Voltammetry ¼®‰ˆ†³•. Basic principle of voltammetry...
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Basic principle of voltammetryVoltammetry: A group of electrochemical methods based on measuring current (i)- applied potential curve during electrolysis - only a small amount of sample (analyte) is used
Polarography: Invented by J. Heyrovsky (Nobel Prize 1959). Differs from voltammetry in that it employs a dropping mercury electrode (DME) as Working electrode to continuously renew the electrode surface.
Read: pp. 716 753 Problems: 25-1,2,3,6,13
Polarographic analysis Electrolytic analysis carried out under special conditions.1. Polarographic analysis process and the conditions for polarographic wave formationspecific characteristicsAa polarized electrode and a depolarized electrode are used as working electrodeBNo stirring Incomplete electrolysis (only a small amount of analyte is consumed)
If the electrode potential has great changes when infinite small current flow through the electrode, such electrode is referred to as polarized electrode. eg. DME ; If the electrode potential does not change with current , such electrode is called ideal depolarized electrode. eg. SCEPolarized electrode and depolarized electrode
Three electrode cell: WorkingReferenceCounter/auxilliary
current flows between working and counter electrodes. Potential controlled by potentiostat between working and reference electrodes.
Two special electrodesSupporting electrolyte : Usually relatively higher concentration of strong electrolytes (alkali metal salts) serves as supporting electrolyteDissolved oxygen is usually removed by bubbling nitrogen through the solutionVoltage scanning Under unstirred state, recording voltage - current curve
residual current electrolytic currentlimiting diffusion currentCd 2+ +2e + Hg = Cd(Hg)2Hg + 2Cl- -2e = Hg2Cl2
-0.2-0.4-0.6-0.8-1.0-1.2-1.4i (A)0.001 M Cd2+ in 0.1 M KNO3 supporting electrolyteV vs SCEidEBase line of residual current
E at iLimiting currentRelated to concentration
Limiting diffusion current -- A basis of polarographically quantitative analysis When the applied voltage exceeds the decomposition voltage, diffusion-controlled current is expressed as:i = K(C-C0)When the applied voltage gets more negative, C0 0, current becomes only diffusion limited, thenid = KCId reaches a limiting value proportional to ion concentration C in bulk solution, and do not changes with applied voltage longer
The potential at which the current is equal to one half the limiting current is called the half-wave potential and given the symbol E1/2.Half-wave potential polarographic qualitative analysis
How it works? The applied voltage is gradually increased, typically by going to a more positive( more negative decomposing potential) A small residual current is observed. When the voltage becomes great enough, reduction occurs at the analytical electrode causing a current. The electrode is rapidly saturated so current production is limited based on diffusion of the analyte to the small electrode.
How it works ?
The reduced species alters the surface of the mercury electrode.
To prevent problems, the mercury surface is renewed by knocking off a drop providing a fresh surface.
This results in an oscillation of the data as it is collected.
2. The diffusion current theory and polarographic wave equationIn above equations, K is called Ilkovic constant, it is expressed as follows:id = KCWe have already known:K = 607 n D1/2m2/3t1/6Thus,id = 607nD1/2m2/3t1/6C
From above equation, we can find that when temperature, matrix solution and capillary characteristic are kept constant, id is proportional to C
polarographic wave equationWhen i = id , log term in above equation is equal to zero, corresponding potential is called halfwave potential E1/2E1/2 independent on the concentrationbasis of qualitative analysis
Residual current (1) redox reactions of impurities in solution (2) charging of Hg drop(non-faradaic current / non-redox current) Migration current The current produced by static attraction of the electrode to sought-for ions
3. Interference current in classical DC polarography
Complex artifactual phenomenonLess likely at low drop rates, in concentrated electrolyte, or low concentration of electroactive speciesLessened by inclusion of surfactants in medium Polarographic Maximum (or malformed peak )
Oxygen wave Dissolved oxygen is easily reduced at many working electrodes. Thus an aqueous solution saturated with air exhibits two distinct oxygen waves. The first results from the reduction of oxygen to hydrogen peroxide: O2 + 2H+ + 2e- H2O2 The second wave corresponds to the further reduction of hydrogen peroxide: H2O2 + 2H+ + 2e- 2H2O Sparge solutions with high purity N2 or Ar for 5-20 min
Factors that affect limiting diffusion currentCharacteristics of capillary hight of HgPotential of dropping Hg electrodeComposition of solutionTemperatureFactors that affect half-wave potentialType and concentration of supporting electrolyteTemperatureForming complexAcidic of solution
Question Why a reference electrode with large area and a dropping mercury electrode with very small area are used to electrolyze in polarographic analysis ? Why large amount of supporting electrolyte is added to sample solution? Why does nitrogen gas pass through the solution before electrolysis ? In the process of polarographic analysis whether or not to carry out stirring the solution? Why?
idavg = KcDirect comparison methodCalibration curve methodStandard addition method4. Polarographically quantitative analytical methods
Fundamental studiesInorganic applicationsOrganic applicationsApplications in pharmaceutical and biochem fields5. Applications