双缩脲检验蛋白质无水硫酸铜有极强的吸水性，把它投入95%乙醇或含水有机物，即吸收水分而恢复为蓝色结晶体。硫酸铜中的铜离子能破坏蛋白质的立体结构，使之变性。测定蛋白质浓度时常在蛋白质中加入碱，再加入硫酸铜溶液，此时溶液会变为紫色，这个反应被称为双缩脲反应。

Biuret test showed that protein anhydrous copper sulfate had strong water absorption. It was injected into 95% ethanol or water-containing organic matter, that is, absorbed water and restored to blue crystals. Copper ions in copper sulfate can destroy the stereostructure of proteins and denaturate them. When the concentration of protein is determined, alkali is often added to the protein and copper sulfate solution is added. At this time, the solution becomes purple. This reaction is called biuret reaction.