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Instrumental Analysis (II)

Electrometric Methods of AnalysisPotentiometry I

3rd Lecture

6 Biological PHCM662

Dr. Raafat Aly

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Objectives

What are electrometric methods of analysis? What are their advantages over other analytical

tools? Survey of the different methods. Potentiometric methods

Basic components Reference electrodes

Ag/AgCl electrode Calomel electrode

Indicator electrodes Metallic electrodes Ion-selective electrodes (ISEs)

How do ISEs work? Glass electrodes

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Introduction

Electroanalytical Chemistry: group of analytical methods concerned with the measurement of electrical quantities, such as potential, current, charge, or conductance and their relationship to chemical parameters.

Electroanalytical measurements requires at least two electrodes (metallic conductors) and a contacting sample solution (electrolyte), which constitute what is known as the electrochemical cell.

One of the two electrodes responds to the target analyte(s) and is thus termed indicator (or working) electrode. The second one, termed the reference electrode, is of constant potential (that is independent of the properties of the test solution).

Electrochemical cells can be classified as electrolytic (when they consumed electricity from an external source) or galvanic ( if they are used to produce electrical energy).

Electrochemical processes occur at the electrode-solution interface where electroactive analyte can donate (oxidized) or accept (reduced) electron(s) at the electrode surface.

Electrometric Methods of Analysis

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Types of Electroanalytical Methods

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a) selective for particular redox state of a species.

e.g. Ce(III) vs. Ce(IV)

b) portable and of low cost - 4,000 $ - 25,000 $ for a good instrument compared to 50,000 $ - 250,000 $ for a good spectrophotometer.

c) measures activity (free ion concentration) not total concentration (free and bounded).

d) fast

e) in situ

f) low detection limits even for very small sample volumes

f) information about:

oxidation states

stoichiometry

rates

charge transfer (kinetics)

equilibrium constants

General Advantages of Electrochemical Methods over Other Analytical Tools:

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I- Potentiometric Methods Introduction

1) Potentiometric Methods: based on measurements of the potential of electrochemical galvanic cells in absence of appreciable currents (i = 0).

In the simplest case, we measure an electroactive species that is part of a galvanic cell. An electroactive chemical can donate or accept electrons at the electrode.• If we have an unknown solution, the electrode used to detect

the analyte is called the indicator electrode.• We can connect the analyte half-reaction to a second electrode

with a fixed composition (known potential).• The second electrode is known as a reference electrode

because of its known and constant potential.

2) Basic Components: a) reference electrode: gives reference for potential measurement.b) indicator electrode: where species of interest is measured.c) potential measuring device.

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If you want to measure the ratio of Fe2+/Fe3+ in solution, you can insert a Pt wire (indicator electrode) and connect this half cell to a 2nd half-cell of constant potential.

]Fe[

]Fe[log05916.0EE 3

2

indicatorindicator

referenceindicatorcell EEE

potentiometer

measured experimentally

reference known potential

can be calculated to get the desired information

Reference electrode

Why is there a need for a reference electrode?

Fe3+ + e- Fe2+

Cathode: electrode at which reduction occurs

Anode: electrode at which oxidation occurs

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1

)a(log05916.0222.0E

a

a.alog

1

05916.0EE

ClAgCl/Ag

AgCl

ClAgAgCl/Ag

1

V 197.0E KCl saturated w/

V 222.0E Cl)s(Age)s(AgCl

:Electrode AgCl|Ag-

The silver-silver chloride electrode used in the previous example

It is clear that the potential of Ag/AgCl electrode depends upon concentration of Cl- in solution. Thus if the composition and concentration of Cl- solution are kept constant, the electrode would exhibit a constant potential

The reference electrode and the salt bridge are enclosed in one body

1. Silver-Silver Chloride Reference Electrode (Ag/AgCl/Cl)

Reference Electrodes

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The calomel electrode is based on the reaction:

the saturated calomel electrode is so frequently used that it is usually abbreviated SCE

The advantage of using saturated KCl is that [Cl] does not change if some liquid evaporates.

