POTENTIOMETRY &

ION SELECTIVE ELECTRODE

DR.RUCHI GOKANI Dept of biochemistry

s.b.k.s.m.i.r.c

Potentiometry In potentiometry, the potential of an electrochemical

cell is measured under static conditions because no current flows while measuring a solution’s potential.

Use of electrodes to measure voltage that provide chemical information.

Concentration of ions in solution is calculated from the measured potential difference between the two electrodes immersed in solution under condition of zero current.

This type of system includes at least two electrodes, identified as an indicator electrode (right half-cell) and a reference electrode (left half-cell ) which act as the cathode and anode respectively.

Each electrode is in contact with either the sample (in the case of the “indicator electrode”) or a reference solution ( in the case of the “reference electrode”).

Electrodes + solution = electrochemical cell

Each electrode represent half cell reaction with correspondance half cell potential.

Potentiometer• A device for measuring the potential of an

electrochemical cell without drawing a current or altering the cell’s composition.

• The potential is measured under static conditions. • Because no current, or only a negligible current

flows while measuring a solution’s potential, its composition remains unchanged.

• For this reason, potentiometry is a useful quantitative method.

• Two electrodes connected to Potentiometer to measure potential difference

• Indicator electrode (Eind) – potential respond to change according to conc. of ions

• Reference electrode (Eref) – half cell potential does not change.

Ecell = Eind ─ Eref + Elj

Schematic diagram of an electrochemical cell of potentiometric measurement

Example 1

System Components

Liquid JunctionReference electrode Indicator or measuring electrodeReadout device (Potentiometer)

Liquid junction – also known as a salt bridge are required to complete the circuit between the electrodes.

Functions:It allows electrical contact between the two solutions.It prevents the mixing of the electrode solutions.It maintains the electrical neutrality in each half cell as ions flow into and out of the salt bridge.

Reference Electrode- is an electrochemical half-cell that is used as a fixed reference for the measurement of cell potentials.

A half-cell with an accurately known electrode potential, Eref, that is independent of the concentration of the analyte or any other ions in the solution

Always treated as the left-hand electrodeExamples:

Normal hydrogen electrodeSaturated calomel electrodeAg-AgCl electrode

Reference ElectrodesCalomel electrode- composed of

mercury/mercurous chloride; It is dependable but large, bulky, and affected by temperature.

Silver/silver chloride- Widely used because simple, inexpensive, very stable and non-toxic.

o reference electrodes are more compact -- overall better & faster

Normal Hydrogen Electrode- consists of a platinized platinum electrode in HCl solution with hydrogen at atmospheric pressure bubbled over the platinum surface.

-determination of pH of the solution.

Saturated calomel electrode

Silver-silver chloride electrode

Standard hydrogen electrode (SHE)

Indicator or measuring electrode• The potential of this electrode is proportional to

the concentration of analyte. • Two classes of indicator electrodes are used in

potentiometry: o metallic electrodes • Electrodes of the first kind• Electrode of the second kind• Redox electrode

o membrane electrodes (ion-selective electrodes) • glass pH electrode

Metallic electrodes Electrodes of the first kind• A metal in contact with a solution containing its

cation. • The potential is a function of concentration of Mn+

in a Mn+ / M. The most common ones:o Silver electrode (dipping in a solution of AgNO3)

• Ag+ + e ↔ Ago Copper electrode • Cu+2 + 2e ↔ Cu 

o Zn electrode• Zn+2 + 2e ↔ Zn

Electrode of the second kind• A metal wire that coated with one of its salts

precipitate.• Respond to changes in ion activity through

formation of complex.• A common example is silver electrode and

AgCl as its salt precipitate. • This kind of electrode can be used to measure

the activity of chloride ion in a solution.

Redox electrode• An inert metal is in contact with a solution

containing the soluble oxidized and reduced forms of the redox half-reaction.

• The inert metal is usually is platinum (Pt).• The potential of such an inert electrode is

determined by the ratio of the reduced and oxidized species in the half-reaction.

• A very important example of this type is the hydrogen electrode.

Indicator Electrode- also called the measuring electrode It is immersed in a solution of the analyte,

develops a potential, Eind that depends on the activity of the analyte.

Is selective in its responseIt is the other electrochemical half-cell that

responds to changes in the activity of a particular analyte species in a solution.

Example:Ion-Selective Electrodes

Ion selective electrode An ion-selective electrode (ISE), also known as a

specific ion electrode (SIE), is a transducer (or sensor) that converts the activity of a specific ion dissolved in a solution into an electrical potential, which can be measured by a voltmeter or pH meter.

indicator electrode based on determination of cations or anions by the selective absorption of these ions to a membrane surface.

