Fe2O3 = 0.00052 g

Mass percent Fe2O3 = 0.00052 x 100/1.022 = 0.052%

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