Today we're going to be talking about the Briggs-Raucher reaction.
This reaction is quite unique because it's one of the very few oscillating chemical reactions that we know of
Three colourless solutions are mixed,
and it slowly becomes darker and darker yellow,
until it suddenly turns black.
The black colour slowly fades and the solution momentarily reverts back to being colourless
it then starts to become yellow again and the cycle repeats itself several times
it's similar to an iodine clock reaction, where a colourless solution quickly turns black.
However the iodine clock clock doesn't oscillate
and the reaction is over once it changes colors.
there's another famous oscillating
reaction known as the Belousav Zhabotinsky
reaction and i plan to
cover it in a future video
this reaction we need to make three
separate solutions and these are all the
ingredients that I used.
For solution A, I use potassium iodate and concentrated sulfuric acid
concentrated sulfuric acid is normally colorless but mine got a
little bit dirty so it has this yellow
tint. Although it's discolored it's still
quite pure and it will work fine for this reaction.
For solution B, I used cornstarch malonic acid and manganese sulfate.
Some guides for this reaction
claim that the malonic acid can be
replaced by acetone but I tried this and
it didn't seem to work.
For solution C, all I need is some 30% hydrogen peroxide.
I'm going to be making 1L of each solution but the
quantities can easily be scaled to any volume.
For example if you instead want
to make a hundred milliliters of each
you can just divide everything by 10
ok so now i need to make the solutions
and I'm going to start with solution A.
into a plastic container i add about 1L
of distilled water.
It's very important to use distilled water because
chloride ions in tap water will ruin
the reaction. I turn the stirring and I
dumping about 43 grams of potassium
iodate. Even with strong stirring this is
going to take several minutes to fully
dissolve
after about 10 minutes it looks like
everything has dissolved so I add the sulfuric acid.
As I said before my sulfuric acid was slightly discolored
so the solution here becomes a little bit yellow.
This is perfectly fine and it
doesn't affect the reaction but it does
make me a little bit sad because all of
the solutions are supposed to be
colorless
I let it mix for something like 30
seconds and then I turned off the
stirring
at this point I'm done making the first
solution so I mark the lid and the container
with a letter A. This is placed on the
side and I move on to making solution B.
The first thing that I need to do is make a starch solution.
So into a beaker i add a small amount of cornstarch
followed by a bunch of distilled water
I mix things up using a glass stir rod but
the corn starch isn't really going to dissolve.
To get it to dissolve I need to
heat it up and the easiest thing to do
is to put it in the microwave
so in the microwave it's brought to a
boil and I continue to boil it for
something like five or ten seconds
although this clears up the solution
quite a bit
it's probably still going to be cloudy
anyway this starch solution is placed on
the side and i continue making the other
part of solution B.
into another plastic container i add about 650 milliliters of
distilled water.
Into the water i add manganese sulfate and I stir until
everything dissolves
When I have a nice and clear solution i add the
malonic acid
Once everything has more or less dissolved I
add the starch solution from before
I stir the solution for a few minutes
to make sure that it's mixed really well
and then I take out the stir bar using a
magnet
I label the lid and the container with
the letter B and I move on to making
solution C
into a freshly clean container i add
about 400 milliliters of 30% hydrogen peroxide
On top of this, I pour in 600 milliliters of distilled water
to give a total volume of 1L.
I turn on the stirring and I
stir it for something like 30 seconds
the stir bar is removed and then everything
is labeled with the letter C
at this point we should be done but
there's actually a problem with solution B
that I need to attend to
in solution B some starch started to
fall out of solution and this needs to
be filtered off
this is very easily done by just passing
it through a coffee filter.
The starch solution was quite hot when it was added
so it heated things up.
as the solution cooled the solubility of starch
decreased and it started to precipitate
some starches are more soluble than
others so depending on the starch that's
used this step may or may not be
required
what I'm filtering here is still
slightly warm and this isn't really a
good idea
it does clean up the solution but it's
kind of pointless because as it
continues to cool more starch is going to
precipitate out the best idea is to just
be patient and to only filter things
after it reaches room temperature
once everything had filtered through the
funnel was removed and I labeled the
container i let it sit for a couple
hours to cool down and i re-filtered it
before moving on
ok so all of the solutions are now made
and I can move on to the reaction
although A is slightly discolored you'll
see in a minute why it doesn't really
matter
the reaction uses an equal amount of
each solution but it's important to mix
A and B first so I fill a beaker about a
third of the way with A and then I add B
The addition of solution B takes away
the yellow color but i'm not exactly sure why.
The impurity that's present is
somehow reacting with solution B and
becoming colorless
if any of you guys think you know what's going on
I'd love to hear your theories in the comments.
Anyway with A and B added i turn
on the stirring and at this point not
much should be happening
the magic happens when I add solution C
which contains the peroxide
ok so now to explain what's going on
it's honestly pretty complicated but i'm
going to try to make it understandable
the major variable in this reaction is
the concentration of iodide when the
concentration is low the process that
generates it is favored and when the
concentration is high the process that
consumes it takes over. This causes the
iodide concentration to fluctuate up and
down and it's responsible for the
oscillating effect. Before i get into
more detail
I'm first going to recap all of the
chemicals that were used.
In solution A we have potassium iodate and sulfuric acid.
In solution B there's starch, malonic acid, and manganese sulfate
And in solution C we have just hydrogen peroxide.
when all of the chemicals are
mixed
there's a whole slew of reactions going
on but they can all be added together to
make one overall equation.
under acidic conditions the iodate ion from
potassium iodate, malonic acid and
hydrogen peroxide all react together to
form iodomalonic acid, oxygen gas and water.
The manganese is acting as a
catalyst and the starch is used as an
indicator
while it's useful to know what's
happening over all this scheme doesn't
really explain why the reaction
oscillates. To really understand this
we're going to have to look at the
intermediates that are being formed. The
overall equation is the sum of two main
steps which I've labeled that equation
one and equation two.
In the first one, iodate reacts with hydrogen peroxide to
form hypoiodous acid, oxygen and water
in the second one
hypoiodous acid is consumed when it reacts
with malonic acid to form iodomalonic
acid and water. The first reaction can
occur by two very different processes
one is called the radical process and
the other one is called the non radical
process. Although they have the same
overall equation their sub reactions are
quite different. The oscillation of this
reaction comes from the switching
between these two process
the process that dominates at any given
time is determined by the concentration
of iodide in the solution.
if we look at the sub reactions of the two processes
we can see that the non radical one
involves iodide in multiple steps but
the radical one doesn't use it at all
when we first start out there's no
iodide present so the radical process
will dominate because it doesn't need it
the radical process is quite fast and it
produces hypoiodous acid faster than it
can be consumed by equation 2.
because of this there is an excess of hypoiodous acid
floating around which can react
with hydrogen peroxide to form iodide
the iodide that forms then reacts with
more hypoiodous acid to form iodine which
gives the solution a yellow color the
yellow color gets darker and darker as
the concentration of iodine increases
the concentration of iodide will
continue to increase until it eventually
starts combining with iodine to form the
triiodide ion when the triiodide forms
it quickly complexes with starch to make
a dark blue-black complex.
at some point after the solution turns black the
concentration of iodide gets high enough
that the non radical process takes over
and the radical process is shut down
the non radical process is slower though and
it doesn't generate hypoiodous acid very quickly.
Because of this the hypoiodous acid is consumed by equation 2
faster than it can be generated
without an excess of hypoiodous acid floating
around we stop generating iodide through
its reaction with the peroxide
The non-radical process slowly uses up the
iodide and the concentration decreases
the triiodide complex with starch falls
apart and the blue color starts to fade
iodine slowly react with malonic acid
and because no more is being made its
concentration drops and the yellow color
fades as well.
the concentration of the iodide eventually gets low enough that
the non radical process is shut down and
the radical process takes over again it
starts to generate iodide again and the
process repeats itself
the reaction oscillates several times but eventually
either the iodate ion or the malonic
acid is used up.
Ok so this was all probably quite
confusing but I hope it was still
somewhat understandable.
Just for fun, this is an original oscillagram from
Briggs & Rauscher in 1972.
they measured the concentration of iodide throughout
the reaction and you can see that it's
constantly oscillating up and down
anyway
now we're done with all the complicated
stuff and we can move on to just playing
with the reaction
one interesting thing to do is to mix
all of the solutions together but to not
stir them.
Without the stirring the reaction isn't uniform and I think it
gives it a cool effect
in the last run the reaction seemed to
be occurring sooner at the top than the
bottom so i wanted to see if I can
exaggerate this effect by using a grad
cylinder. During the reaction it looked
like there was a lot of fluid movement
and I think it kind of mixed itself.
In the other reaction it maintained the
steady gradient from the beginning to
the end but after a couple cycles in
this one it seemed to become more or
less uniform
for the last run I sequentially added it
to four different beakers and I'll play
it back in four-times speed
when the reaction is done the solution just
goes black but if you leave it there for
longer it looks like this.
At the bottom we have solid iodine and the solution
has this brown color
before we can dispose of it we first
have to do a few things.
All of the iodine and the hydrogen peroxide are
neutralized using sodium thiosulfate.
At some point once we've added enough
sodium thiosulfate the purple color will
disappear
however it will probably slowly turn
yellow and revert back to being purple.
This means that there's still iodine
present and I just need to keep adding
more sodium thiosulfate until it stays
colorless
I was running out of space in this small
beaker so I switched it to a 1-liter beaker.
Also i started just adding solid
sodium thiosulfate instead of dissolving it first
eventually i've added enough and the
solution stays colorless.
The next thing that I need to do is remove the
manganese from the solution.
It's very easy to do this by pouring in some
saturated sodium carbonate solution very
quickly the solution becomes opaque and
manganese carbonate starts to
precipitate. The addition of the sodium
thiosulfate generated heat and before
filtering off the manganese carbonate
I'm gonna wait till it cools down a
little.
All of the manganese carbonate that was
caught in the filter was added to my
solid waste. The liquid that passes
through can be poured down the drain
with a bunch of extra water
i finally got my red beakers and I've
opened up an online shop to sell them
the website is just NileRed.ca and
if any of you are interested in checking
it out
there's a link in the description.
Right now the site only accepts orders from
Canada or the US; so if you're from somewhere
else and you want to get something just
send me an email at this address here [admin@nilered.ca]
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