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Author: Subject: I can't get the biuret test to work
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mad.gif posted on 9-4-2011 at 07:23
I can't get the biuret test to work

I'm trying to test for urea content in instant cold packs which according to whoever replied when I tried to contact a company selling them (I couldn't find an email addr to contact the manufacturer at), contains urea and a mixture of nitrate salts. I got this recipe on a university website:


1. Weigh 1.50 g of cupric sulfate pentahydrate (CuSO4 5 H2O) with 6.0 g sodium potassium tartrate tetrahydrate (NaKC4H4O6 4 H2O).

2. Dissolve in 500 ml of H2O.

3. Add 300 ml of 10% NaOH.

4. Make up to total volume of 1 liter. Store in a plastic bottle protected from light.

So I've made biuret reagent with this recipe and also made it using potassium bitartrate instead of rochelles salt and I'm pretty sure what I've made is biuret reagent because the solution turns to a darker shade of blue after adding the NaOH and no copper hydroxide precipitates out. When I mix this stuff with a warm solution of the cold pack mix no colour change occurs. I assume that the solution should turn violet in the presence of urea but its not happening. The only explanation I can think of is maybe there isn't any urea in the cold packs. Before jumping to that conclusion though I want to find out if I really have biuret reagent. Can anyone recommend a household substance that will produce a blatantly obvious colour change in the presence of urea? I have spirulina powder which is packed full of proteins but this stuff turns water green, I need a colourless protein source so I can easily see the solution turn from blue to violet. Any suggestions?
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[*] posted on 9-4-2011 at 07:34

If you followed the recipy to the letter, your solution should (and most probably is) good.

But there are many "Cold packs" that contain no urea at all, only ammonium nitrate. Some have a combination of the two.

I would check out this thread to determine if you have AN or urea in your cold packs:


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[*] posted on 9-4-2011 at 08:34

Prepare a clear protein test solution for your alleged biuret reagent by dissolving the white of an egg in water. About one drop of egg white per mL of water should be enough to give a positive test.

Please stop starting a new thread for every little problem you encounter, OK? There is always the short answer thread if you can't find an existing thread. This could have been added to your other thread(s) on biuret reagent.
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[*] posted on 11-4-2011 at 01:17

I made biuret reagent once. Just plain cows milk gives a good colour change
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[*] posted on 11-4-2011 at 02:51

Are you heating the cold pack material until it melts first?
The biuret test will not work with urea, it must be converted to biuret by heating first.
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