potentiometric methods a.) introduction : 1.) potentiometric methods: based on measurements of the...

Potentiometric Methods A.) Introduction:

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

2.) Basic Components:a) reference electrode: gives reference for potential measurementb) indicator electrode: where species of interest is measuredc) potential measuring device

911014 1http:\\asadipour.kmu.ac.ir

Electrodes and Potentiometry Potential change only dependent on one ½ cell concentrations Reference electrode is fixed or saturated doesn’t change!

Reference electrode, [Cl-] is constant

Potential of the cell only depends on [Fe2+]

& [Fe3+]

Pt wire is indicator electrode whose potential responds to [Fe2+]/[Fe3+]

]Cllog[..]Fe[

]Fe[log

..Ecell

0591602220

1

0591607710

3

2

Unknown solution of[Fe2+] & [Fe3+]


Ecell=Ecathod-Eanod Anod is conventionally reference electode

red

oxi Log

n

0.059– EE cathodcathod

red

oxi Log

n

0.059– EE anodanod

Fe3+ +e- Fe2+AgCl(s) + e- → Ag + Cl-

B.) Reference Electrodes: (Instead of SHE) Need one electrode of system to act as a reference against which potential measurements can be made relative comparison.

Standard hydrogen electrodes are cumbersome- Requires H2 gas and freshly prepared Pt surface

Desired Characteristics:a) known or fixed potentialb) constant responsec) insensitive to composition of solution under studyd) obeys Nernest Equation e) reversible


Electrodes and Potentiometry Reference Electrodes

1.) Silver-Silver Chloride Reference Electrode

Eo = +0.222 V

Activity of Cl- not 1E(sat,KCl) = +0.197 V


Electrodes and Potentiometry Reference Electrodes

2.) Saturated Calomel Reference Electrode (S.C.E)

Saturated KCl maintains constant [Cl-] even with some evaporation

Eo = +0.268 V

Activity of Cl- not 1E(sat,KCl) = +0.241 V


Electrodes and Potentiometry Indicator Electrodes

1.) three Broad Classes of Indicator Electrodes 1) Metal Electrodes

- Develop an electric potential in response to a redox reaction at the metal surface

2) Ion-selective (Membrane) Electrodes- Selectively bind one type of ion to a membrane to generate an electric potential

3) Molecular Selective Electrode

Remember an electric potential is generated by a separation of charge911014 6http:\\asadipour.kmu.ac.ir

1) Metallic Indicator Electrode (3 Main Types)

a) Metallic Electrodes of the First Kindb) Metallic Electrodes of the Second Kindc) Metallic Redox Indicators

a) Metallic Electrodes of the First Kind

i. Involves single reaction

ii. Detection of catione derived from the metal used in the electrode

iii. Example: use of copper electrode to detect Cu2+ in solution

½ reaction: Cu2+ + 2e- Cu (s)

Eind gives direct measure of Cu2+:Eind = Eo

Cu – (0.0592/2) log aCu(s)/aCu2+

since aCu(s) = 1:Eind = Eo

Cu – (0.0592/2) log 1/aCu2+

or using pCu = -log aCu2+:Eind = Eo

Cu – (0.0592/2) pCu911014 7http:\\asadipour.kmu.ac.ir

b) Metallic Electrodes of the Second Kind

i. Detection of anion derived from the interaction with metal

ion (Mn+) from the electrodeii. Anion forms precipitate or stable complex with metal ion (Mn+)iii. Example: Detection of Cl- with Ag electrode

½ reaction: AgCl(s) + e- Ag(s) + Cl- EO = 0.222 V

Eind gives direct measure of Cl-:Eind = Eo – (0.0592/1) log aAg(s) aCl-/aAgCl(s)

since aAg(s) and aAgCl(s)= 1 & Eo = 0.222 V:

Eind = 0.222 – (0.0592/1) log aCl-


c) Metallic Redox Indicatorsi. Electrodes made from inert metals (Pt, Au, Pd)ii. Used to detect oxidation/reduction in solutioniii. Electrode acts as e- source/sinkiv. Example: Detection of Ce3+ with Pt electrode

½ reaction: Ce4+ + e- Ce3+

Eind responds to Ce4+:Eind = Eo – (0.0592/1) log aCe3+/aCe4+


2) Membrane Indicator Electrodes

a) General

i. electrodes based on determination of cations or anions

by the selective adsorption of these ions to a membrane surface.

ii. Often called Ion Selective Electrodes (ISE) or pIon Electrodes

iii. Desired properties of ISE’s 1) minimal solubility – membrane will not dissolve in solution during measurement.

– silica, polymers, low solubility inorganic compounds , (AgX) can be used2) Need some electrical conductivity3) Selectively binds ion of interest


C+ diffuses across the membrane due to concentration gradient resulting in charge difference across membrane


Ion-Selective Electrodes Responds Selectively to one ion

- Contains a thin membrane capable of only binding the desired ion

Does not involve a redox process

Membrane contains a ligand (L) that specifically and tightly binds analyte of interest (C+)

A difference in the concentration of C+ exists across the outer membrane.

