lecture 22 10/26/05. outer curve: 25 ml of 0.100 m i - titrated with 0.0500 m ag + middle curve: 25...
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Outer curve: 25 mL of 0.100 M I- titrated with 0.0500 M Ag+
Middle curve: 25 mL of 0.0100 M I- titrated with 0.00500 M Ag+
Inner curve: 25 mL of 0.00100 M I- titrated with 0.000500 M Ag+
End-point detection for precipitation reactions
• Electrodes• Silver electrode
• Turbidity• Solution becomes cloudy due to
precipitation
• Indicators• Volhard• Fajans
Volhard (used to titrate Ag+)
• As an example: Cl- is the unknown• Precipitate with known excess of Ag+ • Ag+ + Cl- AgCl(s)
• Isolate AgCl (s), then titrate excess Ag+ with standard KSCN in the presence of Fe+3 • Ag+ + SCN- AgSCN(s)
• When all the Ag+ is gone:• Fe+3 + SCN- FeSCN2+
• (red color indicates end point)• Must be acidic….WHY?
Fajans (use adsorption indicator)
• Anionic dyes which are attracted to positively charged particles produced after the equivalence point
• Adsorption of dye produces color change• Fluorescein is common dye• Signals end-point