第五章氧化还原滴定法

Ksp (CuI) = 2.0×10−12(I = 0.1), [I−] =1.0mol⋅ L−1 ϕθ ' = 0.17 + 0.69 = 0.86V > ϕθ (I2 / I−)
Example for Effect of pH on Formal Potential
pH8.0，
ϕθ '(As(V) / As(III)) = ?
• Applications of oxidation/reduction titrations
– Auxiliary oxidizing and reducing reagents – Applying standard reducing agents – Applying standard oxidizing agents
2
Na Li, Peking University
Cont’d
αF(H) = 1 + [H+ ]K H (HF) = 100.4 [F- ] = c(F- ) / αF(H) = 10−1.4
αFe3+ (F) =1+[F− ]β1 +[F−]2 β2 +[F− ]3 β3 =107.7 α = 1 Fe2+ (F)
Oxidation-Reduction Reactions
Ce4+ + Fe2+ = Ce3+ + Fe3+
z Oxidizing agent(氧化剂) has a strong affinity for electrons
z Reducing agent(还原剂) easily donates electrons to another species.
Na Li, Peking University
Chapter V Oxidation/Reduction Titration
• Introduction to Electrochemistry
– Oxidation/reduction reactions in electrochemical cells – Electrode potentials
potential convention by IUPAC,
1953
e
V
e
• Reduction Reaction
• The potential increases as the tendency for reduction increases.
Pt
Salt Bridge
Fe2+, Fe3+
Pt Ce3+, Ce4+
• By convention, the potential of SHE is assigned to a value of 0.000 V at all temperatures.
• An electrode potential is the potential of a cell that has a SHE as the reference.
n
a(R ed )
ϕ =ϕ\ (标准电位）where a(Ox) =1 mol/M and a(Red)=1 mol/L
ϕ
=
ϕٓ+
0 .0 5 9 n
lg
γ (O x) γ (Red)
+
0.059 lg [O x]
n
[R ed]
ϕ=ϕ\c (浓度电位）where [Ox]=1 mol/L and [Red]=1 mol/L
ϕ ٓ(Fe3+/Fe2+ ) = 0.77V, ϕٓ(I2/I- ) = 0.54V
For FeF3, lgI1-3=5.1, 9.2, 11.9 lgKH(HF)=3.1 *(I=0.1)
教程 例 5-2
ϕ = ϕ ∅ + 0.059lg aFe3+ aFe2+
≈
ϕ∅
+
0.059 lg
[Fe3+ ] [Fe2+ ]
• The higher the ϕ\，the stronger the oxidizing agent • The lower the ϕ\，the stronger the reducing agent
Electrode Potential
ϕ = ϕ ٓ+ 0 . 0 5 9 l g a ( O x )
Exceptional Example ： 邻二氮菲（phen), lgβ(Fe(phen)33+)= 14.1, lgβ(Fe(phen)32+)= 21.3
ϕ ′ٓ (Fe3+ /Fe2+ ) = 1.06 V (1 moliL-1 H2SO4 )
Example for Complex Formation
ϕ θ '(Fe3+ /Fe2+ ) = ϕ θc (Fe3+ /Fe2+ ) + 0.059 lg αFe2+ α Fe3+
= 0.77 − 0.059× 7.7 = 0.32V < 0.54V = ϕθ (I2/I− )
Fe3+will not oxidize I-.
Effect of Precipitation
Approach
= ϕ∅
c(Fe3+ ) + 0.059lg c(Fe2+ )
α Fe3+ α Fe2+
= ϕ∅
+ 0.059 lg αFe2+ α Fe3+
+
0.059
lg
c(Fe3+ c(Fe2+
) )
Sort backwards and calculate forwards: αM(F)→[F-]→[H+]
ϕ ϕ =
ٓ
(As(V)/As(III))
+
0.059
lቤተ መጻሕፍቲ ባይዱg
[H
+
]2[H
3
AsO4
]
2
[HAsO2 ]
ϕ x =
ٓ(As(V)/As(III))+ 0.059 log[H+]2
S2O3 2-
2Cu2+ + 4I− → 2CuI ↓ +I2
（Principle for Iodine/Thiosulfate Titration)
教程 例 5-1
ϕ = ϕ ∅ + 0.059 lg aCu2+ aCu+
≈
ϕ
∅
+
0.059 lg
[Cu2+ ] [Cu+ ]
Approach
=
ϕ
∅
+
This approach is seen in everyday life! For example, altitude is given in meters or feet above the see level!
