chapters 22, 23, 24 & 25 electroanalytical chemistry
Post on 19-Dec-2015
259 views
TRANSCRIPT
![Page 1: Chapters 22, 23, 24 & 25 Electroanalytical Chemistry](https://reader033.vdocuments.mx/reader033/viewer/2022061614/56649d395503460f94a129f1/html5/thumbnails/1.jpg)
Chapters 22, 23, 24 & 25
Electroanalytical Chemistry
![Page 2: Chapters 22, 23, 24 & 25 Electroanalytical Chemistry](https://reader033.vdocuments.mx/reader033/viewer/2022061614/56649d395503460f94a129f1/html5/thumbnails/2.jpg)
Electroanalytical Chemistry
![Page 3: Chapters 22, 23, 24 & 25 Electroanalytical Chemistry](https://reader033.vdocuments.mx/reader033/viewer/2022061614/56649d395503460f94a129f1/html5/thumbnails/3.jpg)
Electroanalytical Chemistry...
It encompasses a group of quantitative
analytical methods that are based upon
the electrical properties of a solution of
the analyte when it is made part of an
electrochemical cell.
![Page 4: Chapters 22, 23, 24 & 25 Electroanalytical Chemistry](https://reader033.vdocuments.mx/reader033/viewer/2022061614/56649d395503460f94a129f1/html5/thumbnails/4.jpg)
Why Electroanalytical Chemistry?
• Electroanalytical methods have certain advantages over other analytical methods. Electrochemical analysis allows for the determination of different oxidation states of an element in a solution, not just the total concentration of the element.
• Electroanalytical techniques are capable of producing exceptionally low detection limits and an abundance of characterization information including chemical kinetics information. The other important advantage of this method is its low cost.
![Page 5: Chapters 22, 23, 24 & 25 Electroanalytical Chemistry](https://reader033.vdocuments.mx/reader033/viewer/2022061614/56649d395503460f94a129f1/html5/thumbnails/5.jpg)
Galvanic Electrochemical Cell with Salt Bridge
![Page 6: Chapters 22, 23, 24 & 25 Electroanalytical Chemistry](https://reader033.vdocuments.mx/reader033/viewer/2022061614/56649d395503460f94a129f1/html5/thumbnails/6.jpg)
Electroanalytical techniques are capable of producing exceptionally low detection limits and a wealth of characterization information describing electrochemically addressable systems. Such information includes stoichiometry and rate of interfacial charge transfer, rate of mass transfer, extent of adsorption or chemisorption, and rates and equilibrium constants for chemical reactions.
![Page 7: Chapters 22, 23, 24 & 25 Electroanalytical Chemistry](https://reader033.vdocuments.mx/reader033/viewer/2022061614/56649d395503460f94a129f1/html5/thumbnails/7.jpg)
History
Polarography was first discovered by a czechoslovavian chemist by the name of Heyrovsky in 1920. He won the Nobel prize for it in 1959. He proposed that the current recording generated by a oxidation or reduction in a cell as the A.P. is continuously increased:
Oxidizing Agent + ne Reduction
![Page 8: Chapters 22, 23, 24 & 25 Electroanalytical Chemistry](https://reader033.vdocuments.mx/reader033/viewer/2022061614/56649d395503460f94a129f1/html5/thumbnails/8.jpg)
Oxygen Probe
A.P. – 650 mV Ag|AgCl
Reactions:
O2 + 2H2O + 4e- 4OH-
4Ag + 4Cl- 4AgCl + 4e-
![Page 9: Chapters 22, 23, 24 & 25 Electroanalytical Chemistry](https://reader033.vdocuments.mx/reader033/viewer/2022061614/56649d395503460f94a129f1/html5/thumbnails/9.jpg)
Hydrogen Probe
A.P. +650 mV
Reactions:
H2O2 O2 +2H+ +2e-
2Ag+ + 2e- 2Ag
![Page 10: Chapters 22, 23, 24 & 25 Electroanalytical Chemistry](https://reader033.vdocuments.mx/reader033/viewer/2022061614/56649d395503460f94a129f1/html5/thumbnails/10.jpg)
Daniell Cell
This cell is based on the overall reaction
[Cu(OH2)6]2+
(aq) + Zn --> Cu + [Zn(OH2)6]2+
(aq)
and functions by dissolution of Zn from the anode and deposition of Cu at the cathode. It is therefore very simply represented as
Zn | [Zn(OH2)6]2+
(aq) || [Cu(OH2)6]2+
(aq) | Cu
or just as Zn | Zn(II)(aq) || Cu(II)(aq) | Cu
![Page 11: Chapters 22, 23, 24 & 25 Electroanalytical Chemistry](https://reader033.vdocuments.mx/reader033/viewer/2022061614/56649d395503460f94a129f1/html5/thumbnails/11.jpg)
Galvanic Cells
• A galvanic cell consists of at least two half cells, a reduction cell and an oxidation cell. Chemical reactions in the two half cells provide the energy for the galvanic cell operations. The reactions always run spontaneously in the direction that produced a positive cell potential
![Page 12: Chapters 22, 23, 24 & 25 Electroanalytical Chemistry](https://reader033.vdocuments.mx/reader033/viewer/2022061614/56649d395503460f94a129f1/html5/thumbnails/12.jpg)
Voltaic Cells
• A voltaic cell is an electrochemical cell that external electrical current flow can be created using any two different metals since metals differ in their tendency to lose electrons. Zinc more readily electrons than copper, so placing zinc and copper metal in solutions of their salts can cause electrons to flow through an external wire which leads from the zinc to the copper. The following is a diagram of a voltaic cell.
