Determination of the Iron Content of Iron Supplements

Introduction

In this lab, you will use the oxidation of Fe²⁺ to Fe³⁺ by the permanganate ion, MnO₄^-, to determine the amount of iron in iron supplement tablets. The manufacturer specifies the amount and type of iron in each tablet. Is this an average amount? How much variation is there from tablet to tablet? How much of the variation is due to your experimental technique?

Preparation of the Potassium Permanganate Solution

Weigh about 1.6 g of KMnO₄ into a 1 L beaker and dissolve in 700 mL of DI water. Heat the solution close to boiling for 1 h. Cool and store in a cabinet in the dark overnight. Remove MnO₂ by filtration over a fritted funnel. Place the solution in a container and always store in the dark.

Standardization with Sodium Oxalate

Heat about 2 g of sodium oxalate in an oven for at least 1 h at 110 degrees C. Place the hot sample in a dessicator to cool (hint: do this the lab period before starting the lab). Accurately weigh 0.2-0.3g samples of the sodium oxalate into 400 mL beakers and dissolve each in ~250 mL of 1 M sulfuric acid. Heat each solution to 80-90 degrees and titrate with the potassium permanganate. Keep a thermometer in the solution at all times and do not let the solution temperature drop below 80 degrees. Perform three titrations and determine the potassium permanganate concentration.

Titration of the Iron Supplement Tablets with Potassium Permanganate

Crush a tablet with a mortar and pestle and transfer the contents to a beaker. Rinse the powder remaining on the mortar and pestle with 1 M sulfuric acid into the beaker. Add 1 M sulfuric acid to the beaker up to ~ the 40 mL mark. Titrate this solution with the potassium permanganate to the end point. You need to add the potassium permanganate solution dropwise to ensure an accurate titration. The formation of a brown color in the solution indicates rapid addition. You should have an idea of how much potassium permanganate solution will be needed for the titration before you begin. Check at least 5 iron tablets. Indicate the average, the spread, and the consistency in the iron content of the tablets.

Titration Colors

This experiment was developed by Michael R. Jordan at Oklahoma Baptist University. Reproduction by printing and photocopying for instructional use by educational institutions is permitted.