Re: How do I determine the amount of copper in brass?

The copper can be determined by the indirect iodine method. In the procedure you described, is most important step is listed. After adding acetic acid to the solution, an excess of KI should be added to reduce Cu2+ ions quantitively. I₂, the oxided product, can be titrated by the sodium thiosulfate. The molar relationship between Cu and sodium thiosulphate is 1:1.

A similar process involves following:

• Use 5ml 1:1 HCl, 5ml 30% H₂O₂ to dissolve 0.2g sample in a 250ml conical flask and heat.
• After the sample is dissolved, heat a few more minutes to destroy the remaining H₂O₂.
• When the solution cools down, drip 1:1 NH₃H₂O until the solution becomes opaque, then drip 1:1 acetic acid until the solution becomes clear again and add 1ml more.
• After 1g KI is added into the solution, titrate it using the sodium thiosulfate solution immeditately.
• When the solution turns pale yellow, add 2ml 0.5% starch solution, and continue titrating until the blue color disappears.
• Then add 10ml 10% KSCN, stir the flask,and the blue color appears again.
• Under the condition of rocking the flask violently, continue titrating untill the blue color disappears. It is the end point.

2Cu²⁺ + 4I- ===> Cu₂I₂ + I₂
2S₂O₃ ^2- + I₂ ===> 2I- + S₄O₆ ^2-
Cu₂I₂ + 2SCN- ===> Cu₂SCN₂ + 2I-

Reference:Chemical Analysis & Instrumental Analysis Experiments, Ed. by Qi Qin et al. Nanjing Univ. Press, 1992.