![Page 13: Chapters 22, 23, 24 & 25 Electroanalytical Chemistry](https://reader033.vdocuments.mx/reader033/viewer/2022061614/56649d395503460f94a129f1/html5/thumbnails/13.jpg)
Cathodes and Anodes
![Page 14: Chapters 22, 23, 24 & 25 Electroanalytical Chemistry](https://reader033.vdocuments.mx/reader033/viewer/2022061614/56649d395503460f94a129f1/html5/thumbnails/14.jpg)
Reactions at the Anode
• Some examples are:
Cu(s) <=> Cu2+ + 2e-
Fe2+ <=> Fe3+ + e-
H2(g) <=> 2H+ + 2e-
Ag(s) + Cl- <=> AgCl(s) + e-
![Page 15: Chapters 22, 23, 24 & 25 Electroanalytical Chemistry](https://reader033.vdocuments.mx/reader033/viewer/2022061614/56649d395503460f94a129f1/html5/thumbnails/15.jpg)
Reactions at Cathodes
• Electrons supplied by external circuit via an inert electrode (platinum or gold)
• Some examples are:
Cu2+ + 2e- <=> Cu(s)
Fe3+ + e- <=> Fe2+
2H+ + 2e- <=> H2(g)
AgCl(s) + e- <=> Ag(s) + Cl-
![Page 16: Chapters 22, 23, 24 & 25 Electroanalytical Chemistry](https://reader033.vdocuments.mx/reader033/viewer/2022061614/56649d395503460f94a129f1/html5/thumbnails/16.jpg)
Cells Without Liquid Junctions
• Liquid junction - the interface between 2 different electrolytic solutions
• Cell can contain more than one• A small Junction Potential arises at these
interfaces• Sometimes it is possible to prepare cells
that share a • Common electrolyte to avoid this problem
![Page 17: Chapters 22, 23, 24 & 25 Electroanalytical Chemistry](https://reader033.vdocuments.mx/reader033/viewer/2022061614/56649d395503460f94a129f1/html5/thumbnails/17.jpg)
Concentration Cells
• A concentration cell is an electrochemical cell in which the electrode couple at both electrodes is the same but the concentrations of substances at the two electrodes may differ. The potential difference across a concentration cell can be calculated using the Nernst equation.
![Page 18: Chapters 22, 23, 24 & 25 Electroanalytical Chemistry](https://reader033.vdocuments.mx/reader033/viewer/2022061614/56649d395503460f94a129f1/html5/thumbnails/18.jpg)
Calculation of Cell Potential from Electrode Potentials
• Nernst Equation: The cell potential for a voltaic cell under standard conditions can be calculated from the standard electrode potentials. But real voltaic cells will typically differ from the standard conditions. The Nernst equation relates the cell potential to its standard cell potential.
![Page 19: Chapters 22, 23, 24 & 25 Electroanalytical Chemistry](https://reader033.vdocuments.mx/reader033/viewer/2022061614/56649d395503460f94a129f1/html5/thumbnails/19.jpg)
Cell Potential
• One implication is that the cell potential will be reduced from the standard value if the concentration of Zn2+(aq) is greater than that of Cu2+
(aq) at the standard temperature. An excess concentration of Cu2+(aq) will give a higher voltage. The graph at right shows the increase in cell voltage with increasing concentration of the cation. Note that the horizontal axis is logarithmic, and that the straight line variation of the voltage represents an logarithmic variation with Q. Note that the cell potential is equal to the standard value if the concentrations are equal even if they are not equal to the standard value of 1M, since the logarithm gives the value zero.
