Chapters 22, 23, 24 & 25

Electroanalytical Chemistry

Electroanalytical Chemistry

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.

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.

Galvanic Electrochemical Cell with Salt Bridge

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.

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

Oxygen Probe

A.P. – 650 mV Ag|AgCl

Reactions:

O2 + 2H2O + 4e- 4OH-

4Ag + 4Cl- 4AgCl + 4e-

Hydrogen Probe

A.P. +650 mV

Reactions:

H2O2 O2 +2H+ +2e-

2Ag+ + 2e- 2Ag

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

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

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.

Cathodes and Anodes

Reactions at the Anode

• Some examples are:

Cu(s) <=> Cu2+ + 2e-

Fe2+ <=> Fe3+ + e-

H2(g) <=> 2H+ + 2e-

Ag(s) + Cl- <=> AgCl(s) + e-

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-

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

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.

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.

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.

The Nernst Equation

R = gas constant

T = temperature in Kelvins

Q = thermodynamic reaction quotient

F = Faraday's constant

n = number of electrons transferred

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

Summary of Important Types of Polarography

LCEC:LCEC...

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

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