lecture #18 of 26 - chem.uci.edu
TRANSCRIPT
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617
Lecture #18 of 26
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618
Liquid-Junction Potentials
Chapter 2
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619
Q: What’s in this set of lectures?A: B&F Chapter 2 main concepts:
● “Section 2.1”: Salt; Activity; Underpotential deposition
● Section 2.3: Transference numbers; Liquid-junctionpotentials
● Sections 2.2 & 2.4: Donnan potentials; Membrane potentials;pH meter; Ion-selective electrodes
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620So, in summary, five equations for junction potentials…
… why do they all include “kinetic” transport properties?
(α)
(β)LJ, Type 1
LJ, Type 2
LJ, Type 3(Henderson)
Goldman (GHHK)
Donnan(α)
(β)
the only model that, with one salt and one interface, definitely equilibrates
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621
http://biophys.med.unideb.hu/old/pharmacy/Donnan%20angol2009.pdf
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622… but first, what about “the fifth” equation?
… the Goldman–(Hodgkin–(Huxley)–Katz)) equation!
http://en.wikipedia.org/wiki/Membrane_potential
permeabilities
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623
www.highlands.edu/academics/divisions/scipe/biology/faculty/harnden/2121/notes/nervous.htm
… no need for a squid giant axon (H & H)(diameter ≈ 0.5 mm)…
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624
1900 20001920 1940 1960 1980
1933 – Arnold Beckman markets first pH electrode in Fullerton, CA for measuring acidity of lemon juice…
the Beckman G
1905 Walter Nernst proposes3rd Law of Thermodynamics; In Bern, Einstein describes PE effect,Brownian motion, and Special Rel.
Nernst wins Nobel Prize
1906 – Max Cremer discovers that a thin glass membrane separating two solutions develops a potential related to the difference in pH between them
1920’s – Duncan MacInnes & Malcolm Dole discover a glass suitable for glass pH electrodes…
~110 years of Ion-Selective Electrodes (ISEs)…
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625
the Beckman DU spectrophotometer, 1941
http://www.chemheritage.org/explore/Beckman/beckman.htm
the Beckman Helipot potentiometer, 1942
Beckman also created the first commercial spectrophotometer…
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626… but the glass pH electrode is exceptional in many ways…… while it is not a generic ISE… Why?
porousglass frit
Arnold Orville Beckman
(1900 – 2004)from Wiki
Chemist, Inventor, Investor, Philanthropist
Where are the other two electrodes for a 4-electrode measurement?
… they are not needed due to the high impedance of the circuit and no need to apply a large bias/current… thus, two are good enough!
a thin glass membranetransports cations with high selectivity… … the potential across the thin
glass membrane is measured in a buffered internal solution versus a second reference electrode
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627… protons do not traverse across the glass membrane… their concentration at the glass surfaces is coupled to the concentration of Na+ in the glass, solike before, two (Donnan) equilibria exist (one at each interface), not one!
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628
What type of LJ is this?Type 2!
… protons do not traverse across the glass membrane… their concentration at the glass surfaces is coupled to the concentration of Na+ in the glass, solike before, two (Donnan) equilibria exist (one at each interface), not one!
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629
1900 20001920 1940 1960 1980
1933 – Arnold Beckman markets first pH electrode in Fullerton, CA for measuring acidity of lemon juice…
the Beckman G
1905 Walter Nernst proposes3rd Law of Thermodynamics; In Bern, Einstein describes PE effect,Brownian motion, and Special Rel.
