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Practical chemistry for exams›CP 11 - Redox titration

CP 11 - Redox titration

An overview of CP 11 - Redox titration

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In Edexcel Core Practical 11: Redox titration, the iron(II) content in iron tablets is determined using a redox titration with potassium manganate(VII)

In Edexcel core practical 11: Find the amount of iron in an iron tablet using redox titration, there are several important safety requirements to be aware of (beyond normal lab practice such as wearing goggles).

1.5 mol dm⁻³ sulfuric acid is an irritant.

A step-by-step diagram titled 'EXPERIMENTAL METHOD EDEXCEL CORE PRACTICAL 11' with six numbered steps. Step 1: An image of a pestle and mortar crushing five iron tablets. Text: 'Crush five iron tablets in a pestle and mortar.' Step 2: Shows a weighing boat on a scale, a beaker, and another weighing boat on a scale. Text: 'Weigh the crushed tablets in a weighing boat and transfer to a small beaker. Reweigh the boat, and record the difference.' Step 3: A bottle labeled '1.5 mol dm⁻³ sulfuric acid' pouring into a beaker with crushed tablets, forming a solution. Text: 'Dissolve the crushed tablets in 100 cm³ of 1.5 mol dm⁻³ sulfuric acid, stirring to aid dissolution. This forms an acidified solution of iron sulfate.' Step 4: A funnel pouring the acidified solution into a 250 cm³ volumetric flask, with distilled water added to the 250 cm³ mark. Text: 'Filter the solution into a 250 cm³ volumetric flask, rinsing the beaker with sulfuric acid and adding this to the flask. Make up the solution to the 250 cm³ mark with deionised water. Stopper the flask and invert to mix.' Step 5: A titration setup with a burette labeled 'Manganate (VII) solution' and a conical flask labeled 'Iron(II) solution.' Text: 'Prepare titration apparatus. Pipette 25.0 cm³ of the iron(II) solution into a conical flask. Titrate with potassium manganate (VII) solution, swirling until a permanent pink color appears then stop immediately.' Step 6: A table with columns labeled 'Rough,' '1,' '2,' and '3,' and rows labeled 'Initial reading (cm³),' 'Final reading (cm³),' and 'Titre (cm³).' Text: 'Record the volume of potassium manganate(VII) used. Repeat until concordant results are obtained.'

Avoid unintentional dilution of solutions.

A laboratory pipette with a graduated scale on the side, positioned next to a container labeled NaOH, indicating the presence of sodium hydroxide.

Do

Rinse all glassware used for accurately measuring solution volumes with the solution itself.

A pipette is shown with a container above it labeled 'Distilled water'.

Don't

Rinse precision glassware with water immediately before use – leftover water will dilute the solution.

Prevent parallax errors when reading volumes.

An illustration showing a measuring cylinder with a scale marked from 20 to 25 ml. An eye is depicted on the left side, and a green arrow points to the measurement of 22.14 ml.

Do

Read the burette at eye level and record to

A measurement scale showing values from 20 to 25 ml. Two eyes are illustrated, one pointing to 22.26 ml and the other to 22.02 ml, with red arrows indicating the measurements.

Don't

Read from above or below – this leads to incorrect readings due to parallax error.

Keep the titration mixture well mixed.

A hand holding a conical flask containing a pink liquid, with arrows indicating swirling motion, and a dropper above the flask.

Do

Swirl the conical flask gently and consistently during titration to ensure complete contact between reactants.

A laboratory setup featuring a conical flask with a pink liquid inside, topped with a pipette holder and a blue pipette.

Don't

Leave the flask still during titration – without swirling, the acid and alkali will not mix properly, leading to an inaccurate endpoint.

Combining the two half equations gives the following redox equation:

Determine the ratio of moles of manganate ions that react with iron ions.

Using the average titre calculate the moles of manganate ions used in the titration using the equations:

Calculate the amount in moles of iron in the 25 cm3 sample of iron by looking at the stoichiometric relationship:

Calculate the amount in moles of iron ions in the 250 cm3 graduated flask from the start, by multiplying by 10.

Then calculate the mass of iron in the original five tablets using the equation

Divide this answer by five to calculate the mass of iron in one iron tablet.