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Instrumental Analysis (II)

Electrometric Methods of Analysis

4th Lecture

6 Biological PHCM662

Dr. Raafat Aly

Potentiometry 2

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Objectives

Types of ISEs

Solid state electrodes

Liquid-based electrodes

Compound electrodes

Gas-sensing electrodes

Enzyme-substrate electrodes

(Biosensors)

Specifications for Potentiometry

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Solid-State ISEs Solid-state ISEs are based on an inorganic solid crystal. e.g. LaF3 doped with EuF2. This creates anion vacancies in the crystal (because Eu2+ has less charge than La3+). Thus, F- ions can diffuse into the crystal and “jump” from one side to the other across the vacancies, increasing the conductivity of the crystal.

Out-- ]F[log (0.05916) constantE :electrode F theof Response

0.1 M NaCl

and

0.1 M NaF F- ions in the crystal are exchanged with the F-

ions in the internal and external solution, causing a potential difference to arise across the membrane.

F

F

analyte

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• Another common inorganic crystal electrode uses Ag2Spressed pellet for the

membrane. This electrode responds to Ag+ and S2- ions (essentially ISE for Ag+ ).

• By doping the electrode with AgCl, AgBr, AgI or AgSCN, it is possible to

prepare electrodes selective for Cl-, Br-, I- and SCN- anions, respectively.

• On the other hand, electrodes selective for cations such as Cu2+, Cd2+ and Pb2+ can be designed by doping Ag2S with solid CuS, CdS and PbS, respectively.

• The solid state LaF3 electrode is highly specific for F- and mainly free from

interference. The only interfering species is OH- (very high pH).

• At low pH, F- is converted to HF which tends to form dimers and thus

decreases the F- activity in the solution.

)analyte(S

SAg2SAg2

22

common ion effect

S2- ions in the analyte solution will decrease the solubility of Ag2S. In this

way it will affect the amount of Ag+ exchanged with the membrane and thus in turn the measured potential

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Liquid-Based ISEs Liquid-based ISEs are similar to the solid-state electrodes except that the solid crystal is replaced by a membrane saturated with hydrophobic liquid ion exchanger. The latter acts as a carrier or binder to transport an ion across a membrane.

e.g. for the Ca2+-ISE on the left, the membrane contains Ca-Didecylphosphate (phosphate diester) dissolved in hydrophobic organic solvent.

Didecylphosphate anion (phosphate diester acts as chelator) binds Ca2+ and exchange it across the membrane

Equilibration of Ca2+ with the chelator establishes a voltage at the membrane-solution boundaries related to Ca2+

Out22 ]ca[log

2

0.0591constantE:electrode Ca the of Response

222222 CaPO)RO(2Ca]PO)RO[(

Didecylphosphate, R=C10H21

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Compound ISEs • Compound ISEs contain a normal ISE electrode surrounded by a membrane to isolate (or generate) the analyte to which the electrode responds.

• These electrodes can be used to measure many different gases.

• In CO2 gas-sensing electrode, CO2

diffuses through the gas-permeable membrane where it lower the pH in the electrolyte compartment. The response of the glass electrode to the change in pH is a direct measure of the CO2 outside

the electrode.

• CO2(aq) + H2O H+ + HCO3-

• Other acidic or basic gases, including

NH3, SO2, H2S and NOx can be detected

in the same manner both in solution and

in the gas phase.

1- Gas-sensing electrodes

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• It is a sensor in which an ion-selective electrode

is covered with a coating containing an

immobilized enzyme which reacts with an

organic substance (substrate) resulting in a

species to which the ion-selective electrode

responds.

3HCONH 2 HOH 2 NH-CO-NH 4urease

222

•The well-known example is the selective detection of blood urea nitrogen

(BUN) using urease as the enzyme catalyst. The ammonium ion generated,

can then be detected by an ammonium ion-selective electrode.

2. Enzyme-substrate electrode (Biosensor)

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• Similarly, enzyme reactions generating protons can be followed with glass pH electrode.

• There is a multiplicity of enzyme-electrodes that can be made in this way, with substrates including glucose, penicillin and others.

HgluconateDOHlactone,gluconoDOecosgluD OHoxidaseecosglu 2

222 51

Hacidcpenicilloipenicillin sepencillina

• Additional selectivity (ISE to determine the enzyme) can be attained by using

composite membranes, in which an enzyme present in the outer part of the

membrane catalyses a specific chemical reaction to generate product ions. These

ions can be detected by an internal ion-selective membrane.

• Double selectivity of the enzymatic reaction and the ISE response yields a

procedure that is free from most interference.

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Specifications for Potentiometry

• Advantages

Linear response to analyte

over wide dynamic range

Nondestructive

Short response times

Unaffected by color/turbidity

(limited matrix effects)

Cheaplog a