TYPES OF ION SELECTIVE ELECTRODE

•Glass Membrane Electrode •Solid State Electrode •Liquid Membrane Electrode •Gas Sensing Electrode

 GLASS MEMBRANE ELECTRODE

• Responsive to univalent cations ( H+ , Na+)• Glass electrodes available for Na+, K+, NH4+, Li+, Ag+(cations only) by varying glass composition• The selectivity for this cation by varying the composition of a thin ion sensitive glass membrane.• Glass membrane manufactured from SiO2 with

negatively charged oxygen atom.• Inside the glass bulb, a dilute HCl solution and silver wire coated with a layer of silver chloride.• The electrode is immersed in the solution and pH is

measured• Example: pH electrode

Glass pH electrode• Advantages over other electrodes for pH

measurements:o Its potential is essentially not affected by the

presence of oxidizing or reducing agents.o It operates over a wide pH range.o It responds fast and functions well in

physiological systems.o Selective for monovalent cations only because

polyvalent ions can not penetrate the surface of membrane.

pH electrode

Selective for the detection of hydrogen ions.

The measuring or indicator electrode has a “glass membrane”

pH is then determined from potential between the pH electrode and a standard reference electrode.

SOLID STATE ELECTRODE• Solid state electrode are selective primarily to

anions.• It may be a - homogenous membrane electrode- heterogeneous membrane electrode.• Homogenous membrane electrode: ion-selective

electrodes in which the membrane is a crystalline material (AgI/Ag2S).

• Heterogeneous membrane electrode: ion-selective electrodes prepared of an active substance, or mixture of active substances (silicone rubber or PVC)

• Example: Fluoride ion selective electrode

LIQUID MEMBRANE ELECTRODE• Liquid membrane is a type of ISE based on water-

immiscible liquid substances produced in a polymeric membrane used for direct potentiometric measurement.

• Used for direct measurement of several polyvalent cations (Ca ion) as well as a certain anions.

• Inner compartment of electrode contains reference electrode & aqueous reference solution.

• Outer compartment – organic liquid ion exchanger

•The polymeric membrane made of PVC to separate the test solution from its inner compartment which contains standard solution of the target ion. •The filling solution contains a chloride salt for establishing the potential of the internal Ag/AgCl wire electrode.

GAS SENSING ELECTRODE• Available for the measurement of ammonia,

carbon dioxide and nitrogen oxide.• This type of electrode consist of permeable

membrane and an internal buffer solution.• The pH of the buffer changes as the gas react

with it.• The change is detected by a combination pH

sensor.• This type of electrode does not require an

external reference electrode.

Measurement of PCO2 in routine blood gases

A modified pH electrode with a CO2 permeable membrane covering the glass membrane surface

A bicarbonate buffer separates the membranes

Change in pH is proportional to the concentration of dissolved CO2 in the blood

pco2 electrode

Application of Potentiometric

Measurement

• Clinical Chemistryo Ion-selective electrodes are important

sensors for clinical samples because of their selectivity for analytes.

o The most common analytes are electrolytes, such as Na+, K+, Ca2+,H+, and Cl-, and dissolved gases such as CO2.

• Environmental Chemistryo For the analysis of of CN-, F-, NH3, and NO3

- in water and wastewater.

• Potentiometric Titrationso pH electrode used to monitor the change in

pH during the titration.o For determining the equivalence point of

an acid–base titration.o Possible for acid–base, redox, and

precipitation titrations, as well as for titrations in aqueous and nonaqueous solvents.

• Agricultureo NO3, NH4, Cl, K, Ca, I, CN in soils, plant

material, fertilizers.• Detergent Manufacture

o Ca, Ba, F for studying effects on water quality

• Food Processingo NO3, NO2 in meat preservativeso Salt content of meat, fish, dairy products, fruit

juices, brewing solutions. o F in drinking water and other drinks.o Ca in dairy products and beer.o K in fruit juices and wine making. o Corrosive effect of NO3 in canned foods.

advantages• Relatively inexpensive and simple to use and

have an extremely wide range of applications and wide concentration range.

• Under the most favourable conditions, when measuring ions in relatively dilute aqueous solutions and where interfering ions are not a problem, they can be used very rapidly and easily.

• ISEs can measure both positive and negative ions.

• They are unaffected by sample colour or turbidity.

• Non-destructive: no consumption of analyte.• Non-contaminating.• Short response time: in sec. or min. useful in

industrial applications.

LIMITATION• Precision is rarely better than 1%.• Electrodes can be affected by proteins or other organic solutes.• Interference by other ions.• Electrodes are fragile and have limited shelf life.