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Decomposing hydrogen peroxide

This Project Page first appeared in the September 1995 issue of Chemistry Review, Volume 5, Number 1, Page 30.

NOTE: Project Page is designed to help you think about your investigation. It is not intended to be a set of instructions for practical work and does not include a list of safety precautions. CHEMISTRY REVIEW accepts no responsibility if Project Page is used in any way as a set of instructions.

The catalytic decomposition of hydrogen peroxide provides a range of project opportunities of varying length and complexity. This is because of the variety of catalysts that will increase the rate of decomposition and the methods that can be used to monitor the reaction:

2H₂O₂ → 2H₂O + O₂

Using an enzyme catalyst

In many living organisms hydrogen peroxide is a product of metabolism that must be broken down, since in appreciable concentrations it is toxic. The rate of decomposition is increased by the intra-cellular enzyme catalase.

As a fairly simple project you could compare the effectiveness of the enzyme from different sources such as potato, celery and liver. This might lead you on to a more thorough investigation of other factors that affect the activity of the catalase, including, perhaps, temperature, pH, concentrations of substrate and enzyme and the presence of enzyme inhibitors.

Such experiments may at first sight seem quite straightforward, but as you try to interpret your results you will be led into quite complex, but very interesting, aspects of chemical kinetics and reaction mechanisms.

Using an inorganic catalyst

As an interesting contrast, a similar increase in the rate of decomposition of hydrogen peroxide can be achieved using an inorganic catalyst such as manganese(IV) oxide or lead(lV) oxide.

You could set out to look at how factors such as concentration of the peroxide, amount of the catalyst and temperature affect the rate of the decomposition. Experiments of this kind could lead you towards a possible reaction mechanism and towards the calculation of the activation energy involved.

Contact lens chemistry

The catalytic decomposition of hydrogen peroxide will be very familiar to some students, as it is an integral part of one system for cleaning contact lenses. In this method the l8 December, 2008 clean it and then the remaining solution is decomposed with the help of a platinum-coated catalyst. You might like to investigate how the effectiveness of this catalyst changes under different conditions and compare it with the other catalysts.

Monitoring the decomposition

There are several ways in which you can monitor the decomposition reaction. You might decide to follow the fall in concentration of the hydrogen peroxide. You can do this by taking samples of the reaction mixture at different times and running them into an acidified solution of potassium iodide. The iodine produced can be titrated with a solution of thiosulphate using starch indicator:

2H⁺ + H₂O₂ + 2I^– → I₂ + 2H₂O

I₂ + 2S₂O₃2^– → 2I^– + S₄O₆ ^2–

The volume of thiosulphate solution required for the titration provides a measure of the amount of hydrogen peroxide in the reaction mixture at the different times. As an alternative you might prefer to measure the volume of oxygen produced during the decomposition of the hydrogen peroxide. You can do this by connecting a glass gas syringe to a Buchner flask or Hirsch tube.

There is also another, more novel, approach to monitoring the enzyme-catalysed reaction which you might like to try. First, prepare a catalase extract. Next, use a file paper hole punch to stamp out circles from a filter paper. Soak one of these circles in the enzyme extract, drain, then use a glass rod to poke it to the bottom of a test tube containing the hydrogen peroxide solution. The bubbles of oxygen generated by the decomposition of the peroxide stick to the filter paper circle and carry it to the liquid surface. The time taken for this to happen provides a surprisingly accurate measure of the relative activity of the enzyme under different conditions.

Project tips

A catalase extract can easily be produced by macerating equal amounts of potato or other enzyme source with water in a food processor or blender. Alternatively, you can remove a cylinder of potato with a cork borer and slice off discs of equal thickness.
20 volume hydrogen peroxide is 6% weight/volume or 1.8 mol dm^-3. (Chlorine-free bleach works just as well!)
5 cm³ portions of partially decomposed 1 volume hydrogen peroxide can be added to a mixture of 4 cm³ of 2 mol dm^-3 sulphuric acid and 3 cm³ of 10% potassium iodide solution. Add 1 drop of 3% ammonium molybdate solution and let the mixture stand for 2 minutes. Titrate the iodine released with 0.1 mol dm^-3 sodium thiosulphate solution using 2 cm3 of 1% starch solution as indicator.
A glass syringe sometimes sticks. You can minimise this problem by tapping it continually with a pencil.
Platinum-coated discs are part of the ‘Septicon’ system for cleaning contact lenses. They are available from Boots chemist shops.

The original article was written by Derek Denby. We are grateful to Derek for allowing us to reproduce it here.

This page is free for your personal use, but the copyright remains with Philip Allan Updates. Please do not copy it or disseminate it in any way.

Chemistry Review is indebted to Don Ainley, who has helped to prepare this article for the Web.