Nernst wins Nobel Prize
1906 – Max Cremer discovers that a thin glass membrane separating two solutions develops a potential related to the difference in pH between them
1920’s – Duncan MacInnes & Malcolm Dole discover a glass suitable for glass pH electrodes…
Nernst dies
1961 – Pungor & Hallos-Rokosinyi invent the solid membrane ISE
~110 years of Ion-Selective Electrodes (ISEs)…
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KCl
XY
XY
630… like the pH probe, the elements of an ISE are: (1) an internal reference electrode, (2) a filling solution, (3) an ion transporting element (i.e. membrane), and (4) an external reference electrode…
Bakker, Bühlmann, & Pretsch, Chem. Rev., 1997, 97, 3083, and Chem. Rev., 1998, 98, 1593
Why use a double-junction reference electrode?… To slow KCl salt leakage
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(α)
(β)
631
charge on the ionthat is detected
activity of detected ionoutside the electrode
activity of detected ioninside the electrode(i.e. filling solution)
… “#3: ion transporting element” is the key… it must be engineered to transport one ion selectively… in other words, its transport number is one
(α)
(β)
ideal ISE
Type 1 LJ
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632
… and this offset is determinedby the concentration of the ion inside the ISE
this is the concentrationoutside the ISE, in α…
… “#3: ion transporting element” is the key… it must be engineered to transport one ion selectively… in other words, its transport number is one
(α)
(β)
ideal ISE
Type 1 LJ
(α)
(β)or
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633
ideal ISE
Type 1 LJ
real ISE 𝐸𝑖 = constant +𝑅𝑇
𝑧𝑖𝐹ln 𝑎𝑖 +
𝑗
𝐾𝑗𝑎𝑗
𝑧𝑖𝑧𝑗
selectivity coefficient
activity of interfering ion, j
… “#3: ion transporting element” is the key… it must be engineered to transport one ion selectively… in other words, its transport number is one
(α)
(β)
(α)
(β)or
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634
slope = RT/(ziF)𝑅𝑇
𝑧𝑖𝐹ln
𝑗
𝐾𝑗𝑎𝑗
𝑧𝑖𝑧𝑗
𝐸𝑖 = constant +𝑅𝑇
𝑧𝑖𝐹ln 𝑎𝑖 +
𝑗
𝐾𝑗𝑎𝑗
𝑧𝑖𝑧𝑗
… a generic ISE calibration curve…
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635
for reasons that were not clear (until 1997), the very best ISEs had detection limits of only ~10-6 M
… the magnitude of the selectivity terms dictate the detection limit…
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636
1900 20001920 1940 1960 1980
1933 – Arnold Beckman markets first pH electrode in Fullerton, CA for measuring acidity of lemon juice…
the Beckman G
1905 Walter Nernst proposes3rd Law of Thermodynamics; In Bern, Einstein describes PE effect,Brownian motion, and Special Rel.
Nernst wins Nobel Prize
1906 – Max Cremer discovers that a thin glass membrane separating two solutions develops a potential related to the difference in pH between them
1920’s – Duncan MacInnes & Malcolm Dole discover a glass suitable for glass pH electrodes…
Nernst dies
1961 – Pungor & Hallos-Rokosinyi invent the solid membrane ISE
1966 – Frant and Ross describe the F– selective electrode
~110 years of Ion-Selective Electrodes (ISEs)…
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637
Frant & Ross, Science, 1966, 154, 1553
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638
LOD [F–] ≈ 10-7 M…no other ISE can achievethis small of a LOD…the F– selective electrode is a singularity!
… however, as one might guess, OH– is a strongly interfering ion for this ISE membrane
Warner, Anal. Chem., 1969, 41, 527
The Champion of ISEs: The F– selective electrode…
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639
1900 20001920 1940 1960 1980
1933 – Arnold Beckman markets first pH electrode in Fullerton, CA for measuring acidity of lemon juice…
the Beckman G
1905 Walter Nernst proposes3rd Law of Thermodynamics; In Bern, Einstein describes PE effect,Brownian motion, and Special Rel.
Nernst wins Nobel Prize
1906 – Max Cremer discovers that a thin glass membrane separating two solutions develops a potential related to the difference in pH between them
1920’s – Duncan MacInnes & Malcolm Dole discover a glass suitable for glass pH electrodes…
Nernst dies
1961 – Pungor & Hallos-Rokosinyi invent the solid membrane ISE
1966 – Frant and Ross describe the F– selective electrode
~110 years of Ion-Selective Electrodes (ISEs)…
1967 – Ross demonstrates the first liquid membrane ISE
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640What is a liquid membrane ISE?
(aqueous)
(non-aqueous)
(aqueous)
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641
http://openjurist.org/721/f2d/1540/wl-gore-associates-inc-v-garlock-inc
Discovered in New Zealand in 1966 by John Cropper, and again in 1969 by Wilbert and Robert Gore
What is Gore-Tex?
an expanded, porous PTFE
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642
K+
K+
… dissolved in the organicsolvent is a transporting agent that (ideally)complexes the metal ofinterest reversibly, andwith high selectivity
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643
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644… Nitrate selective electrode…
Thermo Scientific, ISE Manuals
NO3–
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645
Thermo Scientific, ISE Manuals
… Interfering ions (and concentrations) for the nitrate selective electrode…
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646… Calcium selective electrode…
Thermo Scientific, ISE Manuals … you get the idea…
Ca2+
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647
1900 20001920 1940 1960 1980
1933 – Arnold Beckman markets first pH electrode in Fullerton, CA for measuring acidity of lemon juice…
the Beckman G
1905 Walter Nernst proposes3rd Law of Thermodynamics; In Bern, Einstein describes PE effect,Brownian motion, and Special Rel.
