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1、Electroanalytical chemistry,Potentiometry, Voltammetry and Polarography,Electroanalysis,measure the variation of an electrical parameter (potential, current, charge, conductivity) and relate this to a chemical parameter (the analyte concentration) Conductimetry, potentiometry (pH, ISE), coulometry,
2、voltammetry,Potentiometry,the measure of the cell potential to yield chemical information (conc., activity, charge),Measure difference in potential between two electrodes: reference electrode (E constant) indicator electrode (signal analyte),Reference electrodes,Ag/AgCl: Ag(s) | AgCl (s) | Cl-(aq) |
3、 .,Reference Electrodes,SCE: Pt(s) | Hg(l) | Hg2Cl2 (l) | KCl(aq., sat.) |.,Indicator Electrodes,Inert: Pt, Au, Carbon. Dont participate in the reaction. example: SCE | Fe3+, Fe2+(aq) | Pt(s) Certain metallic electrodes: detect their ions (Hg, Cu, Zn, Cd, Ag) example SCE | Ag+(aq) | Ag(s) Ag+ + e- A
4、g(s) E0+= 0.799V Hg2Cl2 + 2e 2Hg(l) + 2Cl- E-= 0.241V E = 0.799 + 0.05916 log Ag+ - 0.241 V,Ion selective electrodes (ISEs),A difference in the activity of an ion on either side of a selective membrane results in a thermodynamic potential difference being created across that membrane,Membrane Struct
5、ure(Examples),Glass Membrane Li2O, Cs2O, La2O3 ,SiO2 , CaO, BaO. KM-I pot or KH-Na = 10-13 pH glass electrode Na2 O (10.6%), Al2O3(10%), SiO2 = (NAS 10.6 10 ) KNa-K = 10-3 Na glass electrode,Single Crystal Lanthanum Fluoride LaF3 doped with Eu, Selective to F- Membrane,Mixed Salts (Ag2 S + AgX) For
6、Anion X, and,(Ag2 S + MS) For Cations,Examples on mixed salts For Anions,Ag2 S+ AgBr for Br-,Ag2 S + AgCl for Cl-,Ag2 S + AgI for I,Ag2 S + AgCN for CN-,Examples on mixed salts For Cations,Ag2 S + PbS for Pb2+,Ag2 S + CuS for Cu2+,Ag2 S + CdS for Cd2+,Ag2 S + NiS for Ni2+,Liquid Ion Exchanger,Calciu
7、m didecyl phosphoric acid For Ca2+,Types of Ion-Selective Electrodes,ISEs,Combination glass pH Electrode,Proper pH Calibration,E = constant constant.0.0591 pH Meter measures E vs pH must calibrate both slope & intercept on meter with buffers Meter has two controls calibrate & slope 1st use pH 7.00 b
8、uffer to adjust calibrate knob 2nd step is to use any other pH buffer Adjust slope/temp control to correct pH value This will pivot the calibration line around the isopotential which is set to 7.00 in all meters,mV,pH,4 7,Calibrate knob raises and lowers the line without changing slope,mV,pH,4 7,Slo
9、pe/temp control pivots line around isopotential without changing it,Liquid Membrane Electrodes,Solid State Membrane Electrodes,Ag wire,Filling solution with fixed Cl- and cation that electrode responds to,Ag/AgCl,Solid state membrane (must be ionic conductor),Nikolsky Equation,Selectivity Coefficien
10、t, KM-I,Response Time Time in mins. or seconds needed for the Potential to reach Equilibrium from the point of changing Concn. (t0 ),Sensitivity of Ion-Selective Electrodes and the Effect of Interferences,Concentration Determination of Ions Using Ion-Selective Electrodes,1- Direct Potentiometry, (Di
11、rect Calibration) The Potential ( E ) of an Ion-Selective Electrode (ISE) is following the concentration of the ion According to simplified Nernst Equation: E = S/n log aA In Analytica Chemistry, it is not easy to use activity (a) but to solve this problem, We Consider this relationship: a = , where
12、 is activity coefficient. In order to make a C we must keep constant. This can be achieved by Using high concentration Ionic strength of an Inert Electrolyte, or better to Use Total Ionic Strength Adjusting Buffer (TISAB). This is prepared from a high Concentration(such as 1 or 2 M ) mixture of (an
13、Inert Electrolyte such as KNO3 + a Buffer to adjust pH + a Ligand to mask metal ion interferences). This solution Is added usually in 1:1 ratio to all the standards and unknowns. The unknowns Will be determined from the Calibration Curve Which will be plotted now from the modified Nernst Eq. E = S/n
14、 log A . E is plotted as A function of logA,2- Standard Addition, According to,This is performed in two ways: a) Single Addition (TISAB is always added) For Simplicity, assume n = 1 and that the analyte is Anion, and E2 = - S/n E2 - E1 = E = S( x ) = , after Manipulation The final Eq. to find Co wil
15、l be :,E1,E2,. E1 = - S/n,b). Multiple Addition ( GranS Plot),. E1 = - S/n If the left side of this Eq. = F then : F Vs If F = 0 Then CoVo = - CsVs which means that F vs Vs is a straight line This is shown in this figure,. E = - S,F,3). Potentiometric TitrationEnd Point Detection,a). First Derivativ
16、e, b) Second Derivative and c) Grans Plot You have already studied methods (a and b), we shall now consider ( c ). Grans Plot: This method Converts the ( S shaped)PotentiometricTitration Curve into a straight line, making the location of (E.P.) more reliable Let Consider this reaction A+ + B- AB Ass
