Nov 16, 2004Introduction to Electroanalytical

Chemistry

Lecture Date: April 27h, 2008

Reading Material

● Skoog, Holler and Crouch: Ch. 22 (An Introduction to Electroanalytical Chemisty)

● See also Skoog et al. Chapters 23-25.

● Cazes: Chapters 16-19

● For those using electroanalytical chemistry in their work, the following reference is recommended:

A. J. Bard and L. R. Faulkner, “Electrochemical Methods”, 2nd

Ed., Wiley, 2001.

Advantages of Electroanalytical Methods

Matched against a wide range of spectroscopic and chromatographic techniques, the techniques of electroanalytical chemistry find an important role for several reasons:

– Electroanalytical methods are often specific for a particular oxidation state of an element

– Electrochemical instrumentation is relatively inexpensive and can be miniaturized

– Electroanalytical methods provide information about activities (rather than concentration)

History of Electroanalytical Methods

Michael Faraday: the law of electrolysis

– “…the amount of a substance deposited from an electrolyte by the action of a current is proportional to the chemical equivalent weight of the substance.”

Walter Nernst: the Nernst equation (Nobel Prize 1920)

Jaroslav Heyrovsky: the invention of polarography: (Nobel Prize 1959)

Walter Nernst(1864-1941)

Michael Faraday (1791-1867)

Jaroslav Heyrovsky(1890-1967)

Main Branches of Electroanalytical Chemistry

Key to measured quantity: I = current, E = potential, R = resistance, G = conductance, Q = quantity of charge, t = time, vol = volume of a standard solution, m = mass of an electrodispensed species

Interfacial methods

Bulk methods

Static methods (I = 0)

Dynamic methods

(I > 0)

Potentiometry (E)

Conductometry(G = 1/R)

Controlled potential

Constant current

Voltammetry (I = f(E))

Amperometric titrations (I = f(E))

Based on Figure 22-9 in Skoog, Holler and Crouch, 6th ed.

Electro-gravimetry

(m)

Coulometric titrations(Q = It)

Main Branches of Electroanalytical Chemistry

Potentiometry: measure the potential of electrochemical cells without drawing substantial current

– Examples: pH measurements, ion-selective electrodes, titrations (e.g. KF endpoint determination)

Coulometry: measures the electricity required to drive an electrolytic oxidation/reduction to completion

– Examples: titrations (KF titrant generation), “chloridometers” (AgCl)

Voltammetry: measures current as a function of applied potential under conditions that keep a working electrode polarized

– Examples: cyclic voltammetry, many biosensors

Electrochemical Cells

Voltmeter

Salt bridge (KCl)

Zn electrodeCu electrode

e-e-

Zn Zn2+ (aq) + 2e-

a Zn 2+ = 0.010Anode

Cu2+ (aq) + 2e- Cu(s)a Cu

2+ = 0.010Cathode

0.010M ZnSO4

solution

0.010M CuSO4

solution

Zinc (Zn) wants to ionize more than copper (Cu).

We can use this behavior to construct a cell:

e-e-

reference electrodecounter electrode

working electrodeindicator electrodedetector electrode

Potentiometry: Measures equilibrium EAmperometry: Control E, measures I as function of timeCoulometry: Control E, measure total Q over a period of time

control measureme

nt

Electrochemical Cells and Analytical Methods

Electrochemical Cells

Galvanic cell: a cell that produces electrical energy

Electrolytic cell: a cell that consumes electrical energy

Chemically-reversible cell: a cell in which reversing the direction of the current reverses the reactions at the two electrodes

Conduction in an Electrochemical Cell

Electrons serve as carriers (e.g. moving from Zn through the conductor to the Cu)

In the solution, electricity involves the movement of cations and anions– In the salt bridge both chloride and potassium

ions move At the electrode surface: an oxidation or a

reduction occurs– Cathode: the electrode at which reduction

occurs– Anode: the electrode at which oxidation occurs

Oxidation occurs when a chemical species loses an electron.LEO = lose electron is oxidation

