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

In summary, the RSC Classic Chemistry Demos use a buffered hydrogen peroxide iodine clock variant at pH 5.8 with ethanoic acid/sodium ethanoate. This is for a demo rather than a serious kinetic study, and the purpose of buffering the reaction is to restrict the variation in H+ concentration, making the clock time more reliable and easier to analyze. Excess H+ can also act as a buffer, but using a classic buffer is an alternative to ensure a reasonable clock time. The conjugate base is not necessary in this reaction as H+ is being consumed rather than formed, and there is no need to keep [H+] constant in the RSC method.
  • #1
Miffymycat
47
0
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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  • #2
Take a look at the reaction equation. The less variables, the better.
 
  • #3
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
 
  • #4
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.
 
  • #5
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!
 
  • #6
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).
 
  • #7
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)
 

1. What is the Iodine Clock reaction?

The Iodine Clock reaction is a chemical reaction that involves the oxidation of iodide ions (I-) by hydrogen peroxide (H2O2) in the presence of an acid (H+). It is called the "Iodine Clock" because the reaction produces a sudden and dramatic change in color, similar to a clock ticking.

2. Why is a buffer used in the Iodine Clock reaction?

A buffer is used in the Iodine Clock reaction to maintain a constant pH level. This is important because the reaction rate is highly dependent on the acidity of the solution. By using a buffer, the pH remains constant and the reaction can be accurately timed.

3. How does a buffer work in the Iodine Clock reaction?

A buffer works by neutralizing any added acid or base, thereby maintaining the pH level. In the Iodine Clock reaction, the buffer prevents the pH level from changing too much, which could affect the rate of the reaction and produce inaccurate results.

4. Can any buffer be used in the Iodine Clock reaction?

No, not all buffers are suitable for the Iodine Clock reaction. The buffer must have a pH close to the desired reaction pH and should not react with any of the other chemicals involved in the reaction. Common buffers used in this reaction include phosphate and acetate buffers.

5. What happens if a buffer is not used in the Iodine Clock reaction?

If a buffer is not used, the pH level of the reaction may change due to the addition of acid or base, which can alter the rate of the reaction. This can result in inaccurate results and make it difficult to determine the exact timing of the color change, which is the basis of the Iodine Clock reaction.

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