Nernst wins Nobel Prize
1906 – Max Cremer discovers that a thin glass membrane separating two solutions develops a potential related to the difference in pH between them
1920’s – Duncan MacInnes & Malcolm Dole discover a glass suitable for glass pH electrodes…
Nernst dies
1961 – Pungor & Hallos-Rokosinyi invent the solid membrane ISE
… little innovation for 30 years!
1967 – Ross demonstrates the first liquid membrane ISE
1966 – Frant and Ross describe the F– selective electrode
~110 years of Ion-Selective Electrodes (ISEs)…
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648
http://www.pretsch.ethz.ch/EP/Pretsch.htmlSokalski, …, Pretsch, J. Am. Chem. Soc., 1997, 119, 11347
6+ orders of magnitude more?!?!?!?! …It’s real, general, and new, after 91 years!… How did he do it?
a breakthrough discovery by Ernö Pretsch (ETH Zürich)…
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649
Sokalski, …, Pretsch, J. Am. Chem. Soc., 1997, 119, 11347
What did Ernö Petsch do?… a “simple” idea…… replace the inner filling solution of the ISE with a metal ion buffer!
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650What’s a metal ion buffer?
… Well, first of all, what’s a H+ (pH) buffer?
a weak acid a weak base
… Henderson–Hasselbalch equation
𝐾a =H3O
+ A−
HA
H3O+ =
𝐾a HA
A−
pH = − log H3O+ = p𝐾a + log
A−
HA
HA + H2O A– + H3O+
make these large…
… which fixes [H3O+]
… even at a small value
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651Case in point: Phosphate = three equilibria, three H+ buffers
(to remove H+…… by adding OH– buffer)
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652
EDTA EDTA–metal chelate
What’s a metal ion buffer? Just like it sounds…
Y4– + Mn+ [MY](n – 4)+
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653
EDTA EDTA–metal chelate
What’s a metal ion buffer? Just like it sounds…
Y4– + Mn+ [MY](n – 4)+
𝐾MY =MY 𝑛−4 +
Y4− M𝑛+ make these large…
… which fixes [Mn+]
… even at a small value
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654
EDTA EDTA–metal chelate
What’s a metal ion buffer? Just like it sounds…
Y4– + Mn+ [MY](n – 4)+
𝐾MY =MY 𝑛−4 +
Y4− M𝑛+
pM = − log M𝑛+ = −p𝐾MY + logY4−
MY 𝑛−4 +
M𝑛+ =MY 𝑛−4 +
Y4− 𝐾MY
make these large…
… which fixes [Mn+]
… even at a small value
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pM
(to remove Mn+…… by adding
EDTA buffer)
655… buffering of Mn+ occurs afterthe equivalence point… Why?
pM = − log M𝑛+ = −p𝐾MY + logY4−
MY 𝑛−4 +
no buffering here…(started with Mn+ in solution)
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656KMY is adjustable based on pH… here is data for Ca2+
(to remove Mn+…… by adding EDTA buffer)
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conventional ISE
657… so, what does a metal ion buffer have to do with an ISE?
filling solution, [Mn+] = 10-4 M
leaked Mn+, [Mn+] ≈ 10-6 M
… so you can’t detect anything lower than this!
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658… so, what does a metal ion buffer have to do with an ISE?
conventional ISE
filling solution,[Mn+]free = 10-10 M
leaked Mn+,[Mn+] ≈ 10-12 M… and so ~1012 M can be detected!
with metal ion buffer
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659
THANK YOU, Ernö!
http://www.pretsch.ethz.ch/EP/Pretsch.htmlSokalski, …, Pretsch, J. Am. Chem. Soc., 1997, 119, 11347
a breakthrough discovery by Ernö Pretsch (ETH Zürich)…
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660
Q: What was in this set of lectures?A: B&F Chapter 2 main concepts:
● “Section 2.1”: Salt; Activity; Underpotential deposition
● Section 2.3: Transference numbers; Liquid-junctionpotentials
● Sections 2.2 & 2.4: Donnan potentials; Membrane potentials;pH meter; Ion-selective electrodes
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661
The Double Layer
Chapter 13