Redox and redox equilibria
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I am investigating the rate of reaction between manganate(VII) and oxalate ions. I am using the colorimeter. It takes time to put the solution into a cuvette. How could i make this accurate? Could I start recording the absorbance after 20 s for each solution to make it fair? I was also wondering how I would investigate the autocatalytic effect. I know I would use MnO, but what do I do with it - vary the concentration maybe?
091008
Corrie writes .....
I would just standardise your procedure and then start timing once the mixture in the cuvette is placed in the colorimeter. It makes it easier - and ensures thorough mixing - if you measure out the manganate(VII) solution and the oxalate solution into separate test-tubes, then pour the one into the other, shake, pour into the cuvette and place it into the colorimeter. You should be able to make the mixing procedure repeatable to within a second or two, and that should be good enough. The uncatalysed reaction is very slow to start with at room temperature, speeding up as the catalyst, Mn2+(aq), (one of the products) is formed. MAKE SURE you always use very clean test-tubes and cuvettes for each run, because any traces of the previous mixture will contain some of this catalyst.
You could investigate the autocatalytic effect by adding small amounts of Mn2+ ions to the mixture at the start. For this you would have to use a solution of a soluble Mn(II) salt, such as the sulphate. My experience is that this can speed up the reaction so much that it is best added - a drop or two - to the mixture already in the colorimeter cuvette, quickly shaken and then placed in the colorimeter.
As I have said in the answer to a previous question, MnO would not be suitable as a catalyst here as (i) it is not one of the products under acid conditions, (ii) not a common oxide of Mn, therefore unlikely to be found in schools - MnO2 is the one usually found in schools, and (iii) it is insoluble, so difficult to make up a solution!
This is a complicated reaction. This A-level website
http://www.docbrown.info/page03/ASA2rates.htm
discusses the reaction between manganate(VII) ions and oxalic acid/oxalate in more detail.
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updated: 10 October 2008
