Standardising potassium permanganate solution with iron(II) sulfate

Prepare a standard solution of iron(II) sulfate 10.254 g of FeSO4.7H2O is weighed in a dry beaker.

The powder is dissolved in water and transferred to a 1 The powder is dissolved in water and transferred to a 1.000 L volumetric flask, then made up to volume with distilled water and mixed well.

Prepare the pipette A pipette filler is used to half-fill a pipette with the solution. Rolling the wheel controls the level of liquid in the pipette.

The pipette is rolled and tilted to coat the inside with the Fe2+ solution. Then this liquid is discarded.

Measure out the Fe2+ The rinsed pipette is filled with the Fe2+ solution. The liquid level is adjusted so that the bottom of the meniscus is touching the line on the pipette.

The solution is transferred to a conical flask.

The liquid is allowed to drain out without blowing. The tip of the pipette is touched to the glass side of the flask. One more drop is pulled out by surface tension.

It is convenient to prepare three flasks at once.

Prepare the burette The potassium permanganate to be standardised is transferred to a clean, dry beaker.

Pour a little of the permanganate solution into a burette. Make sure the tap is turned off before you start adding liquid!

Tilt and roll the burette to coat it with permanganate solution.

Discard this liquid. Rinse the permanganate from the sink with tap water.

Fill the burette with fresh permanganate solution. Make sure there are no air bubbles trapped in the tip of the burette. Finally, remove the funnel from the top of the burette, otherwise it may drip.

The titration Permanganate is so strongly coloured that you cannot read the bottom of the meniscus. For this compound we read the level at the top. What is your reading? 2.9 mL

10 mL of dilute sulfuric acid is added. If no acid is added, what would you expect to happen? The MnO4- will be reduced to brown MnO2 instead of colourless Mn2+.

While swirling the flask, slowly add permanganate solution While swirling the flask, slowly add permanganate solution. At first the purple colour disappears. The endpoint is reached with the first drop of purple that does not turn colourless.

Take a note of the burette reading. 13.2 mL So this first titre was 13.2 – 2.9 mL = 10.3 mL

Record the burette reading just before the next titration. 13.7 mL

This time we can run the liquid in quickly until we’ve added about 9 This time we can run the liquid in quickly until we’ve added about 9.5 mL.

Add the permanganate one drop at a time until the final drop which does not turn colourless.

Record the burette reading. 24.05 mL The second titre is 13.65 – 24.1 = 10.35 mL

If you’re not certain you had reached the endpoint, add another drop. This is definitely overshot.

Calculate the concentration of the standard Fe2+ solution M(FeSO4.7H2O) = 278.0 g mol-1 m(FeSO4.7H2O) = 10.254 g V(FeSO4.7H2O) = 1.000 0 L Least accurate data is 4 sig fig, so answer is quoted to 4 sig fig.

Calculate the amount of Fe2+ solution used in each titration c(Fe2+) = 0.03688 mol L-1 V(Fe2+) = 20.0 mL = 20.0  10-3 L The least accurate data is accurate to 3 sig fig, but we will be using this number in the next calculation, so to avoid rounding errors, record it to 4 sig fig – but do not clear the calculator between calculations.

Write the equation for the redox reaction occurring in the titration Purple MnO4- is reduced to colourless Mn2+: MnO4- + 8H+ + 5e- → Mn2+ + 4H2O Therefore Fe2+ has been oxidised. It will form Fe3+: Fe2+ → Fe3+ + e- Overall ionic equation: MnO4- + 8H+ + 5 Fe2+ → Mn2+ + 4H2O + 5Fe3+

Use the equation to find the amount of MnO4- which reacted MnO4- + 8H+ + 5 Fe2+ → Mn2+ + 4H2O + 5Fe3+ n(Fe2+) = 7.376  10-4 mol, n(MnO4-) = ? Unknown on top

Calculate the concentration of the MnO4- solution First run Second run Initial reading 2.9 mL 13.65 mL Final reading 13.2 mL 24.1 mL Titre 10.3 mL 10.35 mL Average titre 10.33 mL n(MnO4-) = 1.475  10-4 mol V(MnO4-) = 10.33 mL = 10.33  10-3 L Note: although we are using 4 sig fig here, the final answer will only be quoted to 3 sig fig, since that is the accuracy of the pipette.

n(MnO4-) = 1.475  10-4 mol V(MnO4-) = 10.33 mL Don’t forget to change mL into litres. Final answer is quoted to 3 sig fig. The concentration of the potassium permanganate solution is 0.0143 mol L-1.