![Page 20: Chapters 22, 23, 24 & 25 Electroanalytical Chemistry](https://reader033.vdocuments.mx/reader033/viewer/2022061614/56649d395503460f94a129f1/html5/thumbnails/20.jpg)
The Nernst Equation
R = gas constant
T = temperature in Kelvins
Q = thermodynamic reaction quotient
F = Faraday's constant
n = number of electrons transferred
![Page 21: Chapters 22, 23, 24 & 25 Electroanalytical Chemistry](https://reader033.vdocuments.mx/reader033/viewer/2022061614/56649d395503460f94a129f1/html5/thumbnails/21.jpg)
Currents in Electrochemical Cells
• Ohms law is usually obeyed:
E=IR
where E is the potential difference in volts responsible for the movement of the ions, I is the current in amps, and R is the resistance in ohms of the electrolyte to the current
![Page 22: Chapters 22, 23, 24 & 25 Electroanalytical Chemistry](https://reader033.vdocuments.mx/reader033/viewer/2022061614/56649d395503460f94a129f1/html5/thumbnails/22.jpg)
![Page 23: Chapters 22, 23, 24 & 25 Electroanalytical Chemistry](https://reader033.vdocuments.mx/reader033/viewer/2022061614/56649d395503460f94a129f1/html5/thumbnails/23.jpg)
![Page 24: Chapters 22, 23, 24 & 25 Electroanalytical Chemistry](https://reader033.vdocuments.mx/reader033/viewer/2022061614/56649d395503460f94a129f1/html5/thumbnails/24.jpg)
![Page 25: Chapters 22, 23, 24 & 25 Electroanalytical Chemistry](https://reader033.vdocuments.mx/reader033/viewer/2022061614/56649d395503460f94a129f1/html5/thumbnails/25.jpg)
![Page 26: Chapters 22, 23, 24 & 25 Electroanalytical Chemistry](https://reader033.vdocuments.mx/reader033/viewer/2022061614/56649d395503460f94a129f1/html5/thumbnails/26.jpg)
![Page 27: Chapters 22, 23, 24 & 25 Electroanalytical Chemistry](https://reader033.vdocuments.mx/reader033/viewer/2022061614/56649d395503460f94a129f1/html5/thumbnails/27.jpg)
Summary of Important Types of Polarography
![Page 28: Chapters 22, 23, 24 & 25 Electroanalytical Chemistry](https://reader033.vdocuments.mx/reader033/viewer/2022061614/56649d395503460f94a129f1/html5/thumbnails/28.jpg)
![Page 29: Chapters 22, 23, 24 & 25 Electroanalytical Chemistry](https://reader033.vdocuments.mx/reader033/viewer/2022061614/56649d395503460f94a129f1/html5/thumbnails/29.jpg)
LCEC:LCEC...
![Page 30: Chapters 22, 23, 24 & 25 Electroanalytical Chemistry](https://reader033.vdocuments.mx/reader033/viewer/2022061614/56649d395503460f94a129f1/html5/thumbnails/30.jpg)
![Page 31: Chapters 22, 23, 24 & 25 Electroanalytical Chemistry](https://reader033.vdocuments.mx/reader033/viewer/2022061614/56649d395503460f94a129f1/html5/thumbnails/31.jpg)
![Page 32: Chapters 22, 23, 24 & 25 Electroanalytical Chemistry](https://reader033.vdocuments.mx/reader033/viewer/2022061614/56649d395503460f94a129f1/html5/thumbnails/32.jpg)
![Page 33: Chapters 22, 23, 24 & 25 Electroanalytical Chemistry](https://reader033.vdocuments.mx/reader033/viewer/2022061614/56649d395503460f94a129f1/html5/thumbnails/33.jpg)
![Page 34: Chapters 22, 23, 24 & 25 Electroanalytical Chemistry](https://reader033.vdocuments.mx/reader033/viewer/2022061614/56649d395503460f94a129f1/html5/thumbnails/34.jpg)
![Page 35: Chapters 22, 23, 24 & 25 Electroanalytical Chemistry](https://reader033.vdocuments.mx/reader033/viewer/2022061614/56649d395503460f94a129f1/html5/thumbnails/35.jpg)
![Page 36: Chapters 22, 23, 24 & 25 Electroanalytical Chemistry](https://reader033.vdocuments.mx/reader033/viewer/2022061614/56649d395503460f94a129f1/html5/thumbnails/36.jpg)
The Ilkavic Equation:The diffusion current, id, is in the plateau region of a polarogram and, thus, is independent of potential
id = 706n C D1/2 m2/3 t1/6
Where: id = max value of the diffusion current in the life of the drop
n = # of electrons involvedC = Cons. Substitution, mmoles/LD = diffusion coefficient of the ion (cm2/s)m = mass of Hg in mg/st = time between drops
![Page 37: Chapters 22, 23, 24 & 25 Electroanalytical Chemistry](https://reader033.vdocuments.mx/reader033/viewer/2022061614/56649d395503460f94a129f1/html5/thumbnails/37.jpg)
References:http://www.anachem.umu.se/jumpstation.htm
http://userwww.service.emory.edu/~kmurray/mslist.htmlhttp://www.anachem.umu.se/jumpstation.htm
http://www.acs.org
http://www.chemcenter/org
http://www.sciencemag.org
http://www.kerouac.pharm.uky.edu/asrg/wave/wavehp.html