Estimation of Iron(II) in an Iron Tablet using a Standard Solution of Potassium Manganate (VII)

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Equipment:

• 0.005M Potassium Manganate (VII) solution
• Iron tablets
• 1.5M sulphuric acid
• Deionised water
• Balance
• Clock glass
• Dropper
• Pestle and mortar
• Retort stand and clamp
• Pipette and filler
• Burette
• 250cm³ volumetric flask
• Conical flask
• White tile
• Beakers
• Graduated cylinder

Experiment:

1. Find mass of 5 iron tablets.
2. Crush iron tablets using pestle and mortar and dissolve in a beaker of 100cm³ sulfuric acid.
3. Transfer solution and rinsings to volumetric flask and make up to calibrated mark. Stopper and invert.
4. Wash pipette, burette and conical flask with deionised water.
5. Rinse burette with potassium mangate (VII) and pipette with iron (II) solution
6. Pipette 25cm³ iron(II) solution into flask with 10cm³ sulfuric acid
7. Fill burette with potassium manganate (VII), making sure below tap is filled.
8. Carry out one rough and two accurate titrations. Colour change purple to pink
9. Calculate concentration of the iron (II) solution

Calculating mass of iron in an iron tablet

Let’s take 0.0175 as molarity of a solution with 1.9g iron (5 tablets)

Step 1

Find moles in 250 ml

0.0175 moles in 1 litre

0.004375 moles in 250 ml

Step 2

Find mass of iron in 250 ml

0.004375 x 56 = 0.245 g

Step 3

Mass of iron in each tablet

0.245g/5 = 0.049 g

Step 4

Find percentage iron in each tablet

Each tablet weighs 1.9/5 = 0.38g

Percentage iron = 0.049 x 100/0.38 = 12.89%

Possible Questions

1. In this experiment why is dilute sulfuric acid used rather than deionised water to dissolve the iron tablets?

If deionised water were used, the Fe²⁺ in the tablets would be almost immediately oxidised to Fe³⁺. The sulfuric acid prevents this occurring