potentiometry

CHM 4102ELECTROCHEMISTRY

PRINCIPLE OF POTENTIOMETRIC MEASUREMENT Group 6

Siti Nor Rafidah bt Mohd Ali Jinnah 152025 Ong Hui Jin 152161Norasheila bt Mohd Saad 152167Nur Zalikha bt Nasrudin 152843 Fatin Nurul Atiqah bt Osman 153319Siti Nursyafiqa bt Zainul Abidin 153933NurAsiqin bt Iskandar 154652Chan Yun Joo 154663Nur Syazana bt Abdul Rahman 154794Nur Amiera bt Azman 155629

Introduction

• In potentiometry, the potential of an electrochemical cell is measured under static conditions because no current flows while measuring a solution’s potential, its composition remains unchanged.

• Information on a composition of the sample is obtained through the potential appearing between two electrodes.

• The first quantitative potentiometric applications appeared soon after the formulation of Nernst equation.

• In 1889, the Nernst equation relating an electrochemical cell’s potential to the concentration of electroactive species in the cell.

• Potentiometric methods of analysis are based upon measurements of the potential of electrochemical cells under conditions of zero current, where the Nernst equation governs the operation of potentiometry.

Nernst equation:Ecell = E0

cell - (RT/nF) lnQ

Ecell = cell potential under nonstandard conditions (V)E0

cell = cell potential under standard conditionsR = gas constant, which is 8.31 (volt-coulomb)/(mol-K)T = temperature (K)n = number of moles of electrons exchanged in the electrochemical reaction (mol)F = Faraday's constant, 96500 coulombs/molQ = reaction quotient

Basic PrinciplesPotentiometer• A device for measuring the potential of an electrochemical

cell without drawing a current or altering the cell’s composition.• The potential of an electrochemical cell 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.

Potentiometric measurements• Made by using a potentiometer to determine the difference in

potential between a indicator electrode and a reference electrode.Cathode is the sensing electrode. (right half-cell)Anode is the reference electrode. (left half-cell)

Ecell = Ec ─ Ea

Where : Ec is the reduction potential at the cathode.

: Ea is the reduction potential at the anode.

Potentiometric Electrochemical Cells

• To determine difference in potential between sensing electrode and reference electrode

• Separation of the 2 electrodes to prevent the redox from occurring spontaneously on surface of one of electrodes

• Constructed that one of half cells provided a known reference potential of other half cell indicates the analyte concentration

Schematic diagram of an electrochemical cell of potentiometric measurement

Example 1

• Salt bridge contain inert electrolyte such as KCl connects the two half-cells.• The ends of the salt bridge are fixed with porous frits (to

allow the ions of electrolyte to move freely between the half-cells and the salt bridge).• This movement of ions in the salt bridge completes the

electrical circuit.• Reference electrode : left electrode (anode) which

undergoes oxidation.• Sensing electrode : right electrode (cathode) which

undergoes reduction.• When the potential of an electrochemical cell is measured,

the contribution of the liquid junction potential must be included;

Ecell = Ec ─ Ea Elj

Example 2

Introduction to ion selective electrode (ISE)

• The cell consists of both an indicator and reference electrode. • Since the potential of the reference electrode is

constant, the potential developed at the indicator electrode that contains information about the amount of analyte in a sample. • During the measurement, there is little to no current

flow. • An electrochemical cell for making a potentiometric

measurement with a membrane electrode also known as an ion-selective electrode, ISE.

Electrochemical cell for a potentiometric measurement with an ISE.

Ion selectivity

• A specific ion electrode will only respond to the presence of one species.• In reality, ion-selective electrodes can experience

interferences by responding to the presence of other ions.• We can account for the lack of 100% specificity by

incorporating the activity of j and a selectivity coefficient (kij) into this equation:

• This new equation is called the Nikolskii-Eisenman equation:

• The selectivity coefficient is a numerical measure of how well the membrane can discriminate against the interfering ion.• To put this in perspective, if an electrode has equivalent

responses to the two ions, then kij = 1.0.

• From the equation, the smaller the kij values, the less impact the interfering ion will have on the measured potential. • When kij values are less than 1, the ISE is more responsive

to the analyte ion• When kij values are greater than 1, the ISE is more

responsive to the interfering ion. For example, a kij value of 0.01 means that the electrode is 100 times more responsive to ion i over j.

• The selectivity of the ISE is determined by the composition of the membrane.• Ideally the membrane allows the uptake of only one

specific ion into it. • The three main components of making a

measurement at an ISE are• an inner reference, or standard solution• an outer analyte, or sample,• solution separated by a thin membrane.

• The potential developed at the membrane is the result of either an ion exchange process or an ion transport process occurring at each interface between the membrane and solution.

Ion Exchange Process

•Lithium cation displaces a potassium cation from the organic anion, R-:

KR + Li+ LiR + K⇋ +

•We can imbed the lipophilic R- in a membrane and place it in a solution of Li+

KR(mem) + Li+(aq) LiR⇋ (mem) + K+(aq)

• To construct an ion-selective electrode an inner reference solution added to the other side of the membrane. • This solution would contain a fixed concentration of the

ion of interest, Li+ in this example. • This is typically accomplished by placing a thin

membrane at the end of the plastic tube and filling the tube with a standard (known concentration) solution of the analyte.• A reference electrode is placed in the inner solution and

a second reference electrode is in contact with the analyte (outer) solution.

Ion transport

• A membrane, containing an ionophore, between an “unknown” analyte solution and a “known” reference solution .• The ionophore is a neutral “carrier” molecule

represented by the blue oval.