V .E KCl saturated w/

V .E Cl)(Hge)s(ClHg -222

1

2410

2680

)a(log05916.0268.0E

)a(

a.alog

1

05916.0EE

ClClHg/Hg

ClHg

ClHgClHg/Hg

22

21

22

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Similarly, the potential of calomel electrode based upon concentration of Cl.

2. The Calomel Reference Electrode (Hg/Hg2Cl2/Cl)

(SCE)

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The ideal reference electrode:

1. is reversible and obeys the Nernst equation.

2. exhibits a potential that is constant with time. Reference

electrodes usually use sparingly soluble salts (AgCl and Hg2Cl2)

and saturated solution (sat. KCl) which eliminates the possibility that the potential of the reference electrode may change as a function of concentration.

3. returns to its original potential after being subjected to small current i.e., its half reaction is rapid. When current passes across the reference electrode interface, it is shown as a fast electrochemical reaction (exchange of electrons, either oxidation or reduction) without affecting the arrangement of charges on the interface. Therefore, the potential of the reference electrode remains unchanged and independent of the current passes across its surface.

4. is not largely affected by temperature change (Note: calomel electrode can not be used above 60 C because of volatility of Hg)

5. is insensitive to the composition of the solution under study (Note: Ag/AgCl electrode can not be used if the analyte is a protein sample since Ag+ can react with sample components)

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Indicator Electrodes

• There are several types of electrodes that can be used as indicator electrode; we will examine two types:

– Metal electrodes develop potential in response to a redox reaction (electron transfer) on their surface

– Ion-selective electrodes (ISE) allow selective migration of one type of ion (ion-exchange) to generate electric potential and are not based on redox reactions

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1- Metal Electrodes

• Platinum is the most common metal indicator electrode

– Pt is mostly inert, not participating in reactions

– It simply allows electron transfer to/from solution

• Gold is also an inert metal indicator electrode

• Carbon electrodes are often used because many redox

reactions are very fast on a carbon surface

Note that the metal electrodes are not selective electrodes since

they may respond to many ions in solution as a result of electron

transfer (oxidation-reduction) between the metal electrode and the

electroactive species.

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Silver Indicator Electrode (Ag/Ag+)

V 799.0E )s(Age)s(gA -

V 241.0E Cl)(Hge)s(ClHg -222

1

241.0]Ag[

1log059.0799.0EEEcell

]Ag[log059.0558.0Ecell

By measuring the potential of the cell using potentiometer, concentration of Ag+ ion can be determined.

A silver wire can act as a indicator electrode for silver concentration in solution:

If you use a SCE as a reference electrode:

Then the Nernst equation for the cell becomes:

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2- Ion-Selective Electrodes (ISEs) - Composition Ion-selective electrodes are mainly

membrane-based devices, consisting of ion selective - conducting materials, which separate the test sample from the inside of the electrode (On the inside is a filling solution containing the ion of interest at a constant activity.

The membrane is usually nonporous, water insoluble, and mechanically stable.

The composition of the membrane is designed to yield a potential that is primarily due to the ion of interest (via selective binding processes, e.g., ion exchange, which occur at the membrane – solution interface).

The trick is to find a membrane that will selectively bind the analyte ions, leaving co-ions behind.

Membrane materials, possessing different ion recognition properties, have thus been developed to impart high selectivity.

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– ISEs mainly respond to one ion

– They do not involve redox reactions

– Utilize a membrane that binds only one ion.

– Almost all the analyte C+ inside the membrane is bound in the complex LC+, which is in equilibrium with a small amount of free C+ in the membrane. LC+ L + C+

– C+ can migrate across the interface.

– As soon as a tiny number of C+ ions diffuses from the membrane intothe aqueous phase, there is anexcess positive charge in the aqueous phase.

