Test for Proteins
Biuret reagent is recommended for the detection of proteins. The Biuret Reagent is made of sodium hydroxide and copper sulphate. The blue reagent turns violet in the presence of proteins, and changes to pink when combined with short-chain polypeptides.
There are some variations in the preparation of Biuret reagent. One receipe is given below. It is stable. The receipe given in CLEAPSS Receipe cards also works equally as well.
Biuret Reagent: Add, with stirring, 300 mL of 10% (w/v) NaOH to 500 mL of a solution containing 0.3% copper sulfate 5-Waters and 1.2% sodium potassium tartrate, then dilute to one liter. The reagent is stable for a few months but not a year. Adding 1g of potassium iodide per liter and storing in the dark makes it stable indefinitely.
This method may also be made quantitative by using a standard range of protein solutions and a colorimeter. The presence of protein gives a violet color with maximum absorbance around 550-555 nm; we typically read absorbances at 540 nm.
Powdered albumen can be used to make up any protein standards. Albustix reagent strips (available from pharmacists or local educational suppliers) are also useful as a quantitative test for proteins; and give a range of colours indicating some protein concentrations from 0 to 20 g per dm3.
--Ssmith 21:35, 20 August 2006 (BST)