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Comparing quenching & titration and colorimetry

For my coursework, I am comparing the methods quenching and titration and colorimetry to see which one produces the better results and is more accurate. The way I was planning on doing this is seeing which progress curve for each method best fit the expected progress curves. If I got very similar result for those two, how would I be able to choose which method is better? I have four different experiments in my investigation but I was going to choose the best method from the first one and then carry on the method for the other three experiments for my other aims. I can't carry out both methods for the other experiments.

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If you get similar results, you can't say which method is better.

You can compare cost, time involved to do the experiment and so on.

Reply 3

username638250

Original post by Borek

If you get similar results, you can't say which method is better.

You can compare cost, time involved to do the experiment and so on.

Oh, OK. Thanks. Meh. I got told to compare those two methods and I'm just hoping that there is a discernible difference between the results of those two. -_-;

Also, do you know why you don't add starch indicator straight away until the mixture is pale yellow in the titration of sodium thiosulfate and the iodine + propanone reaction mixture?

Reply 4

charco

Study Forum Helper

Original post by yl95

If you add the starch indicator while there is a high concentration of iodine the blue-black complex that is formed can polymerise and become very difficult to discharge.

Reply 5

username638250

Original post by charco

If you add the starch indicator while there is a high concentration of iodine the blue-black complex that is formed can polymerise and become very difficult to discharge.

Do you mind explaining how polymerisation would happen in that case? Also, why does it turn blue-black?

Reply 6

charco

Study Forum Helper

Original post by yl95

Do you mind explaining how polymerisation would happen in that case? Also, why does it turn blue-black?

Scrub that about polymerisation. Starch is already polymeric.

The starch iodide complex is insoluble and if formed in large quantities the iodine cannot get to react with the reducing agent and the black colour does not discharge.

The black colour is caused by the triiodide ion occupying sites in the starch coils causing the starch to absorb light and appear blue-black.

(edited 10 years ago)

Reply 7

username638250

Original post by charco

Thank you! Last question: why must acid be added in the standardisation of iodine with sodium thiosulphate?

Basically, I'm making up 0.01 mol dm-3 and 0.02 mol dm-3 of iodine solution but since there will still be solid iodine left at the bottom I will need to find its exact concentration by titrating with sodium thiosulfate.
http://www.cengage.com/resource_uploads/downloads/0030355230_17074.pdf
If you scroll down to Page 2, you'll know what I'm talking about. :P

And...I want to use 50cm^3 of 0.02 mol dm-3 sodium thiosulfate solution (in burette) and 25cm^3 of roughly 0.01 mol dm-3 iodine solution for the standardisation. Is the volume of sulfuric acid I would need correct? ( I calculated that to be 0.15cm^3 using the link...)

(edited 10 years ago)

Reply 8

Borek

Original post by yl95

Thank you! Last question: why must acid be added in the standardisation of iodine with sodium thiosulphate?

I think you got it wrong - acid is not required for the iodine titration, quite the opposite - it interferes with the titration (see http://www.titrations.info/iodometric-titration )

However, during standardization acid is necessary for the reaction of iodate with iodide, which produces iodine. See "Solution standardization" link in the left menu of the titrations site.

Reply 9

username638250

Original post by Borek

That's good because I decided that there was no point doing that. One problem: when I was titrating iodine against sodium thiosulphate, the starch indicator turned brown instead of blue-black, yet it gave me the result that I expected. Why would that be so?

Also, I was investigating the reaction between iodine and propanone and seeing the order of reaction wrt iodine (meant to be zero) using colorimetry. I got 1% transmittance throughout a run; what could have possibly gone wrong? It certainly worked with distilled water when I tried seeing if it was broken. I got 1% transmittance initially for 0.003 mol dm-3 iodine... For 0.001 mol dm-3 iodine, I got 15.4% initially which went up to 27.8% in 6 minutes....