Letters to the editor

  • The use of the demonstration “A Colorful Catalyst” by Meridith Rahman and Kenneth Lyle, Chem 13 News, March 2014 (pages 12 and 13) provides a great opportunity for a problem-solving moment with your class.

David Katz* taught me this demo many years ago, and I have used it in my classroom for at least 25 years. After doing the demonstration, ask the class the following questions.

“What is the actual catalyst: the cobalt ions, the chloride ions or the combination of these ions?

How can we find out?

What changes can be made to the demonstration to make that determination?”

Let the students think about it and design a method to test for the actual catalyst.

[To remind readers: A pink-coloured solution of cobalt(II) chloride (the catalyst) is added to a warm mixture of sodium potassium tartrate and hydrogen peroxide (the reaction mixture). After some colour changes, the pink colour returns once the reaction goes to completion.]

A good method would be to repeat the experiment isolating the suspected catalyst ions.

  1. Repeat by adding 0.15 M sodium chloride or potassium chloride to the reaction mixture. Ask students “Why not add calcium chloride?” Point out that no calcium ions are involved in the original demo.
  2. Ask students how to test if the cobalt(II) ion is the catalyst. What compound could be added to the reaction mixture? Then add 0.15 M cobalt(II) tartrate which insures no new extraneous ions are introduced.

If chloride ions increase the reaction rate as in the original demo, then chloride ions act as the catalyst. If the cobalt(II) tartrate increases the reaction, then the catalyst is the cobalt(II) ions. If neither reaction above (1) or (2) increases the original demonstration reaction, then cobalt(II) chloride is the catalyst.

Due to the colour changes students will probably guess correctly that the catalyst is cobalt(II) ions. This gives experimental proof and allows for some scientific exploration.

Cobalt(II) tartrate is available from Alfa Aesar [Johnson-Mathey] chemicals, or you can make some by mixing cobalt chloride with sodium potassium tartrate.

*Note: David Katz has been generating ideas for the chemistry classroom since the 1970s and maintains a website with a variety of labs and activities: www.Chymist.com

Andy Cherkas
Retired teacher, Stouffville ON

  • I enjoyed Meridith Rahman and Kenneth Lyle's piece in the March issue of Chem 13 News. They explained the interesting, colourful, Co2+ ion-catalyzed oxidation of tartrate ions by hydrogen peroxide in aqueous solution. One can get a lot of mileage out of this demonstration, catalyst-wise and reaction intermediate-wise. The Royal Society of Chemistry1 suggests the removal — use a large, plastic Beral pipet — of several mL of the green reaction intermediate, with immediate transfer to a test tube sitting in salt/ice water bath. This slows the decomposition of the intermediate to the point where the green colour sticks around for well over an hour. This facilitates discussion of the temperature dependence of reaction rates, and on freezing point depression.


  1. http://www.rsc.org/Education/EiC/issues/2005July/ Exhibitionchemistry.asp

Michael P. Jansen
Crescent School, Toronto ON