Answer:
Because is a reactive for identification protein through a reaction called Biuret
Step-by-step explanation:
Biuret reagent contains CuSO4 in alkaline aqueous solution (thanks to the presence of NaOH). The reaction is based on the formation of a violet compound, due to the formation of a coordination complex between the Cu + 2 ions and the pairs of non-shared nitrogen electrons that are part of the peptide bonds, this if the reaction is positive. When the biuret reaction is negative, it becomes colored blue.
For example: The protein chains present in the egg white when subjected to Biuret reagent, which in its composition has 10% NaOH (which does not participate in the reaction but its presence is of fundamental importance since it provides the basic medium needed) together with CuSO4, it was seen that cupric sulfate reacted with the protein present in the egg albumin solution, and it turns violet which indicates that the reaction was positive.