17、uming that (A & B ) are Monovalent, the electrode is selective for B- and B- is anion. Accordingly: at the E.P. Co Vo = CA Ve Co = CA Ve / Vo CB is the Concn. of B at any point remaining in soln. Substituting the Value of Co and after necessary Manipulations we obtain The following Equation :,CA,Ve,
18、A,VA,E.P,CB,or Co,Vo,B,We mentioned before that the Electrode is Selective for B, So it measures B According to Nekolsky Eq. E = S log.CB Substituting the Value of CB from the previous Eq. we get: E = S log. Then conversion into antilog we obtain the Grans Function: At the End Point Which means that
19、 F = 0,Grans Plot,0,Solid state electrodes,Voltammetry,The measurement of variations in current produced by variations of the potential applied to a working electrode polarography: Heyrovsky (1922): first voltammetry experiments using a dropping mercury working electrode In voltammetry, once the app
20、lied potential is sufficiently negative, electron transfer occurs between the electrode and the electroactive species: Cu2+ + 2e Cu(Hg) There are many types of Polarography, but, we shall Study DC Polarography only.,Mass Transport or Mass Transfer,Migration movement of a charged particle in a potent
21、ial field Diffusion movement due to a concentration gradient. If electrochemical reaction depletes (or produces) some species at the electrode surface, then a concentration gradient develops and the electroactive species will tend to diffuse from the bulk solution to the electrode (or from the elect
22、rode out into the bulk solution) Convection mass transfer due to stirring. Achieved by some form of mechanical movement of the solution or the electrode i.e., stir solution, rotate or vibrate electrode Difficult to get perfect reproducibility with stirring, better to move the electrode Convection is
23、 considerably more efficient than diffusion or migration = higher currents for a given concentration = greater analytical sensitivity,Diffusion,Movement of mass due to a concentration gradient. Occurs whenever there is chemical change at a surface, e.g., O R,Movement of a charged species due to a po
24、tential gradient i.e. Attraction of Opposites Charges Mechanism by which charge passes through electrolyte towards the Electrode Surface,Migration,Convection Movement of mass due to a natural or mechanical force, such as Stirring or gas bubbling,id(max)= 708nD1/2m2/3t1/6c,Ilkovi Equation,id (average
25、)=607nD1/2m2/3t1/6c When 708nD1/2m2/3t1/6 = K, then id = K*C Which is the basis of Quantitative Analysis.,Co,C,The mechanism of producing the Diffusion Current (i) i = (nDFA/ )*(C-Co) n = no.of electrons, D = diffusion Coefficient, F = Faraday, A = surface area, = Thickness of the diffusion layer, C
26、o mole/L = Cocn. on the drop surface & C mol./L= Concn. In the bulk of Solution. When the ion concn. On the drop surface becomes zero, Co= 0 and (nDFA/ ) is constant = K, then i = id (which is the Limiting Diffusion Current)Thus, the equation above becomes id = K*C But this Equation has been replace
27、d by a more sophisticated Ilkofi Equation as shown below:,Dropping Mercury Electrode (DME),The polarogram,points a to b I = E/R points b to c electron transfer to the electroactive species. I(reduction) depends on the no. of molecules reduced/s: this rises as a function of E points c to d when E is
28、sufficiently negative, every molecule that reaches the electrode surface is reduced.,Q What are the Suitable Electrolyte for the Mixture (1,2 and 3) and for the Individuals if Present Alone? And Why?,Effect of Supporting Electrolytes on Polarographic E1/2,Polarography ( Two Electrodes System),Droppi
29、ng Mercury Electrode(DME) Polarography (Three Electrodes System),Renewable surface Potential window expanded for reduction (high overpotential for proton reduction at Hg El.),Polarography,Polarograms,E1/2 = E0 + RT/nF log (DR/Do)1/2 (reversible couple) Usually Ds are similar so half wave potential i
30、s similar to formal potential. Also potential is independent of concentration and can therefore be used as a diagnostic of identity of analytes.,Residual Current,Qualitative and Quantitative Analysis by Polarography,Qualitative,Quantitative,Problems in Polarography and Limitations 1- Dissolved Oxyge
31、n,O2 + 2H2 O + 2e = H2 O2 + 2OH- H2 O2 + 2e = 2OH- In Acidic Medium In Basic Medium O2 + 2H+ + 2e = H2 O2 H2 O2 + 2H+ + 2e = 2H2 O,H2 O2 + 2e = 2OH-,O2 + 2H2 O + 2e = H2 O2 + 2OH-,id,id,Amperometry,Amperometric Titrations,added,COULOMETRY,Coulometry is an analytical method for measuring an unknown c
32、oncentration of an analyte in solution by completely converting the analyte from one oxidation state to another. Coulometry is an absolute measurement similar to gravimetry or titration and requires no chemical standards or calibration.,It is therefore valuable for making absolute concentration dete
