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Lecture #18 of 26 618

Liquid-Junction Potentials

Chapter 2 619

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-junction potentials

● Sections 2.2 & 2.4: Donnan potentials; Membrane potentials; pH meter; Ion-selective electrodes So, in summary, five equations for junction potentials… 620 … why do they all include “kinetic” transport properties? (α) LJ, Type 1 (β)

LJ, Type 2

LJ, Type 3 (Henderson)

the only model that, with (α) one salt and one interface, Donnan (β) definitely equilibrates

Goldman (GHHK) 621

http://biophys.med.unideb.hu/old/pharmacy/Donnan%20angol2009.pdf … but first, what about “the fifth” equation? 622

… the Goldman–(Hodgkin–(Huxley)–Katz)) equation!

permeabilities

http://en.wikipedia.org/wiki/Membrane_potential … no need for a squid giant axon (H & H) 623 (diameter ≈ 0.5 mm)…

www.highlands.edu/academics/divisions/scipe/biology/faculty/harnden/2121/notes/nervous.htm ~110 years of Ion-Selective Electrodes (ISEs)… 624 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… 1933 – Arnold Beckman markets first pH electrode in Fullerton, CA for measuring acidity of lemon juice…

the Beckman G

1905 Walter Nernst proposes 3rd Law of Thermodynamics; In Bern, Einstein describes PE effect, Brownian motion, and Special Rel.

1900 1920 1940 1960 1980 2000

Nernst wins Nobel Prize Beckman also created the first commercial spectrophotometer… 625

the Beckman DU spectrophotometer, 1941

the Beckman Helipot , 1942 http://www.chemheritage.org/explore/Beckman/beckman.htm … but the glass pH electrode is exceptional in many ways… 626 … while it is not a generic ISE… Why? Chemist, Inventor, Investor, Philanthropist

Arnold Orville Beckman (1900 – 2004) porous from Wiki glass frit Where are the other two electrodes for a 4-electrode a thin glass membrane measurement? transports cations with high selectivity… … the potential across the thin … they are not needed due glass membrane is measured in a to the high impedance of buffered internal solution versus the circuit and no need to apply a large bias/current… a second reference electrode thus, two are good enough! … protons do not traverse across the glass membrane… their concentration 627 at the glass surfaces is coupled to the concentration of Na+ in the glass, so like before, two (Donnan) equilibria exist (one at each interface), not one! … protons do not traverse across the glass membrane… their concentration 628 at the glass surfaces is coupled to the concentration of Na+ in the glass, so like before, two (Donnan) equilibria exist (one at each interface), not one!

What type of LJ is this? Type 2! ~110 years of Ion-Selective Electrodes (ISEs)… 629 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… 1933 – Arnold Beckman markets first pH electrode in Fullerton, CA for measuring acidity of lemon juice… 1961 – Pungor & Hallos- the Beckman G Rokosinyi invent the solid membrane ISE

1905 Walter Nernst proposes 3rd Law of Thermodynamics; In Bern, Einstein describes PE effect, Brownian motion, and Special Rel.

1900 1920 1940 1960 1980 2000 Nernst dies Nernst wins Nobel Prize … like the pH probe, the elements of an ISE are: (1) an internal 630 reference electrode, (2) a filling solution, (3) an ion transporting element (i.e. membrane), and (4) an external reference electrode…

Why use a double-junction reference electrode? … To slow KCl salt leakage

KCl

XY

XY

Bakker, Bühlmann, & Pretsch, Chem. Rev., 1997, 97, 3083, and Chem. Rev., 1998, 98, 1593 … “#3: ion transporting element” is the key… it must be engineered to 631 transport one ion selectively… in other words, its transport number is one

(α) Type 1 LJ (β)

activity of detected ion outside the electrode

(α) ideal ISE (β)

activity of detected ion inside the electrode charge on the ion that is detected (i.e. filling solution) … “#3: ion transporting element” is the key… it must be engineered to 632 transport one ion selectively… in other words, its transport number is one

(α) Type 1 LJ (β) this is the concentration outside the ISE, in α…

(α) or ideal ISE (β)

… and this offset is determined by the concentration of the ion inside the ISE … “#3: ion transporting element” is the key… it must be engineered to 633 transport one ion selectively… in other words, its transport number is one

(α) Type 1 LJ (β)

(α) or ideal ISE (β)

푧푖 푅푇 푧푗 real ISE 퐸푖 = constant + ln 푎푖 + ෍ 퐾푗푎푗 푧푖퐹 푗

selectivity coefficient

activity of interfering ion, j … a generic ISE calibration curve… 푧푖 634 푅푇 푧푗 퐸푖 = constant + ln 푎푖 + ෍ 퐾푗푎푗 푧푖퐹 푗

푧 푖 slope = RT/(ziF) 푅푇 푧푗 ln ෍ 퐾푗푎푗 푧푖퐹 푗 … the magnitude of the selectivity terms dictate the detection limit… 635

for reasons that were not clear (until 1997), the very best ISEs had detection limits of only ~10-6 M ~110 years of Ion-Selective Electrodes (ISEs)… 636 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… 1933 – Arnold Beckman markets first pH electrode in Fullerton, CA for measuring acidity of lemon juice… 1961 – Pungor & Hallos- the Beckman G Rokosinyi invent the solid membrane ISE 1966 – Frant and Ross describe the F– selective 1905 Walter Nernst proposes electrode 3rd Law of Thermodynamics; In Bern, Einstein describes PE effect, Brownian motion, and Special Rel.

