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