The counter-ions (R-,A-) can’t cross the membrane and/or have low solubility in membrane or analyte solution

Remember an electric potential is generated by a separation of charge

Potential across outer membrane depends on [C+] in analyte solution






Remember an electric potential is generated by a separation of charge

Potential across inner membrane depends on [C+] in filling solution, which is a known constant

C+ diffuses across the membrane due to concentration gradient resulting in charge difference across membrane

A difference in the concentration of C+ exists across the inner membrane.

innerouter EEE

Electrode potential is determined by the potential difference between the inner and outer membranes:

where Einner is a constant and Eouter depends on the concentration of C+ in analyte solution


][constant Clogn

.E

059160





Electrode Potential is defined as:

where [C+] is actually the activity of the analyte and n is the charge of the analyte


pH Electrodei. most common example of an ISE based on use of glass membrane that preferentially binds H+

ii. Typical pH electrode system is shown pH sensing element is glass tip of Ag/AgCl electrode

Two reference electrodes hereone SCE outside of membrane one Ag/AgCl inside membrane

Combined electrod


Electrodes and Potentiometry pH Electrodes

1.) pH Measurement with a Glass Electrode

Ag(s)|AgCl(s)|Cl-(aq)||H+(aq,outside) H+(aq,inside),Cl-(aq)|AgCl(s)|Ag(s)

Outer referenceelectrode

[H+] outside(analyte solution)

[H+] inside Inner referenceelectrode

Glass membraneSelectively binds H+

Electric potential is generated by [H+] difference across glass membrane911014 15http:\\asadipour.kmu.ac.ir

iii. pH is determined by formation of boundary potential across glass membrane

Boundary potential difference (Eb) = E1 - E2 where from Nernst Equation:

Eb = c – 0.592pH -log aH+ (on exterior of probe or

in analyte solution)

constant

Selective binding of cation (H+) to glass membrane



Glass Membrane Irregular structure of silicate lattice

Cations (Na+) bind oxygen in SiO4 structure



Glass Membrane Two surfaces of glass “swell” as they absorb water

- Surfaces are in contact with [H+]



Glass Membrane H+ diffuse into glass membrane and replace Na+ in hydrated gel region

- Ion-exchange equilibrium- Selective for H+ because H+ is only ion that binds significantly to the

hydrated gel layer

H(0.05916)constant pE

Charge is slowly carried by migration of Na+ across

glass membrane

Potential is determined by external [H+]

Constant and b are measured when electrode is calibrated with solution of known pH911014 19http:\\asadipour.kmu.ac.ir

iii. pH is determined by formation of boundary potential across glass membrane

At each membrane-solvent interface, a small local potential develops due to the preferential adsorption of H+ onto the

glass surface.

Si O-

Glass Surface


Electrodes and Potentiometry Junction Potential

1.) Occurs Whenever Dissimilar Electrolyte Solutions are in Contact Develops at solution interface (salt bridge) Small potential (few millivolts) Junction potential puts a fundamental limitation on the accuracy of direct

potentiometric measurements - Don’t know contribution to the measured voltage

Again, an electric potential is generated by a separation of charge

Different ion mobility results in separation in charge


iv. Alkali Error‚ H+ not only cation that can bind to glass surface

- H+ generally has the strongest binding‚ Get weak binding of Na+, K+, etc‚ Most significant when [H+] or aH+ is low (high pH)

- usually pH $11-12

At low aH+ (high pH), amount of Na+ or K+ binding is significant increases the “apparent” amount of bound H+


v. Acid Error‚ Errors at low pH (Acid error) can give readings that are too high‚ Exact cause not known

- usually occurs at pH # 0.5

c) Glass Electrodes for Other Cationsi. change composition of glass membrane

‚ putting Al2O3 or B2O3 in glass

‚ enhances binding for ions other than H+

ii. Used to make ISE’s for Na+, Li+, NH4+


Example 18: The following cell was used for the determination of pCrO4:

SCE||CrO42- (xM), Ag2CrO4 (sat’d)|Ag

Calculate pCrO4 if the cell potential is -0.386.


16-1 The shape of a redox titration curve

• A redox titration is based on an oxidation-reduction reaction between analyte and titrant.

• Consider the titration of iron(II) with standard cerium(IV), monitored potentiometrically

with Pt and calomel electrodes.

The potentials show above is in 1 M HClO4

solution. Note that equilibria 16-2 and 16-3are both established at the Pt electrode.911014 25http:\\asadipour.kmu.ac.ir

• There are three distinct regions in the titration of iron(II) with standard cerium(IV), monitored potentiometrically with Pt and

calomel electrodes.

1. Before the equivalence point, where the potential at Pt is dominated by the analyte redox pair.

2. At the equivalence point, where the potential at the indicator electrode is the average of their conditional potential.

3. After the equivalence point, where the potential was determined by the titratant redox pair. 911014 26http:\\asadipour.kmu.ac.ir

Before the equivalence point: using analyte’s concentration to calculate E+

At the equivalence point: needs both redox pairs to calculate (why?)

E of the cell 911014 27http:\\asadipour.kmu.ac.ir

After the equivalence point:

Summary• The greater the difference in reduction potential between analyze and

titrant, the sharper will be the end point. • The voltage at any point in this titration depends only on the ratio of

reactants; it will be independent of dilution.

• Prior to the equivalence point, the half-reaction involving analyze is used to find the voltage because the concentrations of both the oxidized and the reduced forms of analyte are known.

• After the equivalence point, the half-reaction involving titrant is employed. At the equivalence point, both half-reactions are used simultaneously to find the voltage.

There are two special points during the above titration process: (1) when V = ½ Ve, [Fe3+] = [Fe2+] and E+ = E°(Fe3+ | Fe2+) ; (2) when V = 2

Ve, [Ce4+] = [Ce3+] and E+ = E°(Ce4+ | Ce3+) = 1.70 V.


• Titration curve.xlsx

911014 http:\\asadipour.kmu.ac.ir 29

potentiometric methods a.) introduction : 1.) potentiometric methods: based on measurements of the...

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