Date: Fri Jan 28 22:19:05 2000
Posted By: Xianlong Wang, Grad student, Chemistry, Nanjing Univ.
Area of science: Chemistry
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:
The reaction equations:
- 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
- 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
- Then add 10ml 10% KSCN, stir the flask,and the blue color
- 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.
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