Reduction is when a species gains an electron.GER = gain an electron is reduction

For example, the chemical reaction

can be decomposed into two half reactions:

“Leo the Lion Says Ger”

Faradaic and Non-Faradaic Currents

Figure 22-2

Mass Transfer occurs by:ConvectionMigrationDiffusion

Faradaic (governed by Faraday’s law): direct transfer of electrons, i.e. oxidation at one and reduction at the other electrode

Non-Faradaic: increasing charge of the double layer

Fundamentals

Electrical charge, q, is measured in coulombs (C). The charge associated with chemical species is related to the number of moles through the Faraday constant, F=96,485.3 (~96,500) C/mole.

Electrical current, I, is measured in Amperes (A). Current is the amount of charge that passes in a unit time interval (seconds).

Ohm's law relates current to potential (E) through the resistance (R) of a circuit by E=IR. The potential is measured in Volts (V) and the resistance in Ohms ().

Power (P) is measured in Watts (W = J/s) and is related to the current and potential by P= IE.

The work is measured in Joules (J) and is related to the potential and the amount of charge by work=q E.

The relationship between the standard Gibb's free energy change, G° (J/mole), and the standard electromotive force (EMF), E° (V), is given by

G°=-n F E°

where n is the number of electrons transferred and superscript on E0 refers to ‘standard state.’

Fundamentals

Fundamentals: The Nernst Equation

● The Nernst equation gives the cell potential E (in volts):

● Q (the activity quotient) is the ratio of products over reactants as in equilibrium calculations. For the generic reaction:

● Q is given by:

● The A’s are activities. For low-concentration solutions (low ionic strengths):

F = faraday (constant)n = # moles electrons in processE0 = standard potential for cell

Electrode Potentials

The reactions in an electrochemical cell can be thought of as two half-cell reactions, each with its own characteristic electrode potential

– These measure the driving force for the reaction

– By convention, always written as reductions

Standard electrode potential (E0): the measure of individual potential of an electrode at standard ambient conditions (298K, solutes at a concentration of 1 M, and gas pressure at 1 bar).

Some Standard Electrode Potentials

Reaction E0 at 298K (Volts)

Cl2(g) + 2e- 2 Cl2 +1.359

O2 (g) + 4H+ + 4e- 2 H2O +1.229

Ag+ + e- Ag(s) +0.799

Cu2+ + 2e- Cu(s) +0.337

Hg2Cl2 + 2e- 2Hg(l) + 2 Cl2 +0.268

2H+ + 2e- H2 (g) 0.000

AgI(s) + e- Ag(s) + I2 -0.151

Cd2+ + 2e- Cd(s) -0.403

Zn2+ + 2e- Zn(s) -0.763

See appendix 3 in Skoog et al. for a more complete list

The Standard Hydrogen Electrode (SHE)

A universal reference, but is really a hypothetical electrode (not used in practice)

– Uses a platinum electrode, which at its surface oxidizes 2H+ to H2 gas.

– Very sensitive to temperature, pressure, and H+ ion activity

Because the SHE is difficult to make, the saturated calomel electrode (SCE) is used instead.

– Calomel = mercury (I) chloride

Q: What is the electrode potential for the electrode Ag/AgI(s)/I-(0.01 M) ?

The overall reaction for this electrode is

This reaction cannot be found in tables of reduction potentials. But the reaction is comprised of two components

Electrode Potentials

We can initially ignore the fact that the electrode contains AgI and find E for the silver ion reduction.

Electrode Potentials

The Glass pH Electrode

● One of the most common potentiometric measurements is pH (a so-called “p-Ion” measurement).

● The common glass pH electrode makes use of junction potentials to determine the hydronium ion concentration in a sample solution.

● A typical glass pH electrode is configured as shown here:

The glass pH electrode is used with a Ag/AgCl reference electrode. For most modern pH electrodes the reference electrode is incorporated with the pH indicator electrode.

A small frit or hole connects the reference electrode and the sample solutions

The Glass pH Electrode

pH Measurements

● A combination pH electrode combines the indicator and reference into a single unit.

