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Membrane Electrodes - 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, Membrane Electrodes, Glass Membrane Electrode, Liquid Membrane Electrode, Solid State Membrane, Permeable Membrane, Combination Ph Electrod

Typology: Slides

2012/2013

Uploaded on 08/30/2013

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Membrane Electrodes
Several types – Glass membrane electrode
- Solid State ““
- Liquid Junction ““
- Permeable “ “
Most important is glass electrode for pH
[H+] = a1[H+] = a2
solution 1 solution 2
potential develops
across membrane
thin glass membrane
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Download Membrane Electrodes - Advanced Analytical Chemistry - Lecture Slides and more Slides Analytical Chemistry in PDF only on Docsity!

Membrane Electrodes

  • Several types – Glass membrane electrode
  • Solid State
  • Liquid Junction
  • Permeable
  • Most important is glass electrode for pH

[H

(^) ] = a+

(^1)

[H

] = a+

2

solution 1

solution 2

across membranepotential develops

thin glass membrane

Glass pH Electrode

  • Combine with reference• E = K’ – 0.0591 pH electrode and meter
  • Half cell voltage proportional to pH
  • Intercept is K’, no E• Nernstian slope

o

  • Calibrate with buffers

with AgClcoatedAg wire Filling solution0.1 M HCl seal connection^ Electrical

Thin glass membrane

Proper pH Calibration (cont.)

nd step is to use any other pH buffer^

  • This will pivot the calibration line around the• Adjust slope/temp control to correct pH value isopotential which is set to 7.00 in all meters

mV

pH

(^4)

7

without changing itline around isopotentialSlope/temp control pivots

  • Other factors influence slope including• Slope is temperature sensitive• Slope comes from RT/nF in Nernst Equation
  • Overall quality of electrode construction– Impurities in glass membrane
  • Many electrodes exhibit “full Nernstian response” while others may give only 90%

Ag Cell for pH measurement (shorthand notation) (s) (^) AgCl

(s) (^) Cl

(aq)^ -

H unk+

HCl(0.1M) AgCl

(sat’d)

(^) Ag

(s)

reference electrode

test soln

glass electrode

Errors in pH Measurement 2

  • Asymmetry potential is another non-ideal H the glass. When both internal & externalpotential that arises possibly from strain in

solutions are the same activity,

potential should be 0 but it’s not

E

cell (^) = E

ind (^) – E

ref (^) + E

j +E a

  • Temperature of electrodes, calibration ATC probes are available for many meterssame primarily because of T in Nernst Eq.buffers and sample solutions must be the

Errors in pH Measurement 3

  • Alkaline Error or Sodium Error occurs when pH is very high (e.g., 12) because Na

raise pH) and Hconcentration is high (from NaOH used to

is very low. Electrode

responds slightly to Na

& gives a lower

most selective for Hthe electrode responds to many ions but isthe concept of selectivity coefficients wherereading than actual pH. This is related to

(^). Problem occurs+

because Na

is 10 orders of magnitude

higher than H

in the solution.

Errors in pH Measurement 4

  • Acid Error – electrode reads slightly higher (not well understood)than the actual pH in very acidic solutions
  • Response Time – related to activity for all activity (concentration) & slow at low conc.potentiometric electrodes & is fast at high
  • Hydration of Glass Surface – glass hrs if it dries out – will cause noisy readingsmeasurement & must be rehydrated for 24electrodes must be kept hydrated for good

Glass Electrode Summary

  • Glass membrane electrodes are very good indicator electrodes in potentiometry
  • Must exercise care in calibration and in maintaining integrity of glass membrane
  • Glass electrodes available for Na• Some errors exist & are unavoidable

, K+

NH

(^4) + , Rb

, Cs+

(^) , Li+

, Ag+

(cations only) by

varying glass composition

  • Combination electrodes combine pH & ref.

Liquid Membrane Electrodes

good exampleCalcium Electrode is

Cagroups selective forwith phosphateorganic compound– water immiscibleLiquid ion exchanger 2+ in a hydrophobic^

membrane

Ag/AgClCaCli.e., fixedsolutionFilling Ag wire 2

membrane(red) to wetexchangerexcess ionfilled withReservoir

Ion

exchanger

Liquid Membrane Electrodes

  • Principle of Ca

2+ electrode is the same as^

for glass electrode, however, since Ca

2+ is^

divalent n = 2

(^) Nernstian slope = 29.5 mV

per 10 fold change in concentration

  • Detection limit for Ca

2+ is approx. 10^

M^

  • Selectivity is:
  • 50 times better for Ca– Independent of pH from 5.5 to 11

2+ than for Mg^

2+

  • 1000 times better for Ca

2+ than Na^

or K

  • Other liquid membrane electrodes available

Table of liquid membrane electrodes

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

Commercially Available Solid State Ion Selective Electrodes (ISEs)

mba = must be absent mr = maximum ratio of interferent to analyte

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