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Lecture 2711/07/05
Two things happening:
First: Titration reaction (Ce4+ + Fe2+ ⇄ Ce3+ + Fe3+)– Goes to completion– Before equiv. point: excess Fe2+
– After equiv. point: excess Ce4+
Second: Reduction of Ce4+ or Fe3+ from cell potential– Negligible effect on concentration– Produces voltage for y-axis
– 2Fe3+ +2Hg(l) +2Cl- ⇄ 2Fe2+ +Hg2Cl2 (s)
– 2Ce4+ +2Hg(l) +2Cl- ⇄ 2Ce3+ +Hg2Cl2 (s)
At the equivalence point Y-axis: [Ce3+]=[Fe3+] and [Ce4+]=[Fe2+] Ce3+ + Fe3+ ⇄ Ce4+ + Fe2+
o
ceriumoironcathode
43
32ocerium
oironcathode
4
3ocerium3
2oironcathode
4
3oceriumceriumcathode
3
2oironironcathode
EE21E
]Ce][Fe[
]Ce]Fe[log05916.0EEE2
]Ce[
]Ce[log05916.0E
]Fe[
]Fe[log05916.0EE2
]Ce[
]Ce[log
1
05916.0EEE
]Fe[
]Fe[log
1
05916.0EEE
At the equivalence point
Ecell = ½(E°Ce + E°Fe) - 0.241
Ecell = ½(1.70 + 0.767) - 0.241
Ecell = 0.99 V
Before the equivalence point Excess Fe Ecell = Ecathode – SCE X-axis: Good place to pick is ½ Ve so that [Fe3+] =
[Fe2+]
V526.0E
241.0767.0E
241.0]Fe[
]Fe[log05916.0767.0E
3
2
After the equivalence point
Excess Ce X-axis: good place to pick is 2Ve since [Ce3+] = [Ce4+]
V46.1E
241.070.1E
241.0]Ce[
]Ce[log05916.070.1E
4
3
Redox indicators
Changes color when changes oxidation state– Look at E° to find good indicator
In(ox) + ne- In(red) E° = 1.147 V – ferroin
Can see red when [In(red)] / [In(ox)] > 10/1 Can see blue when [In(red)] / [In(ox)] < 1/10
V962.0 E
244.010
1log
1
05916.0 - 147.1 E blue when
V844.0 E
244.01
10log
1
05916.0 - 147.1 red when E
Starch-Iodine complex
Not a redox reaction
I2 forms I6 inside amylose helix and turns dark blue
Two things:
1) starch is biodegradable
2) Glucose (product of starch breakdown) is reducing agent
Pre-oxidation
Must be quantitative Must be able to remove/destroy oxidizing agent Peroxydisulfate or persulfate (S2O8
2-)
– Ag+ as catalyst: S2O82- + Ag+ SO4
2- + SO4- + Ag2+
– Boiling: 2S2O82- + 2H2O 4SO4
2- + O2 +H+
AgO– Ag2+ : powerful oxidizing agent
– Boiling: 4Ag2+ + 2 H2O 4 Ag+ +O2 + 4H+
NaBiO3
– BiO3- + 4H+ + 2e- BiO+ + 2H2O
– Filter to remove excess H2O2
– Good oxidizing agent in
– Boiling: 2H2O2 H2O + O2 (gas)