12 – Determining the Percentage of Elemental Iron in an Iron Tablet (LC 2027) (Leaving Cert Chemistry): Revision Notes

12 – Determining the Percentage of Elemental Iron in an Iron Tablet

Introduction and purpose

This experiment allows you to measure the actual amount of iron present in commercial iron tablets and compare your results with the manufacturer's stated values. Iron tablets are commonly used as dietary supplements, and it's important to verify that they contain the advertised amount of elemental iron.

The experiment uses volumetric analysis (titration) to determine iron content. You'll dissolve iron tablets in acid, convert the iron to a measurable form, and then titrate against a standardised potassium permanganate solution.

Background theory

Iron tablets contain iron compounds that must be dissolved and converted to iron(III) sulphate (Fe₂(SO₄)₃) for analysis. When dissolved in dilute sulfuric acid, the iron compounds react to form a solution containing iron ions.

The key principle involves a redox reaction between iron compounds and potassium permanganate (KMnO₄). Potassium permanganate acts as an oxidising agent and must be standardised immediately before use, as it decomposes when exposed to light and heat, causing its concentration to change over time.

Equipment and apparatus

The experimental setup includes:

• Volumetric flask for accurate dilution
• Burette for precise measurement of titrant
• Conical flask for the titration
• Pipette for accurate volume transfer
• Mortar and pestle for tablet preparation

Key experimental procedure

Tablet preparation and dissolution

• Weigh five iron tablets using a clock glass and laboratory balance to determine total mass
• If tablets are in capsule form, empty the contents onto the clock glass and record the mass of the contents from all five capsules
• Grind the tablets using a mortar and pestle to increase surface area and help the iron sulphate dissolve more quickly
• Add some dilute sulfuric acid to the mortar to make grinding easier
• Transfer the ground mixture to a beaker containing some dilute sulfuric acid, taking precautions to ensure complete transfer

Solution preparation

• Stir the solution thoroughly to dissolve the iron sulphate in the paste
• Transfer to a volumetric flask and dilute the solution to the graduation mark with deionised water
• Invert the flask approximately 20 times to ensure thorough mixing
• Clean all glassware (pipette, burette, conical flask) with deionised water

Titration setup and procedure

• Fill the burette with potassium permanganate solution using a funnel, then remove the funnel
• Adjust the solution level so the meniscus sits exactly at the zero mark
• Check for air bubbles in the tap and space below the tap, ensuring they're filled with permanganate solution
• Transfer a measured volume of the iron tablet solution to a clean, dry beaker using a pipette philtre
• Wash the pipette with a small amount of this solution, then fill to the mark with the iron tablet solution and transfer to the conical flask
• Add dilute sulfuric acid in excess to the conical flask using a graduated cylinder
• Carry out the titration in the usual manner until a faint permanent pink colour appears at the endpoint
• Perform multiple titrations - one rough and two accurate titrations for reliability

Observations and explanations

Expected observations

You may notice that the solution appears cloudy rather than completely clear during the experiment.

Scientific explanation

The cloudiness occurs because iron tablets often have an outer coating that doesn't dissolve easily in the acid solution. This coating is designed to slow down the dissolution rate in the stomach, reducing the chance of stomach upset.

Calculations methodology

To determine the percentage of iron in each tablet, you need to work through several calculation steps that connect your titration results to the actual mass of iron present.

The detailed calculation method involves using the balanced chemical equation for the redox reaction and the concentration of the standardised permanganate solution.

Important practical considerations

Safety and technique points

• Handle dilute sulfuric acid with appropriate care and safety equipment
• Ensure complete transfer of tablet material to avoid losing iron content
• Standardise permanganate immediately before use to ensure accurate concentration
• Work efficiently but carefully to minimise errors in measurement
• Record all measurements accurately, including initial masses and titration volumes

Remember!