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As I understand the color change in the biuret test is due to the formation of a complex between the peptide (with more than 2 peptide bonds) and the $\ce{Cu^2+}$ ion, from $\ce{CuSO4}$. But before adding the $\ce{CuSO4}$ we add a base to our solution in this case $\ce{NaOH}$, my book says that's because we want an alkaline environment in order for the reaction to happen.

Shouldn't $\ce{CuSO4}$ react with the $\ce{NaOH}$ and then their product $\ce{Cu(OH)2}$ will precipitate?

Or at least both should happen. Every YouTube video I've seen, of the Biuret test, shows both tubes being clear and only a color change, and no percipitation.

In another case for example a Fehling's test for reducing sugars we add an extra compound (sodium potassium tartrate) in order to prevent $\ce{Cu^2+}$ reacting with the $\ce{OH-}$, why not in the biuret test?

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    $\begingroup$ Don't trust random YouTube videos. Internet is very ueful but it is also full of incorect information. You have to seek reliable sources from the internet for example, pages maintained by universities ending in edu in the address. See this one for example, clear.rice.edu/bioc111/bios111_biuret.htm $\endgroup$
    – AChem
    Feb 5 at 18:15
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    $\begingroup$ It does add potassium tartarate to avoid problems, which you just mentioned in the question. $\endgroup$
    – AChem
    Feb 5 at 18:15
  • $\begingroup$ What you need for this test is $\ce{Cu^2+}$ cation. Hope answer here give you insight. $\endgroup$
    – Mathew Mahindaratne
    Feb 5 at 18:50
  • $\begingroup$ @MathewMahindaratne I've already reviewed that question but still got no answer to mine. Why the Cu2+ ions don't react with the NaOH as well? $\endgroup$
    – CaptainAmerica Whyso
    Feb 5 at 19:06
  • $\begingroup$ @AChem the videos were just the same Biuret test demonstration, i watched them to see if other people had different findings. I was trying to see if percipitation did happen in any of them. $\endgroup$
    – CaptainAmerica Whyso
    Feb 5 at 19:09

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