EDTA as an analytical reagent in classical volumetric analysis Calamine, which i
ID: 1068246 • Letter: E
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, Fe2O3 and other ingredients. A 1.022 gram sample of calamine is dissolved in aqueous acid and diluted to the mark in a 250 mL Class A volumetric flask. Aqueous KF is added to form a very stable fluoride complex with Fest (aq) that is non-reactive with EDTA; under these conditions, when suitably buffered, zn (aq) is titrated quantitatively with EDTA. A 10.00 mL aliquot of the fluoride-treated calamine solution is titrated with Y solution, requiring 38.78 mL of 0.01294 M Y to titrate the Zn. A second 50.00 mL aliquot is used to determine Fe. This is done via a displacement titration; under these conditions, 3+. (aq) reacts quantitatively with znY Under these conditions, the Fe Fe consumes 2.40 mL of a 0.002727 M EDTA solution. A. (5 pts) Write a balanced net ionic equation for the reaction representing the displacement 3+. titration of Fe (aq) reacts quantitatively with ZnYExplanation / Answer
A. Balanced reaction for displacement reaction
Fe3+ + ZnY2- <==> FeY- + Zn2+
B. Displacement reaction,
Fe3+ + ZnY2- <==> FeY- + Zn2+
So for the reaction equilibrium constant becomes,
Keq = [products]/[reactants]
Keq = [FeY-][Zn2+]/[Fe3+][ZnY2-]
C. percentage of ZnO and Fe2O3 in the sample
For Zn2+
Y used = 0.01294 M x 38.78 ml = 0.5018132 mmol
moles of ZnO in 250 ml solution = 0.5018132 x 250/10 = 12.54533 mmol
mass of ZnO = 12.54533 x 81 = 1.0212 g
Percent ZnO in sample = 1.0212 x 100/1.022 = 99.92%
for Fe3+
EDTA used = 0.002727 M x 2.40 ml = 6.5448 x 10^-3 mmol
moles of Fe3+ in original solution = 6.5448 x 10^-3 x 250/5 = 0.032724 mmol
mass of Fe2O3 in sample = 0.032724 x 159.69/2 = 0.002612 g
percent Fe2O3 in sample = 0.08%
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