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Determination of Ethanol Content Using Titration

by maysj18
Tags: content, determination, ethanol, titration
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maysj18
#1
Dec4-12, 01:37 AM
P: 7
Hi everyone! So, something sort of weird happened in analytical lab and I was wanting some guidance. I was trying to determine the ethanol concentration in a 4 Loko, a red wine, and a beer through acid/base titration. I pipetted a little bit of a diluted alcohol sample into a small beaker and suspended it over a potassium dichromate solution made from sulfuric acid, water, and potassium dichromate. I left it for two days, then I took the sample out, squirted water down the sides of the flask and titrated the potassium dichromate solution with thiosulfate. The thiosulfate determined how much potassium dichromate did not react with ethanol (I had blanks that help me determine this). Well, one indicator I used was KI. Apparently, it is used to oxidize the chromium. Once it faded from a rust color back to the bright yellow it was prior to adding the KI, I then added starch which turned the solution dark blue and continued titrating. The end point was when the dark navy changed to a transparent blue color.

Now, my issue here is this. The first time I did this, I got terrible results (5% ethanol for the wine, 9% for the 4 Loko, and 0% for the beer). I figured it was because of how much they were diluted so I did more samples with far smaller dilution factors. Well, for the first set of trials the color of the acid chromate layer was what it was supposed to be prior to titration: bright yellow. For the second set, it was either a weird blue/green or light blue. I figured that some of the suspended solution dripped down into the acid layer which it reacted with the starch in the beverage, but that doesn't make sense because the starch wouldn't react without KI being present.

Maybe I'm confused, but if anyone can shed any light on what these colors mean, I would appreciate it.

The colors for the titration, if correct, go like this:
Highlighter yellow --add KI--> Rusty brown/red --titrated w/ thiosulfate----> back to yellow
--add starch--> dark navy --titrated with thiosulfate--> light blue

BUT, after the second trials incubated, the colors for most of the solutions ranged from light green to light blue rather than bright yellow. Maybe it was a fluke, but maybe there is some interaction in there I don't know about. The samples were only diluted with water and the acid dicrhomate solution that the alcohol samples were suspended over contained potassium dichromate, sulfuric acid, and water. Again, the colors were right for the first trials (even though my results were horrible), but were really weird for the second set of trials. The only differences were the dilution factors. Some of them, when I added KI, turned to a yellow-y color but then after two drops of thiosulfate, they were light blue. -____- confused!

One other thing was that for the first trials, the dichromate solution was fresh and had just been made; however, the second trials were done maybe two weeks later using that same solution. It had been left in the hood over break. Not sure if this changes anything, but figured I'd throw that out there. Thanks!
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chemisttree
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Dec4-12, 10:30 AM
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Go ahead and ask your question. I don't think you'll get help on this forum if you ask, "Tell me what is going on here?"
maysj18
#3
Dec4-12, 10:36 AM
P: 7
That's just the thing. It doesn't make any sense. All the literature I have talks nothing about this particular thing happening. I'm assuming that means it was just a fluke, but I really want to cover my bases.

chemisttree
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Dec4-12, 10:43 AM
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Determination of Ethanol Content Using Titration

Quote Quote by maysj18 View Post
That's just the thing. It doesn't make any sense. All the literature I have talks nothing about this particular thing happening. I'm assuming that means it was just a fluke, but I really want to cover my bases.
What particular thing?

Ask your question and we'll see where it goes from there.
maysj18
#5
Dec4-12, 11:07 AM
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The acid chromate solution that the alcohol was suspended over top of was supposed to be a bright yellow after incubation. Instead, they were green or blue.
chemisttree
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Dec4-12, 12:17 PM
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Quote Quote by maysj18 View Post
The acid chromate solution that the alcohol was suspended over top of was supposed to be a bright yellow after incubation. Instead, they were green or blue.
The acid chromate solution started out as a bright yellow. Ethanol was added and the mixture was incubated. When you were finished, the solution is green or blue. What do you think happened to the chromate in solution? What happened to the ethanol? What color did you expect?
Borek
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Dec4-12, 12:35 PM
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Please try to write reaction equations for all reactions taking place. You are wrong on so many accounts when it comes top your understanding of the processes taking place it is hard to even start the discussion.

It is not an acid/base titration, iodine is not oxidizing chromium, there is no acid layer and so on.
maysj18
#8
Dec4-12, 05:23 PM
P: 7
Well, it's a redox titration, not acid/base. If I said acid/base, that was my mistake.

2Cr2O7(2-) + 16H+ + 3C2H5OH --> 4Cr3+ + 11H2O + 3CH3COOH
Cr2O7(2-) + 14H+ + 6I- --> 2Cr3+ + 3I2 + 7H2O
2S2O3(2-) + I2 --> S4O6(2-) + 2I

This is the overall reaction that is supposed to take place.

I just have no idea what is even in that solution to turn it blue or green in the first place. Water and ethanol slowly evaporate and as the ethanol comes in contact with the dichromate it first dissolves, and then is oxidized. Eventually, all the ethanol has dissolved and reacted with the dichromate. The dichromate remains yellow through the incubation process and then is titrated after incubation to determine how much of it reacted with ethanol. Stoichiometry was then used to determine how much ethanol was in the sample to begin with (assuming that it all reacted).
Borek
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Dec4-12, 05:46 PM
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titrations.info/iodometric-titration-end-point-detection
maysj18
#10
Dec4-12, 05:52 PM
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I understand that.

But there was no KI even added to the solution at this point, so that doesn't explain why the dichromate layer was green/blue.
Borek
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Dec4-12, 06:03 PM
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What does dichromate get reduced to? What is the color of the product?
maysj18
#12
Dec4-12, 06:18 PM
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Cr 3+
and it should be yellow.
chemisttree
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Dec4-12, 09:25 PM
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Nope. Kelly green.

In the next step, KI gets oxidized by Cr+6 that remains. So potassium dichromate must be present in an accurately known quantity and it needs to be present in excess relative to ethanol. What remains oxidizes the KI. From there, it is a typical iodine titration with thiosulfate using starch as an indicator.
maysj18
#14
Dec4-12, 09:50 PM
P: 7
Ohhhhhhhhhhhhhhhhhhhhh, so that green I saw means that all the chromium reacted and that it was not in excess. Right?!

What about the blue? What do you think happened there? I definitely follow you on why one solution was green. It wasn't diluted hardly at all, so I bet that's why.
chemisttree
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Dec4-12, 10:55 PM
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Quote Quote by maysj18 View Post
Ohhhhhhhhhhhhhhhhhhhhh, so that green I saw means that all the chromium reacted and that it was not in excess. Right?!
I don't know. The only way to tell is to add KI and starch indicator, which is WAAY darker at low concentrations than green Cr+3.

I have no idea about why you obtained a blue solution after treatment with Cr+6. It could be a charge transfer complex, which can be any color under the sun, of either the Cr+3 or Cr+6. Only way to tell is to add KI and finish the experiment.


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