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Na Li, Peking University
The Nernst Equation
Ionic strength is considered.
ϕ = ϕ \C + 0.059 lg α Red + 0.059 lg c(O x)
n
α Ox
n
c(R ed)
ϕ ٓ′(条件电位）
ϕ =ϕ \ |(条件电位/克式电位）Formal Potential where c(Ox)=1 mol/L and c(Red)=1mol/L. Side reaction occurs.
This approach is seen in everyday life! For example, altitude is given in meters or feet above the see level!
Measurement of the Electrode Potential for an Ag Electrode
H3AsO4 2.1 H2AsO4-
6.7 8.0 HAsO42- 11.2 AsO43-
HAsO2
9.1 AsO2-
pH
pKa
Half reaction
H3AsO4 + 2H+ + 2e- q HAsO2 + 2H2O
教程 例 5-3
When H+ or OH- are in involved in the half-cell reaction， Nernst Equation will include [H+] or [OH-]
0.059
lg
[Cu2+ ] Ksp (CuI) [I- ]
= ϕ ∅ + 0.059 lg
1
+ 0.059 lg[Cu2+ ]
Ksp (CuI) [I- ]
Ksp (CuI) = 2.0 ×10−12 (I = 0.1), [I− ] = 1.0 mol ⋅ L−1 ϕθ ' = 0.17 + 0.69 = 0.86 V > ϕθ (I2 / I− )
z Reaction unit– electron电子
Electrochemical Cell and Half-cell Reaction
Ecell = ECe-EFe
Half-cell reaction: Ox + ne- = Red ϕ(Ox/Red)
eS
The Gibbs-Stockholm electrode
Example:
At 25℃, [KI] = 1mol·L-1, Calculate formal potential for Cu2+/Cu+ (Disregard 忽略effect of ionic strength.)
ϕ ٓ(Cu2+/Cu+) = 0.17V, ϕ ٓ(I2 / I-) = 0.54V
Introduction to Electrochemistry
– Oxidation/reduction reactions in electrochemical cells
– Electrochemical cells – Electrode potentials/Half-cell potentials
The Standard Hydrogen Reference Electrode
• The standard hydrogen electrode (SHE) serves as a reference electrode so that relative electrode potential data can be widely applicable and useful. 2H+ (aq) +2e- q H2 (g)
Factors Affecting ϕ\′ -Ionic Strength of Solution
ϕ
⊖(
F
e
(
C
N
)
3 6
-
/
F
e
(C
N
)
4 6
-
)
=
0 .3 5 5 V
I
0.00064 0.00128 0.112
1.6
ϕ \′
0.3619 0.3814 0.4094 0.4584
Other than pointed out, Ionic Strength is not considered in most cases, that is, ϕ\c ≈ ϕ\
• Application of Standard Electrode Potential
– Calculating Potentials of electrochemical cells – Calculating redox equilibrium constant – Constructing redox titration curves – Oxidation/reduction indicators
Iodine/thiosulfate titration is used for titrating Cu2+ at pH 3.0. If trace amount of Fe3+ is in presence and c(F-)=0.1mol·L-1, calculate ϕ\′(Fe3+/Fe2+)
Ox + ne- q Red Reversible
ϕ (Ox / Red) = ϕ θ (Ox / Red) + 0.059 lg a(Ox) n a(Red)
ϕ\ (The standard electrode potential) is a measure of the relative strength of the oxidizing agents as electron acceptors or the reducing agents as electron donors.
Complex Formation
For Fe3+/ Fe2+, ϕ\(Fe3+/Fe2+)=0.77 V
Media( 1 mol·L-1)
ϕٓ (′Fe3+ / Fe2+ ) /V
HClO4 0.75
HCl 0.70
H2SO4 H3PO4 HF 0.68 0.44 0.32
与Fe3+的络合作用增强 Complex of Oxidizing form is more stable, which makes the electrode potential decrease.