[yoyowong007]

I had a problem with a test for a sample. The sample consists of glucose, vitamin C and proteins.
But when I use Biuret test to test for the presence of proteins. After adding Cu(II)SO4 into the sample.
It turns cloudy and the colour turns yellow. Are there any errors in my experiment? Why the sample doesn't
turn blue finally? I had already followed the instructions. After adding NaOH, then add some drops of Cu(II)SO4.
I had done all of this, but why can't I get the correct result?

[deadally]

Copper could be binding to your proteins, and that might prevent it from turning blue (no free copper?)

I'm not entirely sure, but that seems like a possibility

You mentioned the solution turning cloudy, also.  I'm aware that heavy metals (IE Copper) have a propensity to precipitate protein out of solution, which might account for the cloudiness.

Or it might just be an experimental error

[yoyowong007]

First of all, I would like to give thanks to deadally.

My teacher explained to me that the test may be affected by vitamin C in the solution.
As vitamin C is a reducing agent, once we added Cu(II)SO4 into the solution.
Cu(II) ions may be reduced to Cu(I). Therefore, the result may be different from the expected one.

[DocSuleman]

You are right dued!. The vitamin C due to its reducing power also give false Benedict Test (thats for glucose etc). I've been asked this question during my viva voce in last examination.