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


by Miffymycat
Tags: buffer, clock, h2o2 or i or h, iodine
Miffymycat
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#1
Nov30-13, 11:29 AM
P: 32
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
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Borek
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#2
Nov30-13, 11:49 AM
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Take a look at the reaction equation. The less variables, the better.
Miffymycat
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#3
Nov30-13, 12:18 PM
P: 32
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
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#4
Nov30-13, 01:06 PM
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Iodine Clock H2O2/I-/H+ ... why buffer it?


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
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#5
Dec1-13, 02:49 AM
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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
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#6
Dec1-13, 03:03 AM
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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
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#7
Dec2-13, 04:54 PM
P: 32
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)


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