The alkaline copper solution is primarily used in protein analysis because it directly measures the mass of proteins present in a sample, without the need for any additional reagents or methods.

The alkaline copper solution, also called the Biuret test, works by forming a violet complex when copper ions in a strong base bind to the peptide bonds in proteins. This color change tells you that protein is present, but it does not give you the exact mass or amount of protein. To estimate protein concentration you must compare the color intensity to a standard curve made with known amounts of protein. For example, if a sample turns deep violet, you compare its color to standards and calculate the protein amount from the curve, rather than measuring mass directly. Thus the test indicates presence, not an absolute mass, and it still needs standards and a spectrophotometer.