Applied Science BTEC Nationals/Chemical Laboratory Techniques/Iron tablet

To Investigate The Percentage Of Iron In “Iron Tablets.”

The experimental details given here are given in good faith and are believed to be safe and workable methods. However, the authors cannot take responsibility for the consequences of performing these experiments.

The experiments are written for experienced science teaching staff to use as instructions for a supervised class of students. The experiments are not designed for students or inexperienced members of the public to perform without supervision. If you wish to attempt the experiments, ensure that you have completed a legally adequate risk assessment beforehand and that you work within the constraints of the risk assessment.

Discussion.

Iron tablets are prescribed for anaemia. The iron in commercial iron tablets is in the form of Fe²⁺. This can be oxidised to Fe³⁺ by the manganate (VII) ('permanganate') ion. This is the reaction which will form the basis of our titration.

MnO₄^- + 8 H⁺ + 5 e^- → Mn²⁺ + 4 H₂O

Fe²⁺ → Fe³⁺ + e^-

5 Fe²⁺ + MnO₄^- + 8 H⁺ → 5 Fe³⁺ + Mn²⁺ + 4 H₂O

First we need to prepare a standard solution of sodium ethanedioate and titrate it with the given potassium manganate (VII) solution to determine the concentration of the manganate (VII) solution

Apparatus

beakers, 250 cm³

weighing bottles

burettes, 50 cm³

measuring cylinder, 25 cm³

pipettes, 25 cm³

thermometer

volumetric flasks, 250 cm³

Bunsen burner, tripod & gauze

conical flasks, 250 cm³

Reagents

potassium manganate (VII) solution

sodium ethanedioate

sulphuric acid

iron tablets

Precautions

Wear eye protection and laboratory coat at all times. Tie back long hair when using Bunsen burners.

Standard Solutions Procedure.

1. Weigh out accurately 1.65 - 1.70 g of sodium ethanedioate Na₂C₂O₄ into a 250 cm³ beaker by the following procedure:

a) Place a weighing bottle on the top-pan balance and tare the balance.

b) Add the required quantity of salt.

c) Now weigh the bottle plus salt on an analytical balance, tip the salt into the beaker and weigh the bottle "empty."

2. Add about 100 cm³ de-ionised water and stir the mixture to dissolve the solid. Transfer this solution to a 250 cm³ volumetric flask and make up to the mark with de-ionised water, observing all the normal precautions for making up a standard solution.

3. Pipette 25cm3 aliquots into a 250 cm³ conical flask and add about 25 cm³ dilute sulphuric acid.

4. Heat the mixture to about 70 °C and titrate with manganate (VII) solution until a faint permanent pink colouration is obtained.

5. Repeat twice more to obtain accurate titrations.

Calculations

1. Use your weighings to determine the mass of sodium ethanedioate used to make up the solution and hence determine its concentration.

2. The fully balanced equation for the standardisation titration is:

5 C₂O₄^2- + 2 MnO₄^- + 16 H⁺ → 10 CO₂ + 2 Mn²⁺ + 8 H₂O

Use this equation to determine the concentration of the manganate (VII) solution used in this experiment.

Questions

1. Suggest a reason why the titration flask is heated to 70 °C before the titration is carried out.
2. Write out the half ionic equations for the reactions and identify, giving your reasons, the oxidising and reducing agents.
3. This particular reaction is said to proceed by a process of "autocatalysis". Explain what this term means.

Iron Tablet Analysis Procedure.

1. Accurately note the mass of one iron tablet.
2. Crush this tablet to a fine powder in a mortar with a pestle.
3. Rinse all of the crushed tablet into a conical flask and add about 25 cm³ of dilute sulphuric acid. Add more water and / or warm if you experience difficulty in getting the tablet to dissolve completely.
4. Titrate this solution with the standard potassium manganate (VII) solution in as accurate a manner as possible until a pink colour is obtained.
5. From your result, decide if have obtained a suitable titre. Do you need to use more than one tablet or use a dilute solution as the equivalent of using less than one tablet?
6. Repeat the titration accurately in the chosen manner.
7. For each titration, calculate the mass of iron in one tablet.
8. Calculate the percentage mass of iron in a tablet
9. Compare your values with other members of the group and find average values.

Questions.

1. Compare and contrast this technique with that used in the previous titrations. Consider carefully the sources of errors and mistakes in the two techniques.
2. Describe briefly how this experiment would be used in an industrial or commercial context.