Core practical 11: Find the amount of iron in an iron tablet using redox titration

What is a redox titration?

A titration of a reducing agent by an oxidising agent (or vice versa).

What equipment is used to carry out a titration?

● A pipette and pipette filler are used to accurately measure out the volume of a reactant before transferring it to a conical flask.

● A burette is a controlled way to add small volumes of one reactant to the other reactant (until the reaction has reached completion).

How do you carry out a titration?

● Once the pipette has been used to place one reactant into the conical flask, fill the burette with the other reactant. Record initial volume.

● Add a few drops of indicator to the conical flask.

● Open the burette tap and allow the reactant to flow into the conical flask, swirling it to mix the contents.

● Close the burette tap once the expected colour change occurs. Use a white tile so the colour change is easy to identify.

● Record final burette volume.

● Repeat until you get concordant results, then calculate a mean titre.

How do you do a titration with potassium manganate (VII)?

● Add the potassium manganate(VII) solution into the burette.

● The other reactant solution is acidified with dilute H2SO4 and placed into a conical flask.

● The potassium manganate(VII) solution flows into the flask and as it reacts, it becomes colourless. When there is the first trace of a permanent pale pink solution, close the burette tap.

How would you find the amount of iron in an iron tablet experimentally?

● Titrate potassium manganate (VII) solution against acidified (H2SO4 ) crushed iron tablets.

● Calculate mean titre using concordant results

. ● Calculate number of moles of MnO4 - in the mean titre.

● Using stoichiometry calculate the number of moles of Fe2+ reacted and in the original solution

. ● Using this calculate the mass of Fe in solution and mass per tablet.

How would you reduce the uncertainty in this experiment?

● Use a balance with a greater resolution

● Use pipettes and burettes etc. with a greater resolution.

● Use greater masses and volumes so percentage uncertainty is reduced.

What is the weighing by difference technique (used to measure mass of the iron tablets)?

● It is a method to weigh materials accurately.

● Mass of substance = Mass of weighing dish and substance - Mass of dish after substance has been transferred.

y procedural errors

Procedural errors:

● Stirring may not be sufficient to ensure that all the iron dissolves – warming the solution may help.

● Transfer of the solution and filtering – ensure that the beaker and the filter paper are rinsed with water.

● The solution may not be mixed – invert the volumetric flask several times to ensure thorough mixing.

● Glassware measurements may not be read accurately – read glassware marks from the bottom of the meniscus.

● The end-point may not be clear – use a white tile so that you can see the end-point clearly.