Brass analysis. Image 2 of 7. One gram of brass in nitric acid (HNO3). Brass contains copper. When brass is mixed with nitric acid, the blue solution containing Cu2+ ions is produced. The amount of light absorbed by this solution is proportional to the concentration of Cu2+. This is known as the Beer- Lambert law, and can be used to find the proportion of copper in the brass. Alternatively further analysis can be performed on the solution using potassium iodide (KI). This analysis can be seen in images A500/566-572.