Iodine Clock H2O2/I-/H+ why buffer it?

[Miffymycat]

The RSC Classic Chemistry Demos list the method for the hydrogen peroxide iodine clock variant as being buffered at pH5.8 with ethanoic acid/sodium ethanoate. Can anyone explain why it might be useful to buffer a clock reaction in this way? It seems this is for a demo rather than a serious kinetic study, but still seems unusual.
Thanks in advance

 

[Borek]

Take a look at the reaction equation. The less variables, the better.

 

[Miffymycat]

OK, but H+ concentration varies - as does any reactant - as it is consumed. Just don't see why trying to restrict this will improve the outcome in any way - the clock time will be just as reliable based on a non-buffered initial acid concentration ...
Thanks again

 

[Borek]

I don't think it is important for the experiment, but this is often part of some lab in which kinetics data is further analyzed. Then constant [H⁺] makes the description much easier.

 

[Miffymycat]

Yes - agreed. But then why not simply have excess H+, rather than a buffer, as is usually the case when keeping a reactant constant or "isolated" in kinetic studies? Seems a little curious!

 

[Borek]

Excess H⁺ IS a buffer (compare the plot at buffer capacity page).

Then, excess H⁺ means high H⁺ concentration, for obvious reasons it can unnecessarily speed up the reaction, making it harder to observe (and measure the time).

 

[Miffymycat]

Thanks Borek - makes more sense now. So using a classic buffer (ie weak acid + conjugate base here) is an alternative to simply using excess H+, in order to arrive at a reasonable clock time. Very helpful buffer capacity reference - thanks again. (In this reaction, H+ is being consumed rather than formed, so technically, the conjugate base isn't needed ... and no kinetic studies are intended in the RSC method, so no real need to keep [H+] constant ... hey ho, whatever)