1900 1920 1940 1960 1980 2000 Nernst dies Nernst wins Nobel Prize 637

Frant & Ross, Science, 1966, 154, 1553 The Champion of ISEs: The F– selective electrode… 638

… however, as one might guess, OH– is a strongly interfering ion for this ISE membrane

LOD [F–] ≈ 10-7 M… no other ISE can achieve this small of a LOD… – the F selective electrode is a singularity! Warner, Anal. Chem., 1969, 41, 527 ~110 years of Ion-Selective Electrodes (ISEs)… 639 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… 1933 – Arnold Beckman markets first pH electrode in Fullerton, CA for measuring acidity of lemon juice… 1961 – Pungor & Hallos- the Beckman G Rokosinyi invent the solid membrane ISE 1966 – Frant and Ross describe the F– selective 1905 Walter Nernst proposes electrode 3rd Law of Thermodynamics; In Bern, Einstein describes PE effect, 1967 – Ross demonstrates Brownian motion, and Special Rel. the first liquid membrane ISE

1900 1920 1940 1960 1980 2000 Nernst dies Nernst wins Nobel Prize What is a liquid membrane ISE? 640

(aqueous)

(non-aqueous)

(aqueous) What is Gore-Tex? 641

an expanded, porous PTFE

Discovered in New Zealand in 1966 by John Cropper, and again in 1969 by Wilbert and Robert Gore

http://openjurist.org/721/f2d/1540/wl-gore-associates-inc-v-garlock-inc 642

K+

… dissolved in the organic solvent is a transporting agent that (ideally) complexes the metal of interest reversibly, and with high selectivity K+ 643 … Nitrate selective electrode… 644

– NO3

Thermo Scientific, ISE Manuals … Interfering ions (and concentrations) for the nitrate selective electrode… 645

Thermo Scientific, ISE Manuals … Calcium selective electrode… 646

Ca2+

Thermo Scientific, ISE Manuals … you get the idea… ~110 years of Ion-Selective Electrodes (ISEs)… 647 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… 1933 – Arnold Beckman markets first pH electrode in Fullerton, CA for measuring acidity of lemon juice… 1961 – Pungor & Hallos- the Beckman G Rokosinyi invent the solid membrane ISE 1966 – Frant and Ross describe the F– selective 1905 Walter Nernst proposes electrode 3rd Law of Thermodynamics; In Bern, Einstein describes PE effect, 1967 – Ross demonstrates Brownian motion, and Special Rel. the first liquid membrane ISE

1900 1920 1940 1960 1980 2000 Nernst dies Nernst wins Nobel Prize … little innovation for 30 years! a breakthrough discovery by Ernö Pretsch (ETH Zürich)… 648

6+ orders of magnitude more?!?!?!?! … It’s real, general, and new, after 91 years! … How did he do it? http://www.pretsch.ethz.ch/EP/Pretsch.html Sokalski, …, Pretsch, J. Am. Chem. Soc., 1997, 119, 11347 What did Ernö Petsch do? 649 … a “simple” idea… … replace the inner filling solution of the ISE with a metal ion buffer!

Sokalski, …, Pretsch, J. Am. Chem. Soc., 1997, 119, 11347 What’s a metal ion buffer? 650 … Well, first of all, what’s a H+ (pH) buffer?

– + HA + H2O A + H3O

a weak acid a weak base

+ − H3O A 퐾a = HA make these large… + … which fixes [H3O ] 퐾 HA … even at a small value H O+ = a 3 A− A− pH = − log H O+ = p퐾 + log 3 a HA … Henderson–Hasselbalch equation Case in point: Phosphate = three equilibria, three H+ buffers 651

(to remove H+… … by adding OH– buffer) What’s a metal ion buffer? Just like it sounds… 652

Y4– + Mn+ [MY](n – 4)+

EDTA EDTA–metal chelate What’s a metal ion buffer? Just like it sounds… 653

Y4– + Mn+ [MY](n – 4)+

EDTA EDTA–metal chelate

MY 푛−4 + 퐾MY = Y4− M푛+ make these large… … which fixes [Mn+] … even at a small value What’s a metal ion buffer? Just like it sounds… 654

Y4– + Mn+ [MY](n – 4)+

EDTA EDTA–metal chelate

MY 푛−4 + 퐾MY = Y4− M푛+ make these large… … which fixes [Mn+] 푛−4 + … even at a small value 푛+ MY M = 4− Y 퐾MY

Y4− pM = − log M푛+ = −p퐾 + log MY MY 푛−4 + pM (started with M (started with here… no buffering pM = − log n+ M 푛 in solution) + the equivalence point the … = buffering − p 퐾 MY + log of of (to (to remove M M n MY + occurs Y 푛 4 … − − 4 W + after n+ hy … ? … by adding EDTA EDTA buffer) 655 2+ 656 KMY is adjustable based on pH… here is data for Ca

(to remove Mn+… … by adding EDTA buffer) … so, what does a metal ion buffer have to do with an ISE? 657

conventional ISE

filling solution, [Mn+] = 10-4 M leaked Mn+, [Mn+] ≈ 10-6 M

… so you can’t detect anything lower than this! … so, what does a metal ion buffer have to do with an ISE? 658

conventional ISE with metal ion buffer

filling solution, n+ -10 [M ]free = 10 M leaked Mn+, [Mn+] ≈ 10-12 M … and so ~1012 M can be detected! a breakthrough discovery by Ernö Pretsch (ETH Zürich)… 659

THANK YOU, Ernö! http://www.pretsch.ethz.ch/EP/Pretsch.html Sokalski, …, Pretsch, J. Am. Chem. Soc., 1997, 119, 11347 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-junction potentials

● Sections 2.2 & 2.4: Donnan potentials; Membrane potentials; pH meter; Ion-selective electrodes 661

The Double Layer

Chapter 13