33、rminations of standards. Coulometry uses a constant current source to deliver a measured amount of charge. One mole of electrons is equal to 96,485 coulombs of charge, and is called a faraday.,Schematic of a coulometric cell,Coulometric Titration Due to concentration polarization it is very difficul
34、t to completely oxidize or reduce a chemical species at an electrode. Coulometry is therefore usually done with an intermediate reagent that quantitatively reacts with the analyte. The intermediate reagent is electrochemically generated from an excess of a precursor so that concentration polarizatio
35、n does not occur. An example is the electrochemical oxidation of I- (the precursor) to I2 (the intermediate reagent). I2 can then be used to chemically oxidize organic species such as ascorbic acid.,The point at which all of the analyte has been converted to the new oxidation state is called the end
36、point and is determined by some type of indicator that is also present in the solution. For the coulometric titration of ascorbic acid, starch is used as the indicator. At the endpoint, I2 remains in solution and binds with the starch to form a dark purple complex. The analyte concentration is calcu
37、lated from the reaction stoichiometry and the amount of charge that was required to produce enough reagent to react with all of the analyte.,Coulometry and Coulometric Titrations,Coulometric Measurement,Two Main procedures are Usually applied for measuring Coulometry: 1- Using Electronic Coulometry,
38、2- Chemical Coulometry (which will be discussed). Chemical Coulometry: Here we discuss the Theory of this Method, which depends on Faraday Law, and water Electrolysis:,Total No of moles of,One Eq.Wt. of O2 =,Mole O2,One Eq.Wt. of H2 = mole H2,1 F will evolve 1 Eq.Wt. of( O2 + H2 ),Thus: 1F will evol
39、ve,We must find No of F,Applications of Coulometry,1-When the Deposition is not possible and 2- When Deposition is not Successful,Fe2+ = Fe3+ + e Anodic AsO3 + H2 O AsO43- + 2H+ + 2e Anodic Cl3 CC OO- + H+ + 2e Cl2 CHC OO- + Cl- Cathdic,Examples on Coulometric Titratios,Reagent Generation,1- Determi
40、nation of Sulphide S2-,E.P. Detection,Example 1- Determination of Sulphide S2-,2-,End Point Detection,After switching on The Current (i) will start on Pt Electrode and Br2 will generate on the Anode,+,-,On the Pt anode which will Oxidise AsO33- to AsO43-,This Redox reaction Continues and measure Amp
41、erometrically Until the First sign of Xss. Br2 at the End-Point will send a Signal to Switch Off the Reagent Generation. The time t (s) and the Constant Current ( i)amp. Are then Measured and No. of Coulombs(q) is Calculated. q = it from here Faraday F is found F = q/96500. Can You Now find the Weig
42、ht (Wg) of the Analyte Unknown? A problem: A constant current of 0.8 A, was passed through a Cu2+ solution for 20 min. Find the weight of Deposited Cu on Cathode and Vol. of Librated O2 on the Anode.,Why Electrons Transfer,EF,Eredox,Net flow of electrons from M to solute Ef more negative than Eredox
43、 more cathodic more reducing,Reduction,Oxidation,Net flow of electrons from solute to M Ef more positive than Eredox more anodic more oxidizing,E,E,Steps in an electron transfer event,O must be successfully transported from bulk solution (mass transport) O must adsorb transiently onto electrode surf
44、ace (non-faradaic) CT must occur between electrode and O (faradaic) R must desorb from electrode surface (non-faradaic) R must be transported away from electrode surface back into bulk solution (mass transport),Nernst-Planck Equation,Ji(x) = flux of species i at distance x from electrode (mole/cm2 s
45、) Di = diffusion coefficient (cm2/s) Ci(x)/x = concentration gradient at distance x from electrode (x)/x = potential gradient at distance x from electrode (x) = velocity at which species i moves (cm/s),Diffusion,Ficks 1st Law,Solving Ficks Laws for particular applications like electrochemistry invol
46、ves establishing Initial Conditions and Boundary Conditions,I = nFAJ,Simplest Experiment Chronoamperometry,Simulation,Recall-Double layer,Double-Layer charging,Charging/discharging a capacitor upon application of a potential step,Itotal = Ic + IF,Working electrode choice,Depends upon potential windo
47、w desired Overpotential Stability of material Conductivity contamination,Polarography,A = 4(3mt/4d)2/3 = 0.85(mt)2/3,Mass flow rate of drop,Density of drop,We can substitute this into Cottrell Equation,i(t) = nFACD1/2/ 1/2t1/2,Giving the Ilkovich Equation: id = 708nD1/2m2/3t1/6C I has units of Amps
48、when D is in cm2s-1,m is in g/s and t is in seconds. C is in mol/cm3 This expression gives the current at the end of the drop life. The average current is obtained by integrating the current over this time period iav = 607nD1/2m2/3t1/6C,We also replace D by 7/3D to account for the compression of the diffusion layer by the expanding drop,Other types of Polarography,Examples re