Title: Chem 3253 Introduction to Electrochemistry March 31, 2004
1Chem 3253Introduction to ElectrochemistryMarch
31, 2004
2Four Electroanalytical Methods
- 1) Potentiometric
-
- 2) Voltammetric, Polarographic, Amperometric
- 3) Electrolysis (including electrogravimetric
and coulometric) - 4) Conductiometric
- We will consider the details of each of these
- separately.
3Potentiometry
- 1) Potentiometric - The electrical potential of a
galvanic - (spontaneous cell) is related to the
concentration of the - analyte by the Nernst Equation.
- E Eo RT/nF log Q
- where Eo is the standard voltage for the cell,
- R is the gas constant (8.314 J/mol K)
- T is the absolute temperature (K)
- n is the number of electrons in the net redox
equation - F is the charge of 1 mole of electrons, and
- Q is a term like Keq except the actual
concentrations and - pressures are used instead of equilibrium values.
4Potentiometry
- In order to make potentiometric measurements one
must have - both a reference and an indicator electrode. The
- measurement is the voltage difference between the
two - electrodes.
- Reference Electrodes Primary Reference
Electrode - The point of beginning for the construction and
definition of - reference electrodes is with the standard
hydrogen electrode - (SHE). It is based on the half-cell reaction
- H(aq, 1 M) e ½H2 (g, 1 atm).
-
5Standard Hydrogen Electrode
Standard Cu2/Cu half-cell
Experimental setup to measure the potential of a
half-cell vs. the standard hydrogen electrode.
6Reference Electrodes Primary Reference Electrode
- For the SHE when the H 1.00 m and pH2
1.00 atm this - electrode has a defined potential of 0.0000
volts. Unfortunately - the SHE is not a very convenient reference
electrode. It is quite - fragile, has the hazard of using hydrogen gas,
and the standard - acid solution is difficult to maintain at 1.00 m.
Although it is not - used in the laboratory for measurements, it is
the primary - reference electrode against which all other
electrodes are - ultimately standardized.
7Secondary Reference Electrode Saturated Calomel
- There are 2 commonly used secondary reference
electrodes. - 1) The first is the saturated calomel electrode
(SCE) whose - half-cell reaction is
- Hg2Cl2(s) 2e 2Hg(l) 2Cl?(aq)
- This half-cell is measured against the SHE with
saturated KCl as - the electrolyte its potential is 0.2445 volts _at_
25oC. The potential - is also dependent of the concentration of
chloride, so the - saturated KCl is the most commonly used because
it is - easiest to maintain. The literature gives the
potential at other - temperatures, as well as with other
concentrations of KCl.
8The Calomel Reference Electrode
9Secondary Reference Electrode Silver-silver
Chloride
- 2) The other widely used laboratory reference
electrode is - the silver-silver chloride electrode whose
half-cell reaction is - AgCl(s) e Ag(s) Cl?(aq).
- The potential of the Ag-AgCl system is 0.2223
volts _at_ 25oC and - a saturated solution of KCl is used as its
electrolyte. As with the - calomel reference electrode, the literature gives
the potential at - other temperatures.
10The Silver-Silver Chloride Reference Electrode
11Indicator Electrodes
- Indicator electrodes may be classified according
to the process - that produces the electrode potential.
- Metal indicator electrodes develop a potential
dependent on - the position of the equilibrium of the redox
half-reaction at the - surface of the electrode.
- Membrane indicator electrodes develop a potential
- determined by the difference in the concentration
of particular - ions across a special thin layer known as a
membrane.
12Metal Indicator Electrodes
- Metal indicator electrodes are classified as
either first-order or - second-order electrode.
- A first order electrode involves the metal in
contact with - Its own ions, such as Ag, Ag or Zn, Zn2. Only a
few metals - give reproducible potentials as first-order
electrodes due to - crystalline irregularities on their surfaces and
the ease of forming - oxides.
- A second-order electrode is one that responds to
the - presence of a precipitating or complexing ions.
For example, a - silver wire could serve as the indicator
electrode for chloride. - The use of second-order indicator electrodes is
somewhat - limited.
13Inert Electrodes
- The most commonly used indicator electrodes are
known as - inert. These electrodes are not involved in the
half-cell reactions - of the electrochemical species. Typical inert
electrodes are - platinum, gold, and carbon. Inert electrodes are
responsive to - any reversible redox system these are widely
used in - potentiometric work.
14An inert electrode of Platinum Responding to the
Sn4/Sn2 half-cell
15Membrane Indicator Electrodes
- There are several different types of membrane
indicator - electrodes according to the type of membrane and
the - specific use of that electrode. Membranes may be
made of - glass, polymers, or crystals.
16The Glass Electrode
- The first and most widely used membrane indicator
electrode is - the glass electrode, most commonly used as the pH
electrode. - The membrane is a very thin layer of specially
composed glass, - generally containing CaO, BaO, Li2O and Na2O as
well as SiO2. - The internal solution is of known constant
hydrogen ion activity, - such as pH 7.0. When this system is placed in an
external - solution of other hydrogen ion activity, a
Nernstian potential - develops across the glass membrane. Although the
response - follows the Nernst equation
- E 0.05916/1 log (Hexternal /Hinternal)
- there are three important facts that must be
considered.