Chemistry 11
ANALYSIS OF ZINC TABLETS : PREPARATION
For a class of 24 students (12 pairs)
Chemicals
Chemical
2000 mL of 0.0250 M potassium ferrocyanide
Preparation
Dissolve and dilute 21.12 g of K4Fe(CN)6•3H2O to 2000 mL and
divide among 6–500 mL bottles, labeled “0.0250 M K4Fe(CN)6”.
2000 mL of 0.0500 M zinc sulphate
Dissolve and dilute 28.75 g of ZnSO4•7H2O to 2000 mL and
divide among 6–500 mL bottles, labeled “0.0500 M ZnSO4”.
25 mL of diphenylamine indicator solution
Dissolve 0.25 g of diphenylamine in 12.5 mL of concentrated
sulphuric acid. Slowly, with continual stirring, add the acidic
diphenylamine solution to 12.5 g of ice in a 250 mL beaker.
Care: the mixture can become VERY HOT! Divide among 6
dropping bottles, labelled “Diphenylamine indicator”.
500 mL of 3.0 M sulphuric acid
Carefully dilute 84 mL of concentrated sulphuric acid to 500mL.
Divide among 6–100 mL bottles, labeled “3.0 M H2SO4”.
25 ml of freshly prepared potassium ferricyanide Dissolve and dilute 0.25 g of K3Fe(CN)6 to 25 mL. Divide
among 6 dropping bottles, labeled “1% K3Fe(CN)6”.
36—50 mg zinc tablets
Place in a large petri plate, by the high precision balance.
Equipment
12—50 mL burettes
12—stands
12—burette clamps
12—25 mL pipettes
12—mortars and pestles
12—spatulas
SPECIAL NOTES
1. The zinc sulphate must be both very pure and relatively fresh. ZnSO4•7H2O effloresces, lowering the molar
mass and increasing the moles of Zn2+ contained in 28.75 g. In addition, K4Fe(CN)6•3H2O is slightly
efflorescent and will have less than the calculated number of water molecules per mole. If the ZnSO4•7H2O is
sufficiently fresh and pure but the K4Fe(CN)6•3H2O has decomposed somewhat, the following method can be
used to adjust the amount of K4Fe(CN)6•3H2O used.
The calculated volume of ZnSO4 required to titrate 25.00 mL of 0.0250 M potassium ferrocyanide is
18.75 mL. Assume the solutions require 21.10 mL of ZnSO4 to achieve end point. After the initial titration, re–
make the K4Fe(CN)6•3H2O solution as follows.
mass of K4Fe(CN)6•3H2O to be used = 21.12 g x
18.75 mL
21.10 mL
= 18.77 g
2. The 1% K3Fe(CN)6 is added to oxidize the indicator into its coloured form in the absence of ferrocyanide.
3. If the Zn tablet in Part II contains an abnormally high amount of Zn, the solution may turn blue without the
addition of ZnSO4 . In this case, the student will have to pipet an extra 10.00 mL of K4Fe(CN)6 into the mixture
and adjust the calculations accordingly.