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Topic: Andrews' Titration  (Read 24610 times)

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kemistrykid

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Andrews' Titration
« on: March 02, 2006, 04:53:42 PM »
Hey guys, I'm a 2nd year chemistry student and I am writing up my lab report for an andrews' titration experiment. It goes something like this:

Make standard solution (called solution "S"): 25ml of 0.1M Sodium thiosulphate, 25ml of 0.05M I2 in aqueous NaI (approx. 0.15M in NaI), dilute up to 250ml.

This is the reaction:
2S2O32- + I2 (+NaI) ---->  2I- + S4O62-

Now the titration: Take 25ml of "S", add 25ml of H2O, 25ml of conc. HCl and 2ml of chloroform, and titrate against 25ml of 0.05M KIO3.

This is the reaction:

IO3- + 6H+ + 4e------>I+ + 3H2O

Also, since this reaction is happening in a vast excess of hydrochloric acid, this reaction also happens:

I+ + 2Cl------->ICl2-

OK, my question is, I have to work out the reaction ratio for S4O62-/IO3-. I have worked it out to be 1:7, but I can't work out the exact equation for this mechanism, namely because I am not sure how they react.

Any ideas?

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Re:Andrews' Titration
« Reply #1 on: March 02, 2006, 06:06:03 PM »
IO3- + 6H+ + 4e------>I+ + 3H2O

Something is wrong here.

IO3- + 5I- + 6H+ -> 3I2 + 3H2O
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kemistrykid

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Re:Andrews' Titration
« Reply #2 on: March 02, 2006, 07:39:38 PM »
Something is wrong here.

IO3- + 5I- + 6H+ -> 3I2 + 3H2O

No, its definitey what I put. Thats the whole point of this experiment, well, at least one of them. We go from I------>I2------>I+

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Re:Andrews' Titration
« Reply #3 on: March 03, 2006, 03:07:31 AM »
I don't get whole experiment setup. It doesn't make chemical sense.

What do you use iodine for at the very beginning?

You add thiosulfate to reduce iodine to iodides - at least that's what your first equation suggests. But you have also added iodides to the S solution. Why do you need two sources of iodides?

Usually iodideds are added to iodine solution to inxrease I2 solubility (in form of I3-). But you don't add thiosulfate in such situation, as it will reduce iodine. You may use thiosulfate as a titrant, to determine amonut of iodine, but that's completely different story.

And the reaction equation I have posted is definitely the one used for determination od iodides with iodate solution in iodometry. There is no single reason for other reaction to take place in the solution you described.
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Re:Andrews' Titration
« Reply #4 on: March 03, 2006, 08:25:24 AM »
Writing all these equations was what I call a time-consuming process.  :D

IO3- + 5I- + 6H+ -> 3I2 + 3H2O

6S2O32- + 3I2 ->  6I- + 3S4O62-

IO3- + 5I- + 6H+ + 6S2O32- -> 6I- + 3S4O62- + 3H2O


To answer your question, the ratio is 1:6

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Re:Andrews' Titration
« Reply #5 on: March 03, 2006, 10:38:40 AM »
Writing all these equations was what I call a time-consuming process.

Not with proper tools  :P

Quote
IO3- + 5I- + 6H+ + 6S2O32- -> 6I- + 3S4O62- + 3H2O

Huh? It is not a correct reaction equation.
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Offline Albert

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Re:Andrews' Titration
« Reply #6 on: March 03, 2006, 10:48:40 AM »
IO3- + 6H+ + 6S2O32- -> I- + 3S4O62- + 3H2O


What do you think about it, now? However, I agree with you: the first two equations sound more correct to me and they show the ratio, while this one is maybe a faster :uhhuh: way to display the ratio.

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Re:Andrews' Titration
« Reply #7 on: March 03, 2006, 11:49:08 AM »
Now it is correctly balanced and obey rules. The question is whether it describes real reaction that takes place in the solution. I doubt.

Besides, I still don't know what was the lab procedure, let's wait for kemistrykid.
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Offline Albert

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Re:Andrews' Titration
« Reply #8 on: March 03, 2006, 01:42:03 PM »
I don't get whole experiment setup. It doesn't make chemical sense.

Have a look at page 36:
http://www.nzfsa.govt.nz/science/research-projects/iodine-fort/iodine-fort-foods.pdf

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Re:Andrews' Titration
« Reply #9 on: March 03, 2006, 02:39:54 PM »
I don't see there anything explaining experiment as described by kemistrykid, besides, I was aware of the chemistry described.

Solution S doesn't make sense to me. Either it have to contain iodides (if so - what iodine and thiosulfate are used for?) or iodine (if so - what iodide and thiosulfate are used for?) or thiosulfate (if so - what iodine and iodides are used for?). Produced S4O62- is of no use too, as it will be not titrated by IO3- - so what is this solution for?
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kemistrykid

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Re:Andrews' Titration
« Reply #10 on: March 14, 2006, 02:13:07 PM »
The S4O62- is directly titrated against the KIO3 solution. I've worked out the reaction ratio is 7:1 using the number of moles used. Strange thing is some people have been getting 7:2. O.o

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Re:Andrews' Titration
« Reply #11 on: March 14, 2006, 02:31:42 PM »
The S4O62- is directly titrated against the KIO3 solution.


This is not consistent with what you wrote in the opening post.

Could you write the chemical equation that describes the reaction behind the titration of S4O62- against KIO3?
« Last Edit: March 14, 2006, 02:31:57 PM by Albert »

kemistrykid

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Re:Andrews' Titration
« Reply #12 on: March 14, 2006, 03:14:42 PM »
I said titrate solution S (i.e. S4O62-) against the KIO3. Since I worked out the reaction ratio to be 7:1.

kemistrykid

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Re:Andrews' Titration
« Reply #13 on: March 15, 2006, 09:53:35 AM »
OK, the next thing is that i add 2ml of 0.5M barium chloride to one of the titrated solutions and it turns yellow and then cloudy. I do the same to a solution of 0.25ml KIO3 and 10ml of conc. HCL, diluted to 40ml. It turns yellow, but not cloudy.

Apparently I can work out the oxidation half-reaction in the tetrathionate/iodate reaction, and the overall reaction, but I can't quite see how...

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Re:Andrews' Titration
« Reply #14 on: March 15, 2006, 09:58:09 AM »
What reaction do you expect to take place between S4O62- and IO3-? S4O62- is already in oxidized form, it is product of thiosulfate oxidation with iodine.

My guess is that there is some error in your description of the experiment. Perhaps S solution was made without use of iodine, just with iodide? It may make some sense, as iodate plus iodide produces iodine which in turn reacts with thiosulfate.
« Last Edit: March 15, 2006, 10:01:53 AM by Borek »
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