Chapters 22-25: Electroanalytical Chemistry Flashcards
What is the branch of chemistry concerned
with the interrelation of electrical and chemical effects.
Electrochemistry
General Advantages of Electroanalytical Methods
How particular are electrochemical measurements?
particular for specific oxidation
states.
Can distinguish between Fe2+ and Fe3+
General Advantages of Electroanalytical Methods
Are electrochemical instrumentation expensive or inexpensive?
Electrochemical instrumentation is relatively inexpensive.
Typically electrochemical instrumentation costs between $10,000-30,000.
A complete spectroscopic instrument may cost
$50,000 - $250,000.
Applications of
Electroanalytical/Electrochemical
Methods
- Determine rate constants of reactions
- Electroless deposition
- Electrosynthesis and catalysis
- Measure mass transfer
- Photoelectrochemistry (photovoltaic devices)
- Analysis of ions (pH) and heavy metals
- Coatings, galvanization, electroplating, corrosion
- Examine the electron transfer process of molecules
DEEM PACE
Terminology for Electrochemistry
What is a measure of the available energy to drive charge externally between two electrodes and is measured in volts (V), where 1 V = 1 joule/coulomb (J/C)?
Cell potential (E)
Terminology for Electrochemistry
What is a measure of the amount of charge that passes in a unit time interval (i.e. rate of flow of electrons) and is measured in ampere (A), where 1 A = 1 coulomb/second (C/s)?
Electrical current (I)
Terminology for Electrochemistry
What is a measure of
a materials opposition to the flow of
electrical current and is measured in
ohms (Ω)?
Electrical resistance (R)
Terminology for Electrochemistry
What relates potential (volts), current (amps), and resistance (ohms). Potential = current × resistance (V=IR) ?
Ohm’s Law
Terminology for Electrochemistry
What is a measure of the number electrons passed in an electrical the circuit. Electrical charge (Q) is measured in terms of the total charge expressed in units of coulombs (C), where 1 C is equivalent to 6.24 × 1018 electrons?
Electrical charge (Q)
Terminology for Electrochemistry
What relates charge and the amount of product formed where 96485.4 C causes 1 equivalent of reaction (e.g. - consumption
of 1 mole of reactant or production of 1 mole of product in a oneelectron reaction.) ?
Faraday’s law
Terminology for Electrochemistry
What is a measure of the rate at which electrical energy is transferred by an electric circuit and is measured in watts (W), where 1 W = 1 J/s?
Power (P)
Fundamental Concepts of Electrochemistry
What is a chemical species that loses an electron (becomes more positive, less negative)?
Oxidation
Fe2+ → Fe3+ + e-
Fe2+ + Ce4+ → Fe3+ + Ce3+
Fundamental Concepts of Electrochemistry
What is a chemical species that gains an electron (becomes more negative, less positive)?
Reduction
Ce4+ + e-→ Ce3+
Fe2+ + Ce4+ → Fe3+ + Ce3+
Classification of Electrochemical Cells
How are electrochemical cells classified?
Electrochemical cells can be classified as either a galvanic cell or electrolytic cells.
Classification of Electrochemical Cells
What are galvanic cells?
A GALVANIC CELL is one in which reactions occur spontaneously at the electrodes when they are connected externally by a conductor. Often employed in converting chemical energy into electrical energy. Commercial applications include fuel cells, secondary (rechargeable) cells (e.g. - Pb-PbO2 storage batteries), and primary (nonrechargable) cells.
Classification of Electrochemical Cells
What are electrolytic cells?
An ELECTROLYTIC CELL is one in which reactions are affected by imposition of an external voltage greater than the open-circuit potential of the cell. These cells are frequently employed to carry our chemical reactions by expending electrical energy. Commercial processes include electrolytic synthesis (e.g. -
production of chlorine and aluminum), electrofining (e.g. - copper purfication), and electroplating (e.g. - silver and gold).
