2009-11-23T09:53:00.002-08:00

Briggs-Rauscher oscillating reaction

I did not expect to write my first entry about this topic. Indeed, this topic is complicated. I don't know whether I can manage to finish it. Correct me if I got any mistake. (Grammar mistake included)

Briggs-Rauscher oscillating reaction is a well-known oscillating clock reaction, though Briggs and Rauscher are not, and I have limited information about them, if anyone have something more about them, please send me a message.

Briggs–Rauscher oscillating reaction is a oscillating clock reaction, implies that it carry both Oscillating and Clock reaction properties.

Clock reaction is a reaction in which one of the reagents has a visible color, crossing a concentration threshold can lead to an abrupt color change, which may be used to tell time. (Definition from Wikipedia)

Oscillating reaction is a series of reaction, in which the concentration of products and reactants of oscillatory chemical systems can be approximated in terms of Damped oscillations. (In simple term)

The above graph gives an idea on how the concentration of Iodide ion varies with time,this Oscillogram was made in July 1972 by Briggs and Rauscher.

To investigate this experiment, one should know the initial conditions.

The initial aqueous solution contains hydrogen peroxide, an iodate, divalent manganese (Mn2+) as catalyst, a strong chemically unreactive acid, eg. sulfuric acid, and an organic compound with an active hydrogen atom attached to carbon which will slowly reduce free iodine (I2) to iodide (I−), Malonic acid (IUPAC Propanedioic acid) usually servers this purpose. These chemicals mixed sequentially and will be discussed later.

This reaction reacts with limited colour change, hence a indicator with abrupt colour change is needed. In this case, Starch is added as an indicator to show the abrupt increase in Iodide ion concentration as a sudden change from amber (free iodine) to dark blue ("Iodine-starch complex", which requires both Iodine and Iodide)

This reaction shows periodic change in colour, sudden change from amber to dark blue, gradual change from dark blue to amber.

Procedure of this reaction is not very complicated, it requires mix of solution only. However, please be reminded that, Hydrogen peroxide is added at the last moment, as everything will be triggered when Hydrogen peroxide is added.

This reaction may include but not limited to above chemicals. A wide range of chemicals will work, Organic compound and Acid are common variants. To study Oscillating Clock reaction in a deeper sense, throughout experiments along different factors, such as Organic/Acid substrate, Concentration, temperature, etc, should be investigated.

On the Chemical mechanism, this reaction can be generally simplified to two process. A "non-radical" process A and a "Radical" process B.
A "non-radical process": The slow consumption of free iodine by the malonic acid substrate in the presence of iodate. This process involves the intermediate production of iodide ion.

B "radical process": A fast autocatalytic process involving manganese and free radicalintermediates, which converts hydrogen peroxide and iodate to free iodine and oxygen. This process also can consume iodide up to a limiting rate.

IO₃^- + 2 H₂O₂ + H⁺ --> HOI + 2 O₂ + 2 H₂O

This reaction can occur by a B radical process only when I^- concentration is low, or by a nonradical process when the I^- concentration is high. Both processes reduce iodate to hypoiodous acid. The radical process forms hypoiodous acid at a much faster rate than the nonradical process.

HOI + CH₂(CO₂H)₂ --> ICH(CO₂H)₂ + H₂O

Where Malonic acid slowly consume free Iodine, hence the reaction forms a loop.

Process A slowly generates the intermediate iodide ion out of the free iodine at an increasing rate proportional to its (i.e. I₂) concentration.

At a certain point, Concentration of Iodide overwhelms process B, stopping the production of more free iodine, which is still being consumed by process A. Thus, eventually the concentration of free iodine (and thus iodide) falls low enough for process B to start up again and the cycle repeats as long as the original reactants hold out.

To summarize,

The amber color results from the production of the I₂. The I₂ forms because of the rapid production of HOI during the radical process. When the radical process is occurring, HOI is created faster than it can be consumed. Some of the HOI is used while excess is reduced by hydrogen peroxide to I^-. The increasing I^- concentration reaches a point at which the nonradical process takes over. However, the nonradical process does not produce HOI nearly as fast as the radical process, so the amber color begins to clear as I₂ is consumed more quickly than it can be created. Eventually the I^- concentration drops low enough for the radical process to restart so the cycle can repeat itself.
It repeats as long as the original reactants is fully consumed.

Reference:
en.wikipedia.org/wiki/Briggs-Rauscher_reaction
en.wikipedia.org/wiki/Iodine_clock_reaction
en.wikipedia.org/wiki/Chemical_clock
en.wikipedia.org/wiki/Malonic_acid
chemistry.about.com/cs/demonstrations/a/aa050204a.htm

I am not so confidence that everything is correct, I just express and summarize what I understand from the refernce and my experience.

C for Chemistry