4
$\begingroup$

I am trying to complete the Bray-Liebhafsky reaction in the lab so I can use it in the future for demonstrations.

The above Wikipedia link quote:

A fundamental property of this system is that hydrogen peroxide has a redox potential which enables the simultaneous oxidation of iodine to iodate:

$$\ce{5 H2O2 + I2 -> 2 IO3− + 2 H+ + 4 H2O}$$

and the reduction of iodate back to iodine:

$$\ce{5 H2O2 + 2 IO3- + 2 H+ -> I2 + 5 O2 + 6 H2O}$$

Between these two reactions the system oscillates causing a concentration jump of the iodide and the oxygen production. The net reaction is:

$$\ce{2 H2O2 -> 2 H2O + O2}$$

necessitating a catalyst and $\ce{IO3-}$.

I am trying to set it up in a very simple manner, in a beaker with a stirrer and hotplate. From what I have seen from reading numerous papers it should just need a heated solution of sulfuric acid and potassium iodate and then be mixed with another solution of hydrogen peroxide and the reaction should proceed.

However, in my attempts so far I have been unable to get the desired colour changes. How do you run this reaction?

There seems to be lots of information on how to run both the BZ and BR oscillating reactions but I have been unsuccessful in finding meaningful information for this one.

I am wondering if I need an indicator to be able to visibly see the oscillations?

UPDATE: Current experimental procedure. a 50 mL solution of 0.125M potassium iodate and 0.24M sulfuric acid was made and heated to 65 degrees in an oil bath. After half an hour 1.3 mL of 30% w/w hydrogen peroxide was added.

I have also tried other varying concentrations of iodine/sulfuric acid and hydrogen peroxide with no success as yet also I have done this at temperatures of 25, 50 and 60 degrees.

$\endgroup$
10
  • 1
    $\begingroup$ I suspect there is no colour change without some indicator that shows strong colour change when iodine is present. $\endgroup$
    – matt_black
    May 3, 2023 at 11:15
  • $\begingroup$ Try different starting points, different ratios, different mixing order. $\endgroup$
    – Poutnik
    May 3, 2023 at 11:41
  • 1
    $\begingroup$ The reaction equally is known as iodine-clock. Did you venture out references like wikipedia, or -- because it is appealing -- in the Journal of Chemical Education (some of the hits here may address it in detail)? $\endgroup$
    – Buttonwood
    May 3, 2023 at 14:03
  • $\begingroup$ @Buttonwood I will look into it thankyou. I was under the impression that the iodine clokc was slightly different as it uses sodium thiosulfate. However, I notice they use starch as an indicator. Perhaps I can do that with this one. $\endgroup$ May 4, 2023 at 1:26
  • 1
    $\begingroup$ @HeathDimsey Starch is notoriously known indicator for free iodine, forming intense dark blue even for small iodine concentrations. Search for practical preparation, as it may be tricky $\endgroup$
    – Poutnik
    May 4, 2023 at 6:49

1 Answer 1

1
$\begingroup$

In a paper by 2017, Cervellati and Greco provide some context about said discovery. Bray's first publication by 1921 for research up to 1917/18 already describes how the initial concentration of sulfuric acid affects the rate of reaction:

enter image description here

enter image description here

Normality (as e.g. $\pu{0.0916 N}$ about trace III in figure 1) is a description of concentration no longer used (for quite some time, actually). In contemporary terms – because sulfuric acid can provide two protons – a solution of $\ce{H2SO4} (\pu{2 N})$ is a solution of $\ce{H2SO4} (\pu{1 M})$, or $\pu{1 mol/L}$. Within this context, the initial concentrations about trace III in figure 1 are: $\ce{H2O2}$ ($\pu{190 mmol/L}$), $\ce{KIO3}$ ($\pu{94 mmol/L}$), and aqueous $\ce{H2SO4}$ ($\pu{45.8 mmol/L}$).

(1) Cervellati, R.; Greco, E. Periodic Reactions: The Early Works of William C. Bray and Alfred J. Lotka. J. Chem. Educ. 2017, 94, 195–201. doi 10.1021/acs.jchemed.6b00342.

(2) Bray, W. C. A periodic reaction in homogeneous solution and its relation to catalysis. J. Am. Chem. Soc. 1921, 43, 1262–1267. doi 10.1021/ja01439a007. There equally is an open access copy of this seminal paper on zenodoo (link).

$\endgroup$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.