Electrochemically Driving Equilibrium with Ferrocene
Electrochemical reaction for Ferrocene
Ferrocene (Fc) ⇌ Ferrocinium (Fc+) + e-
Typically starting conditions:
- 0.5 mM Fc + 0.5 mM Fc+
- 0.1 M TBAP
Typically starting conditions:
- acetonitrile solvent (ACN)
E0 for Fc → Fc+ is + 0.31 V SCE
Experimental Aspects of Electroanalytical Methods
The overall chemical reaction taking place in a cell is made up of two independent
half reactions, which describe the real chemical changes at the two electrodes.
However, most of the time, electrochemists are interested only in one of these
reactions. The electrode at which this occurs is called the WHAT?
Working (indicator) electrode
Experimental Aspects of Electroanalytical Methods
To focus on the working electrode, chemists standardize the other half reaction of
the cell using a WHAT?, which has a known potential that approaches ideal characteristics.
Reference electrode
Experimental Aspects of Electroanalytical Methods
What happens if the passage of current does not affect the potential of the reference electrode (i.e. - the resistance is
small say 1-2 mV) ?
The two electrode cell can be used for electrochemical measurements.
Experimental Aspects of Electroanalytical Methods
If the passage of current does affect the potential of the reference electrode, a
WHAT? is preferred. This is used in most electrochemical experiments.
three-electrode cell
Experimental Aspects of Electroanalytical Methods
In the three-electrode cell arrangement, the current is passed between the working electrode and a counter (auxiliary) electrode. The counter electrode can be made of any material as its electrochemical properties WHAT? affect the behavior of the working electrode.
do not
Experimental Aspects of Electroanalytical Methods
In the three-electrode cell the potential of the WHAT? electrode is monitored relative to a separate WHAT? electrode, which is positioned nearby.
working, reference
Experimental Aspects of Electroanalytical Methods
In the three-electrode cell the reference electrode has a high input WHAT? so that a negligible current is drawn through the reference electrode.
impedance
Experimental Aspects of Electroanalytical Methods
What is the measure of the
opposition that a circuit presents to a current when a voltage is applied (similar to
resistance applied to an AC circuits)?
impedance
Working Electrode (WE)
Composition of WE
- Solid inert/noble metals (Pt, Au, etc.)
- Liquid Hg drops
- Carbon (HOPG, glass (amorphous) carbon, etc.)
- Transparent materials: Indium Tin Oxide (ITO)
SLCT
Working Electrode (WE)
Characteristics of a WE
- Chemically inert
- Relatively expensive
- Known geometry
- Temperature stable
CRKT
Counter Electrodes (CE)
Composition of CE
- Most common: Pt wire
- Any material that does not produce substance by electrolysis that will reach the
working the electrode surface.
Counter Electrodes (CE)
Characteristics of a CE
- Chemically inert
- Conductive
- The surface area of the counter electrode should be larger than the surface area of the working electrode.
Its purpose is to provide enough surface for a redox reaction to balance the one occurring at the working electrode. - Does not need to be polished as we do
not care/measure this reaction.
Reference Electrodes (RE)
Reference electrodes measure the potential of the working electrode against a
known reaction with a known standard potential. Think of it as a WHAT?
voltage sensor
Reference Electrodes (RE)
The international accepted reference electrode is the standard hydrogen electrode (SHE) or normal hydrogen electrode (NHE). What are the details of the two?
- Measures the oxidation of hydrogen, H2 (g) → 2H (aq)
- Not convenient/practical from an experimental standpoint and therefore other reference electrodes are measured and quoted and subsequently compared back to a SHE or NHE.
Alternative Reference Electrodes
The saturated calomel electrode (SCE) is commonly used electrode. Provide details.
- The SCE potential is 0.242 V versus a NHE.
- Hg electrode/Hg2Cl2/KCl (saturated in water)
Alternative Reference Electrodes
The silver-silver chloride electrode (SSCE) is another commonly used electrode. Provide details.
- The SSCE is 0.197 V versus a NHE
- Ag/AgCl/KCl (saturated in water)
Alternative Reference Electrodes
The silver wire is an example of quasireference electrode. Provide details.
- Ag/Ag+ reaction
- Simple and cheap, but can change from day to day.