– This charge imbalance creates an electric potential difference that opposes diffusion of more C+ into theaqueous phase.

analyte

Binding Agent

Mechanism - How ion-selective electrodes work?

Liquid based ion selective electrode

Internal reference solution (0.1 M C+)

C+

C+

C+

C+

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The electric potential difference across the external boundary between the membrane and analyte solution can be described:

The potential difference across the whole membrane is:

outer

memout ]C[

]C[log

n

0591.0ttanconsE

Inouter

meminoutmembrane E

]C[

]C[log

n

0591.0ttanconsEEE

inoutermeminoutmembrane E]C[logn

0591.0]C[log

n

0591.0ttanconsEEE

constantconstant

• Electric potential across the membrane depends on the concentration of analyte.

Potential Difference for an ISE

The observed potential is thus a kind of boundary potentials that develop across the membrane that separates the analyte solution from the reference solution

• The electric potential difference across the ion selective membrane is measured by two reference electrodes (one internal and the other external).

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)analyte(]A[logn

0.0591KE

analyte of chargen )analyte(outer]C[logn

0.0591KE

This equation applies to any ion-selective electrode. If the analyte is anion,

the sign for n is negative.

– But almost the first, second and fourth terms in the previous equation are

constants, so we can simplify the equation significantly:

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Glass Electrode (pH Meter)

• The glass electrode is the most common ISE and mainly used to measure pH (H+ concentration).

)s(Ag|)s(AgCl|)aq(Cl),inaq(H|membraneglass|)outaq(H||)aq(-Cl|(s)AgCl|(s)Ag

The combined glass electrodes uses two Ag/AgCl reference electrodes to measure the potential across a glass membrane that allows only H+ to pass through.

H+ can replace cations bound to oxygen in glass in what is called an ion-exchange equilibrium.

Outer reference electrode Inner reference electrode

Combined glass electrode

Tetrahedral structure of the glass

Oxygen

silicon

Cations like Na+, Li+ or Ca2+

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The Glass Membrane • H+ is the only ion that bindssignificantly to the outer(hydrated) layer of glass

• Monovalent cations, Na+ orLi+ (glass membrane) can move through the silicate lattice.

• So, metal ions in the hydratedgel layer diffuse out of theglass membrane into thesolution, while H+ can diffuseinto the membrane to replacethe metal ions.

pH 0.0591KE

out]H[log 0.0591KE

Response of the electrode

H+ + Na+ H+ + Na+

soln. glass glass soln.

Ion-exchange equilibrium

(aH+ = 0.1) a(aH+ is variable)

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Calibration of a glass electrode

A glass electrode must be calibrated with two or more standard buffers close to the pH of the unknown. There are obviously many buffers that can be used for this purpose.

It is also important that the glass electrode be kept in solution when not in use. If not, the hydrated gel layers of the glass will dry out and the electrode will have to be reconditioned for several hours before use.

0.05 M

potassium

hydrogen

phthalate

0.025 M

KH2PO4,

0.025 M

Na2HPO4

0.025 M

NaHCO3

0.025 M

Na2CO3

pH value at 25 C 4.008 6.865 10.062

pH Values of National Institute of Standard and Technology Buffers

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Sources of Error in pH measurement

1. Alkaline error or sodium error

When the concentration of H+ is very low and the concentration

of Na+ is very high, the electrode responds to Na+ and the

apparent pH is lower than the true pH.

2. Acid error

In strong acid, the measured pH is higher than the actual pH,

perhaps because the glass surface is saturated with H+ and can

not be protonated at any more sites.

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Other Ion Selective Electrodes

1. Glass membrane electrodes (H+)

2. Glass membrane electrodes for other monovalent cations,

Na+, K+, Ag+ and NH4+.

3. Solid state electrodes

4. Liquid-based electrodes

5. Compound electrodes

ISEs are used for many analytes. There are a number of different types of ISEs: