encipher
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Iodine Clock Kinetics
Hi,
There has been something bugging me for quite some time now with the iodine clock reaction (The Iodate variation). For those who don't know, the
following are the three main reactions involved:
IO3- (aq) + 3HSO3-(aq) → I- (aq) + 3HSO4-(aq) (step 1)
IO3-(aq) + 5I-(aq) + 6H+(aq) → 3I2 + 3H2O(l) (step 2)
I2(aq) + HSO3- (aq) + H2O(l) → 2I-(aq) + HSO4-(aq) + 2H+(aq)
(step 3)
I won't go into details, but basically, when all the bisulfite is consumed, the I2 cannot be reduced back to iodide, and starch (the indicator) will
form the starch-triiodide complex which is deep blue in color.
My issue is with the rate of the reaction with respect to concentration. For some reason, when I decrease the concentration of Iodate to a certian
point, the reaction time become inaccurate i.e. Not predictable by equations. It's consistent, but consistently off. The following is the
concentrations and quantities of reagent I used:
.02M KIO3 --- 10mL + 10mL H2O
.012M NaHSO3 --- 10mL + 10mL H2O
2mL starch indicator.
Using that combination, the reaction time was 7.4 seconds.
My next run, I decreased the KIO3 to 5mL and increased the H2O to 15mL.. I kept the Bisulfite the same. The time I got for that run was 14.6 seconds,
Which is a very good result. I tried multiple times in between all with good results and minimal error.
According to multiple sources, and my experimentation with the iodine clock reaction, the reaction is FIRST ORDER with respect to the iodate.
Now, when I decreased the quantity of KIO3 to 2mL and 18mL of H2O.. I got 46.7 seconds consistently. I cannot figure out why?! I made sure my
measurements were accurate, using 2mL volumetric pipett and others for different volumes.. and I still cannot get the predicted time.
Unless I am doing my calculations all wrong, does someone have any idea what might be the reason for this discrepancy in time? I also forgot to
mention above, but I have kept the ionic strength the same by using KNO3 instead of H2O for the Iodate and Na2SO4 instead of H2O for the Bisulfite. I
ran the same reaction with H2O and the results arent as consistent (which is understandable).
Any help appreciated, thanks !
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12AX7
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Ionic strength? Le Chatelier's principle? Chemistry being chemistry? (Remember the equilibrium equation is only an approximation, and chemistry
just loves to not obey equations.)
Tim
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chemrox
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please post the details or a ref for them.. it looks like a fun demo thing .. I used to have a collection of clock reax but seems to be gone
(Contrived probably took it home).
"When you let the dumbasses vote you end up with populism followed by autocracy and getting back is a bitch." Plato (sort of)
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pantone159
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Shakhashiri, Chemical Demonstrations, Vol 4, 10.1 has a discussion of this reaction.
According to this, the empirical relationship between the clock period and initial concentrations of HSO3- and IO3- ions, at 25 C, and 'with no added
acid' is:
P = (0.003 s-mol-mol/liter-liter) / ( [IO3-]0 [HSO3-]0 )
So for initial iodate = 0.025 M and initial bisulfite = 0.013 M, this gives about 9 sec. (I calculate 9.23 sec). It does mention that if there are
more than 3 times as many bisulfite as iodate, there won't be a color change and the equation is not applicable.
I'm not sure this equation matches your results, however.
I wouldn't add sulfate ion, it appears in the reaction equations so I would expect it to interfere with the timings.
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encipher
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| Quote: | Originally posted by pantone159
Shakhashiri, Chemical Demonstrations, Vol 4, 10.1 has a discussion of this reaction.
According to this, the empirical relationship between the clock period and initial concentrations of HSO3- and IO3- ions, at 25 C, and 'with no added
acid' is:
P = (0.003 s-mol-mol/liter-liter) / ( [IO3-]0 [HSO3-]0 )
So for initial iodate = 0.025 M and initial bisulfite = 0.013 M, this gives about 9 sec. (I calculate 9.23 sec). It does mention that if there are
more than 3 times as many bisulfite as iodate, there won't be a color change and the equation is not applicable.
I'm not sure this equation matches your results, however.
I wouldn't add sulfate ion, it appears in the reaction equations so I would expect it to interfere with the timings. |
The iodate must be in excess or else there will be none left to oxidize the iodide formed. I made sure that my concentrations were within that limit.
@ chemrox:
Peroxide Variation - http://jchemed.chem.wisc.edu/JCESoft/CCA/CCA3/MAIN/CLOCKRX/P...
Iodiate Variation - http://www.uni-regensburg.de/Fakultaeten/nat_Fak_IV/Organisc...
There is also one using persulfate, but I only have it in a lab manual. I can scan it if you want however.
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