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Membrane - Advanced Analytical Chemistry - Lecture Slides, Slides of Analytical Chemistry

Indicator Electrodes, Potentiometry, Electrochemistry, Electrode Kinetics, Electrogravimetry, Polarography, Square Wave Voltammetry, Wavelength Selection, Types of Spectroscopy, Detectors, Theory of Molecular Absorption, Absorption by Organic Molecules, Applying UV-vis Spectroscopy are major topics of this course. Main points from these slides are: Membrane Electrodes, Liquid Junction, Solid State, Permeable, Glass Ph Electrode, Ph Calibration, Combination Ph Electrode, Liquid Membrane Electrode

Typology: Slides

2012/2013

Uploaded on 08/30/2013

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bg1
Electrical
connection
seal
0.1 M HCl
Filling solution
Ag wire
coated
with AgCl
Thin glass membrane
Membrane Electrodes
Several types
- Glass membrane electrode
- Liquid membrane electrode
- Solid State membrane
electrode
- Permeable membrane
electrode
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with AgClcoatedAg wire Filling solution0.1 M HCl seal connectionElectrical

Thin glass membrane

Several types Membrane Electrodes

electrode- Permeable membraneelectrode- Solid State membrane- Liquid membrane electrode- Glass membrane electrode

Combination pH Electrode

Ag wire Solid State Membrane Electrodes

Ag/AgClresponds toelectrodecation that[Cl-] andwith fixedsolutionFilling Solid state membrane

(must be ionic conductor)

Pb 2+

Ag

(^2) S + PbS

Cd 2+

Ag (^2) S + CdS

Cu 2+

Ag (^2) S + CuS

Ag (^) , S+

2-

Ag (^2) S

Ag (^) , I+

AgI

Ag (^) , Br+

AgBr

Ag (^) , Cl+

AgCl

F , La-

3+

LaF

3

Ion Determined

MembraneSolid State Membrane Chemistry

Permeable Membrane Electrodes

Gas Permeable Membrane Electrodes

Gas Sensing Electrodes

  • Membrane that is permeable to a gas (e.g., NH

(^3) ) is the key component of electrode

  • Membrane is part of a small chamber which housed insideencloses a filling solution with a pH electrode
  • Filling solution has “fixed” [NH

(^4) + ] which

responds to changes in [NH

(^3) ] passing

membrane according to

NH

3

  • H

(^2) O

NH

(^4) +

  • OH

Membrane ElectrodesGas Permeable

(blue)membraneGlass electrode^ pH

(yellow)permeablegasmembrane -Hydrophobic

(gray)solutionFilling

  • NH- Electrode immersed in test solution

(^3) diffuses through membrane

  • NH

(^3) in test solution equilibrates

with NH

(^3) in filling solution

NH (^3)

  • H (^2) O

NH (^4) +

  • OH

[NH

(^4) + ][OH

]-

K b = ----------------------

[NH

(^3) ]

K b

[OH

] = --------- [NH-

(^3) ]

[NH

(^4) + ]

pH = 14 – pOH = pNH

3

Diagram of CO

2

Reference electrodeshowing internalgas sensing probe is hydrophobicbecause membranein the filling solutionbehind membranemust be locatedReference electrode

e.g., Urea ElectrodeEnzyme Electrode

(blue)membraneGlass electrode^ pH

(yellow)permeablegasmembrane -Hydrophobic

(gray)solutionFilling

on the surface of the NHa thin layer of the enzyme ureasecan be prepared by immobilizingAn electrode sensitive to urea

3 electrode

O

H (^2) N-C-NH

(^2)

  • H (^2) O

2 NH

(^3)

  • CO

2

  • Urea is broken down to NHimmobilized on the surface- Urea comes in contact with urease

3

& CO

2 in this enzyme layer

-NH

(^3) diffuses through

membrane to give response

Enzyme layer

Potentiometry - Conclusion

  • Electrochemical (galvanic) cell with essentially no current flow
  • Requires a solution that is conductive i.e., contains a “supporting electrolyte”
  • Laboratory pH/millivolt meters should be capable of measuring + 0.1 mV
  • Measure “free” or uncomplexed ions not total• Electrodes measure activity not concentration• This corresponds to 0.4 x n % uncertainty

In all electrochemical methods, the rate of which depend on:at electrode surface (electrode kinetics),2) kinetics of the electron transfer processreach electrode surface (mass transport)1) rate & means by which soluble speciesoxidation & reduction depend on:

c) temperatureb) nature of electrode surfacea) nature of the reaction

(we don’t have much control over #2)

Mass Transport or Mass Transfer

Migration – movement of a charged

In most cases migration is undesirable and(important for conductance & electrophoresis)particle in a potential field – generally bad

migrating, not the analytereduced) Inert electrolyte does theelectrochemically inert – not oxidized orexcess of an inert electrolyte (i.e.,can be eliminated by adding a 100 fold

Voltammetry

  • Apply potential or current to electrochemical• Methods based on an electrolytic cell reactionssurface due to oxidation & reductioncell & concentrations change at electrode
  • Measure current or voltage• Stirred or unstirred solution• Can have 2 or 3 electrodes

In all electrochemical methods, the rate of which depend on:at electrode surface (electrode kinetics),2) kinetics of the electron transfer processreach electrode surface (mass transport)1) rate & means by which soluble speciesoxidation & reduction depend on:

c) temperatureb) nature of electrode surfacea) nature of the reaction

(we don’t have much control over #2)

Fick’s Laws describe diffusion

st Law^

JJ JJ Where

cm= flux of material i.e., moles passing a 1

(^2) plane at point x & time t (mol/cm

/sec)^2

DDDD

(^) = diffusion coefficient (cm

/sec)^2

CCCC

(^) = concentration

tt tt

= time (sec) from when power is turned on

xxxx

(^) = distance from electrode surface (cm)