Analysing iron tablets

Experiment 3
Analysing iron tablets
Aim
Iron tablets bought at a pharmacy usually contain iron (II) sulphate (ferrous sulphate), a cheap and soluble form of iron. This experiment will determine the percentage of iron in a sample of this iron salt. Assuming all the iron in the tablets is in the form Fe2+, it is possible to estimate the iron content by titration against potassium manganate (VII), KMnO4.

Equipment list
• 50 cm3 burette
• 25 cm3 pipette with pipette filler
• 250 cm3 volumetric flask
• 250 cm3 conical flask
• 100 cm3 beaker
• Funnel
• White tile
• Burette clamp and stand
Chemicals required
• Pestle
• Mortar
• 1.0 M sulfuric acid
• 0.01 M potassium manganate (VII)
• Iron tablets
• Spatula

Method
Experiment part 1 – preparing the solution (This is prepared by the technician)
1. Accurately weigh 5 iron tablets
2. Use mortar and pestle for crushing the tablets to a powder.
3. Add sulfuric acid and stir to dissolve them in about 100 cm3 of 1.0 moldm-3 sulfuric acid in a beaker. This may require heating gently.
4. Filter the solution into a 250 cm3 volumetric flask, washing out the beaker and filtering the washings into the flask. Make up to 250 cm3 with distilled water.
Experiment part 2 – titration with potassium manganate (VII)
1. Pipette 25 cm3 of the iron solution into a conical flask and add 25 cm3 of 1.0 moldm-3 sulfuric acid.
2. Add the standard solution KMnO4 to burette.
3. Titrate with 0.01 M potassium manganate (VII) solution until three consistent results are obtained. Make note of the EXACT concentration of the potassium manganate (VII) solution when the color change stays for at least a minute or longer.
4. Calculate the mean concentration of the titre values.

Figure 1: Redox titration experimental set-up (adopted from BYJU’s.com)

Safety precautions
• Safety goggles and gloves must always be worn.
• Potassium manganate (VII) is irritant so avoid contact with the skin. If the skin comes into contact, wash immediately.
• Sulfuric acid is corrosive and should be treated with the same precautions as alkalis.
Analyses of results
The results from each titre can be recorded in a table like the one below. More columns may be added until 2 concordant results are obtained:
Rough titre Titre 1 Titre 2 Titre 3
Initial burette volume (cm3) eg. 35.4
Final burette volume (cm3) 6.4
Titre (cm3) 29.0

• Write a balanced redox equation for the reaction.
• Use the mole relationship between iron and manganate ions to calculate the mole of iron and find its mass using the molar mass.
• Determine the total mass of iron in each tablet.
• How does this compare to the value stated on the packet of iron tablets?

Questions
• Iron tablets bought from a pharmacy have an insoluble coating on them. Why is this?
• Why is the solution of iron tablets made with sulfuric acid and distilled water and not just with distilled water?
• Why did this experiment not require an indicator for the titration process?
• Why did the color change from pink to colorless over time?
• Identify the elements that were oxidized and reduced.