• The ionophore cannot diffuse out of the membrane and but can “trap” the analyte ion (A+) at the interface between the solution and membrane.•Without the ionophore, the analyte would be unable to

partition into the organic membrane. • As with the ion-exchange process, equilibrium isestablished at both solution-membrane interfaces. Theresulting charge separation at each interface leads to aphase-boundary potential.• Now potential develop across the membrane.

Reference electrode

• It has a standard potential on its own and its potential does not change to whichever solution it is dipped. • Always treated as the left-hand electrode (anode)• Example of reference electrode :• Standard hydrogen electrode (SHE)• Saturated calomel electrode• Silver-silver chloride electrode

Standard hydrogen electrode (SHE)

• Defined as the potential that is developed between H2

gas adsorbed on the Pt metal and H+ of the solution .• It is used for • determination of electrode potential of metal

electrode system• determination of pH of the solution

Pt,H2 (g, 1atm) | H+ (aq, a = 1.00) ||2 H+ (aq) + 2 e ─ ↔ H2 (g)

Saturated calomel (Hg2Cl2) Electrode (SCE)

• Contains of an inner jacket and outer sleeve. • Inner jacket has wire contact with Hg and plugged with a mixture of

calomel Hg2Cl2 & KCl.• Outer sleeve has crystals of KCl & porous plug of asbestos• Application: pH measurement, cyclic voltammetry and general

aqueous electrochemistry.• Advantages: ease of construction and stability of potential.

Hg(l) | Hg2Cl2 (sat’d), KCl (aq, sat’d) || Hg2Cl2(s) +2e– ↔2Hg(l ) + 2Cl-(aq)

Silver-silver chloride electrode

• Widely used because simple, inexpensive, very stable and non-toxic.• Mainly used with saturated potassium chloride (KCl)

electrolyte.• Advantages : easy to use• Disadvantage : difficult to prepare

Ag(s) | AgCl (sat’d), KCl (x M) ||AgCl(s) + e– ↔ Ag(s) + Cl- (aq)

Sensing Electrodes

• The potential of the sensing electrode in a potentiometric electrochemical cell is proportional to the concentration of analyte. • Two classes of indicator electrodes are used in potentiometry: • metallic electrodes • Electrodes of the first kind• Electrode of the second kind• Redox electrode

• 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:• Silver electrode (dipping in a solution of AgNO3)• Ag+ + e ↔ Ag

• Copper electrode • Cu+2 + 2e ↔ Cu

• 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.

Membrane electrodes

Glass pH electrode• Advantages over other electrodes for pH measurements:• Its potential is essentially not affected by the presence

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

systems.

Glass pH electrode

Principle: • For measurement, only the bulb needs to be submerged. • There is an internal reference electrode and electrolyte

(Ag| AgCl| Cl─) for making electrical contact with the glass membrane, its potential is necessarily constant and is set by the concentration of HCl.• A complete cell, then, can be represented by:

Theory of the glass membrane potential

• Both the inside and outside surfaces of the glass membrane in the GE bulb have SiOH groups.

• The interior surface of the glass membrane is in contact with a constant concentration of HCl, and so the number of SiO– groups on the interior surface remains constant.

• By contrast, the number of SiO– groups on the exterior of the glass membrane will change when the pH of the solution the glass membrane is immersed in changes.

• The difference in charge on the inside and outside of the glass membrane results in a membrane potential.

• If we can set up an experiment to measure the membrane potential, then this corresponds to measuring the pH of the solution in which the glass electrode is immersed.

Alkaline Error

• Systematic error occurs when using glass pH electrode to measure pH of extremely alkaline solution

• Glass pH electrode responds very selectively to H+ ions, but, sensitive to alkali metal ions too

• Caused by interference of high concentration of alkaline metal ions, e.g: Li+, Na+, K+

Alkaline Error cont.

• At high pH where [H+] <<< [Na+], electrode begins to respond to [Na+].

• Ion exchange reaction occurs at membrane surface

• Alkaline ions will replace H+ ions completely/partially in outer gel layer of glass membrane.

• Result: pH value measured < actual pH• Usually noticeable: - pH> 12 - [Li+ /Na+] ≥ 0.1 mol/litre

Figure 2: Cross-section of glass pH membrane. Alkaline metal cations will compete with H+for free spaces in solvated layer.

Alkaline Error cont.

Figure 1: Deviation from linear pH dependence due to alkaline error

• Alkaline error ↑ pH value ↑ alkaline concentration↑

Application of Potentiometric Measurement

• Clinical Chemistry• Ion-selective electrodes are important sensors for clinical samples

because of their selectivity for analytes in complex matricies.• The most common analytes are electrolytes, such as Na+, K+, Ca2+,H+, and Cl-,

and dissolved gases such as CO2.

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

- in water and wastewater.

• One potential advantage of an ion-selective electrode is the ability to incorporate it into a flow cell for the continuous monitoring of wastewater streams.

• Potentiometric Titrations• Use a pH electrode to monitor the change in pH during the titration.• For determining the equivalence point of an acid–base titration.• Possible for acid–base, complexation, redox, and precipitation

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

• Agriculture• NO3, NH4, Cl, K, Ca, I, CN in soils, plant material, fertilizers and

feedstuffs

• Detergent Manufacture• Ca, Ba, F for studying effects on water quality

• Food Processing• NO3, NO2 in meat preservatives• Salt content of meat, fish, dairy products, fruit juices, brewing

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

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