- Use an internal standard to measure the poteitnal. - For example, Ferrocene/Ferrocinum (Fc/Fc+) has a
measured potential of 0.31 V versus a SCE. To calibrate a Ag wire, one measures the sample, adds ferrocene, andthen measures again looking for ferrocene oxidation.
Reference Electrode Issues
Both the SCE and SSCE have frits, which are membranes that allow ions to flow
across. However, frits are a pain!!! Provide details.
- There a junction resistance (e.g. - voltage drop) across the frit that one must
account for or at least recognize that it exists. - Sample contamination as this contacts the electrochemical solution
- Frits can get clogged, wear out, and must be kept wet
Reference Electrode Issues
SCE and SSCE potentials are WHAT? dependent.
Temperature and thus the temperature must reported and adjusted accordingly.
Supporting Electrolyte
The WHAT? reduces the resistance of the solution by carrying charge between the various electrode surfaces.
supporting electrolyte
Supporting Electrolyte
What are the ideal characteristics of a supporting electrolyte?
- Very soluble in the solvent (0.1 to 1.0 M)
- Electrochemically intert
- Very pure (e.g . - 99.9999% KOH)
Supporting Electrolyte
Examples of supporting electrolyte
Protic (water soluble) electrolytes: KNO3, H2SO4, NaOH, KOH, KCl, KClO4
Supporting Electrolyte
Aprotic (organic soluble) electrolytes: tetraalkyl ammonium salts (tetrabutyl
ammonium), LiClO4, PF6. Provide details.
- Aprotic solvents include acetontirile, benzonitrile, DMF, THF and must distilled
several times for purity.
Every Electrochemistry Experiment has Background (Noise)!!!
Background noise is caused by:
1) Solvent breakdown
- oxidation/reduction of the solvent called “solvent window”
- depends on the purity, type of solvent, and electrode surface
2) Oxygen
- electroactive species that reduces at -0.9 V vs SCE
3) Capacitive charging (double layer)
Double Layer Charging (Electrical Double Layer)
Electrochemical measurements involve heterogeneous systems because an electrode can only donate or accept electrons from a species that is present in a layer of solution that is immediately adjacent to the electrode. Thus the layer may have a composition that differs from that of the bulk solution.
double layer charging
Double Layer Charging (Electrical Double Layer)
The charged solution has two parts:
1) Compact inner layer (d0 to d1) where the potential deceases linearly.
2) Diffuse layer (d1 to d2) where the potential decreases exponentially.
Electrochem Experiment Conduct Current Over Electrode/Solution Interface
WHAT? is the direct transfer of electrons via an
oxidation reaction at on electrode and a reduction reaction at the other. These process are governed by faraday’s law and the amount
of current passed is related to the amount of chemical reaction.
Faradaic processes
results in change of potential and must be
Electrochem Experiment Conduct Current Over Electrode/Solution Interface
What is due to the adsorption and desorption of species at the electrode interface, changes in the structure of the electrode/solution interface (double layer).
Nonfaradaic processes
results in change of potential and must be
Reversibility for Types of Rxn Occurring at Electrode/Solution Interface
Can a species change its chemical structure and return to its previous form when oxidized and reduced (or vise-versa)?
Chemically Reversibility
Reversibility for Types of Rxn Occurring at Electrode/Solution Interface
Does a species procedure the same number of electrons during oxidation and reduction? Depends on the time scale of the experiment
(rate constant of electron transfer, k0).
Electrochemically Reversibility
Mass Transfer in the Cells with the Passage of Current
What is the movement of materials from one location in the solution to another?
Mass Transfer
Mass Transfer in the Cells with the Passage of Current
Because an electrode can only probe a very thin layer of solution at the
electrode surface (compact inner layer, d0 to d1), a faradaic current requires
continuous mass transfer of reactive species from the bulk of the solution to the
electrode surface. What are the three mechanisms to accomplish this:
1) Convection is the mechanical motion of the solution (stirring!!!)
2) Migration is the movement of ions through a solution brought about by
electrostatic attraction between ions and the charged electrode.
3) Diffusion is the motion of a species brought about by a concentration gradient.
What Measures Current as a Function of Potential and is One of the Most Common
Electroanalytical Methods?
Voltammetry
