85 Voltammetry 85-1 Basic principle of polarograpy Voltammetry a group of analytical methods based on determining current flow-voltage curve during electrolysis Several types of methods based on Electrode type How the potential is applied How the current is measured
§5 Voltammetry §5-1 Basic principle of polarograpy Voltammetry A group of analytical methods based on determining current flow – voltage curve during electrolysis Several types of methods based on Electrode type How the potential is applied How the current is measured
polarogra phy The cell crrent is measured during electrolysis at various potentials Most common electrode type is a dropping merry electrode (DME) A current vs. potential arve is produced
When applied voltage reaches the B decomposition voltage of metal ion: Reduction reaction on cathode: Cd2++ 2e Cd Oxidation reaction on anode: 2OH-2eH2O+1202 1→U外U=iR 电解装置 Where Uy denotes applied potential E-电源R可变电阻器 Ud is decomposition potential, R is 一伏特计A-安培计 total resistance of electrolysis circuit, PL一铂电极/-溶液(Cd2) i is current through the circuit. 只—电磁搅拌器
When applied voltage reaches the decomposition voltage of metal ion: Reduction reaction on cathode: Cd2+ + 2e- Cd Oxidation reaction on anode: 2OH- -2e H2O + 1/2 O2 U外 ∝ i Where U外 denotes applied potential, Ud is decomposition potential, R is total resistance of electrolysis circuit, i is current through the circuit. U外- Ud= iR (Cd2+)
When following conditions are obeyed, Uyp -i graph is showed in following figure Lower current density during electrolysis Adequately stirring to remove concentration gradient No quantitative relationship between current(or potential and concentration of 外加电压 interested ions 大面积铂片电极(阴极) 溶液经充分搅拌
When following conditions are obeyed, U外 – i graph is showed in following Figure Lower current density during electrolysis Adequately stirring to remove concentration gradient No quantitative relationship between current (or potential) and concentration of interested ions
To find the analytical relationship between the current and the concentration of sought-for ions, following measurements are adopted OMICROPLATINUMELECTRODE Or DROPPING MERCURY ELECTRODE ( in most common use) to insure high current density ○ NO STIRRING to insure enough high concentration difference between the electrode surface and the solution bulk Then RT E=Eo+ nF in cm
To find the analytical relationship between the current and the concentration of sought-for ions , following measurements are adopted: ●MICROPLATINUM ELECTRODE Or DROPPING MERCURY ELECTRODE ( in most common use) to insure high current density ● NO STIRRING to insure enough high concentration difference between the electrode surface and the solution bulk Then E = Eo + ln CM RT nF
When the current flowed for only a short period through the electrode, ion concentration on the surface of the electrode reduce suddenly. The difference in ion concentrations between the surface and the bulk solution is equivalent to an electrochemical cell. which is called concentration polarization(浓差极化).thus, there is a voltage that is equivalent to this concentration change called concentration polarization potential(浓差电 D When a microplatinum electr s electrode is used and no stirri 极限电流 concentration polarization tal imiting current polarogram is obtained A 外加电压 微铂电极(阴极),电解时不 搅拌溶液
When the current flowed for only a short period through the electrode, ion concentration on the surface of the electrode reduce suddenly. The difference in ion concentrations between the surface and the bulk solution is equivalent to an electrochemical cell, which is called concentration polarization(浓差极化). thus, there is a voltage that is equivalent to this concentration change called concentration polarization potential（浓差电势） When a microplatinum electrode or dropping mercury electrode is used and no stirring is carried out, concentration polarization takes place soon, following polarogram is obtained Limiting current A B C D
olara Battery galvanometer net voltomm Sample Professor Jaroslav heyrovsky (1890-1967)and his polarogra which he first described in 1922 The output was recorded on photographic film
When applied voltage does not reach reduction voltage of metal ion, only a little current flow, called residual current produced by reduction of impurities and charging current, eg AB in the Figure Arriving on decomposition voltage, the electrode reactions occur. the current rises rapidly with increasing voltage. bC section in the figure Cathode dme: 极限电流 miting current Cd+ 2e- +Hg Cd(Hg) B amalgam△ 外加电压 Anode(calomel or pool mercury electrode with a big area) 微铂电极(阴极),电解时不 2Hg -2e"+ 2CI-Hg2 Cl 搅拌溶液
When applied voltage does not reach reduction voltage of metal ion, only a little current flow, called residual current produced by reduction of impurities and charging current, eg AB in the Figure Limiting current A B C D Arriving on decomposition voltage, the electrode reactions occur, the current rises rapidly with increasing voltage, BC section in the figure Cathode (DME): Cd2+ + 2e- +Hg Cd(Hg) (amalgam) Anode (calomel or pool mercury electrode with a big area): 2Hg - 2e- + 2Cl- Hg2Cl2
When it reaches C point voltage, due to concentration polarization, the current arrives at a limiting value and does not rise markedly, called limiting current, consisting of residual current and diffusion current 极谱波可以用U外表示曲线表示,也可以用i~Et线来表示,从下 面的讨论可以看出,二者是基本重合的。 U=(ESCE -E de+ir i and r are very little i polarographic electrolysis u esce -Ede =-edel vs sce To remove completely influence of iR, three electrode system is usually used
When it reaches C point voltage, due to concentration polarization, the current arrives at a limiting value and does not rise markedly, called limiting current, consisting of residual current and diffusion current 极谱波可以用i~ U外表示曲线表示，也可以用i ~ Ede曲线来表示，从下 面的讨论可以看出，二者是基本重合的。 U = ( ESCE -Ede ) + i R ∵ i and R are very little in polarographic electrolysis U = ESCE -Ede = -Ede( vs. SCE) To remove completely influence of iR, three electrode system is usually used
√ Very little oscillation 间 在长冑期检流计上观蔡 到的电流一阿曲线
Very little oscillation