● The potential of this cell is:

● where Eij and Eoj are the junction potentials at the inner and outer layers of the glass membrane.

● Junction potential: occurs at the interface of two electrolytes, caused by unequal diffusion rates of cation and anions across the boundary (e.g the frit in a salt bridge)

More About pH Measurements

● The surface of the glass is hydrated, which allows exchange of hydronium ions for the cation in the glass (sodium or lithium).

● There are four interface regions, the external solution and hydrated glass, hydrated glass and dry glass on the outside, dry glass and hydrated glass on the inside, and hydrated glass and the internal solution.

● If the glass is uniform, the two hydrated glass/dry glass interfaces should be identical and should have the same junction potential.

● Since the glass interface junction potentials then cancel each other, the junction potential is then the difference between the internal and external solutions.

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pH Measurements

pH Measurements

For a real electrode, the two surfaces will not be identical and the constant k needs to be determined experimentally. The constant k is termed the asymmetry potential. The constant is termed the electromotive efficiency.

pH Measurements

Q: Why does the pH change the interfacial potential of the glass/aqueous interface?

A: The motion of the sodium ions leave behind a negatively charged glass layer that is neutralized to a lesser or greater extent according to the pH.

More explanation about how a pH meter really works: The sodium ions must move through the dry part of the membrane and this process is slow. For this reason, the membrane is made very thin. Also, a nonperturbing (low-current) voltmeter is used to read the cell voltage so that only a few sodium ions must move through the dry glass in a given time period.

pH Measurements

Errors in pH measurements with glass electrodes arise from the following effects:

– Calibration problems (e.g. drift, or error in the calibration)

– Junction potential

– High [Na+] interacting with electrode

– High acid concentration

– Equilibration time

– Temperature control

Typical electrodes have the following performance:

– Accuracy = +/- 0.02 pH units

– Precision = +/- 0.002 pH units

pH Electrodes: Errors, Accuracy and Precision

Modern pH electrodes are usually of the "combination" type, meaning that a single cylinder contains both the reference electrode, and a glass membrane electrode.Schematically, the total cell may be expressed as

SCE//test solution ([H3O+]=a1)/glass membrane/[H3O+]=a2, Cl-/AgCl(s)/Ag

The Combination pH Electrode

A Modern Combination pH Electrode

Electrochemical pH Measurements Concluded

Consider a typical problem related to the use of the combination pH electrode. Recall that

Ecell = L - 0.0592 V pH

QUESTION: If Ecell = -0.115 V at a pH of 4.00,

what is the pH of a solution for which Ecell is

-0.352 V?

ANSWER: First, find L from the measurement of the standard:

-0.115 V = L -0.0592 x pH

-0.115 V = L -0.0592 x 4.00

Therefore, L = 0.122 V

Second, use this value of L to find pH:

-0.352 V = 0.122 V - 0.0592 V x pH

pH = (0.122 V -(-0.352 V))/0.0592

pH = 7.84

QUESTION: What does the pH meter read if the pH is 7.00 in a 1 M salt solution having 1 M Na+ ions present?

ANSWER:

[H+]obs = 1 x 10-7 + 1 x 10-12

Conclusion -- the pH meter reads the true pH under these conditions.

The Ion Selective Electrode (ISE)

● An ISE generally consists of the ion-selective membrane, an internal reference electrode, an external reference electrode, and a voltmeter.

● Example: an ISE for fluoride (F-)

Automatic pKa and log P DeterminationpKa (ionization constant) and log P (octanol/water partition) are important physical parameters that play critical roles in determining how compounds behave in physiological environments and how they interact with enzymes, receptors and cell membranes

The Sirius GLpKa system:

combination pH electrode

sample tray

reagents

liquid dispensors

Conductometry

Conductometry: Detection of electrical conductivity

– Key analytical applications: conductometric detection in ion-exchange chromatography (IEC or IC) and capillary electrophoresis (CE)

Used to detect titration endpoints

Homework Problems (for Study Only)

Chapter 22:

– 22-1

Chapter 23:

– 23-11