Estimation of PErcentage of Copper in Brass

Exp 8

What are the constituents of brass?

Brass is an alloy of copper (50 to 90%) and zinc (20 to 40%), and it also contains small quantities of tin, lead, and iron

How is a brass solution prepared?

A brass solution is prepared by dissolving the brass foils in a minimal amount of concentrated nitric acid (HNO₃).

What is the purpose of adding urea to the brass solution?

The oxides of nitrogen present in the brass solution are destroyed by adding urea. The presence of nitrogen oxides would cause the liberation of extra iodine from potassium iodide (KI), as they are also good oxidizing agents.

Why is NH4OH added to the brass solution?

The nitric acid present in the brass solution is neutralized by adding ammonium hydroxide. This leads to the formation of a bluish-white precipitate of copper hydroxide (Cu(OH)₂). If this step is not done, the strong oxidizing agent, nitric acid, would also liberate iodine from KI.

Question 5: Why is CH3COOH added to the brass solution in the determination of copper?

Acetic acid neutralizes the excess NH4OH and provides a slightly acidic medium, which is necessary for the quantitative oxidation of KI to I2. Other acids can lower the pH too much, making the iodine release from Cu2+ less quantitative.

Why is KI added to the brass solution?

Cu²⁺ ions do not react with sodium thiosulfate solution directly. However, Cu²⁺ oxidizes KI, causing the liberation of iodine. During this reaction, Cu²⁺ (which is blue) is reduced to Cu⁺ (which is colorless), and the iodine liberated is then titrated with standard sodium thiosulfate, using starch as an indicator.

Why does the blue solution of brass turn brown upon adding KI?

The blue solution turns brown because iodine is released from KI, indicating the oxidation process.

Why is starch added towards the endpoint in iodometric titration?

Why is starch added towards the endpoint in iodometric titration?
Answer: Starch is added near the endpoint because, at high concentrations of iodine, starch can form an insoluble starch-iodine complex. Also, iodine can get adsorbed onto colloidal starch particles, and this adsorbed iodine is not easily released by sodium thiosulfate. By adding starch near the endpoint, we minimize errors.

What is the white precipitate left at the endpoint?

The white precipitate at the endpoint is copper iodide

What is the color change at the endpoint in the determination of copper in brass?

Starch reacts with iodine to form a blue-colored complex. At the endpoint, when all the iodine is consumed by the sodium thiosulfate, the blue color disappears, and the solution turns white. This means that the copper iodide precipitate is formed, and the color change is from blue to white.