lecture #3 of 17ardo/echem/uci-chem248-2019s_lectur… · 4/15/2019 2 119 7+ 6+ 4+ 3+ 2+ 0...

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116

Lecture #3 of 17

117

Looking forward… our review of Chapter “0”

● Cool applications

● Redox half-reactions

● Balancing electrochemical equations

● History of electrochemistry

● IUPAC terminology and Ecell = Ered – Eox

● Nernst equation and Common reference electrodes

● Standard and Absolute potentials

● Latimer and Pourbaix diagrams

● Calculating Ecell under non-standard state conditions

● Conventions

118

A Latimer diagram is a summary of the E0 values for an element; it is useful for visualizing the complete redox series for an element and for determining when disproportionation will occur.

from Wiki

Oxidation Reduction

1,69 V

Two diagrams of empirical standard potentials…

Wendell Mitchell Latimer

(1893–1955)http://academictree.org/chemistry/peopleinfo.php?pid=24644

Chemist

Latimer, The oxidation states of the elements and their potentials in aqueous solution, 1938

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119

7+ 6+ 4+ 3+ 2+ 0

Oxidation Reduction

1,69 V



from Wiki



Chemist


120

Disproportionation – spontaneous and simultaneous reduction and oxidation of a molecule

7+ 6+ 4+ 3+ 2+ 0

Oxidation Reduction

1,69 V



from Wiki



Chemist


121

from Wiki

7+ 6+ 4+ 3+ 2+ 0

Oxidation Reduction

(1) Does Mn2+ disproportionate?(2) What is the standard reduction potential of MnO4

– to MnO2?

1,69 V




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122

Reduction: Mn2+ Mno Eo = +1.18 V

Oxidation: Mn2+ Mn3+ Eo = +1.51 V

from Wiki

7+ 6+ 4+ 3+ 2+ 0

Oxidation Reduction

(1) Does Mn2+ disproportionate?(2) What is the standard reduction potential of MnO4

– to MnO2?

1,69 V




123

from Wiki

7+ 6+ 4+ 3+ 2+ 0

Oxidation Reduction

(1) Does Mn2+ disproportionate? NO. Eo = Ered – Eox = 1.18 – 1.51 = – 0.33 V(2) What is the standard reduction potential of MnO4

– to MnO2?

1,69 V

Reduction: Mn2+ Mno Eo = +1.18 V

Oxidation: Mn2+ Mn3+ Eo = +1.51 V




124

from Wiki

7+ 6+ 4+ 3+ 2+ 0

Oxidation Reduction

(1) Does Mn2+ disproportionate? NO. Eo = Ered – Eox = 1.18 – 1.51 = – 0.33 V(2) What is the standard reduction potential of MnO4

– to MnO2?ΔGo = -nFEo = -3FEo

1,69 V




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125

from Wiki

7+ 6+ 4+ 3+ 2+ 0

Oxidation Reduction

(1) Does Mn2+ disproportionate? NO. Eo = Ered – Eox = 1.18 – 1.51 = –0.33 V(2) What is the standard reduction potential of MnO4


ΔGo = -nFEo1 + -nFEo

2 = -F((1 x 0.56 V) + (2 x 2.26 V)) = -F(5.08 V)

1,69 V




126

from Wiki

7+ 6+ 4+ 3+ 2+ 0

Oxidation Reduction

(1) Does Mn2+ disproportionate? NO. Eo = Ered – Eox = 1.18 – 1.51 = –0.33 V(2) What is the standard reduction potential of MnO4


ΔGo = -nFEo1 + -nFEo

2 = -F((1 x 0.56 V) + (2 x 2.26 V)) = -F(5.08 V)Set them equal to each other, and thus, 3Eo = 5.08 and Eo = 1.69 V


1,69 V



… you do not need to do this process for #1 above, because the reaction is always balanced/equal in the number of electrons

1,69 V

127

from Wiki

7+ 6+ 4+ 3+ 2+ 0

Oxidation Reduction



Recall from before…

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1,69 V

128

from Wiki

7+ 6+ 4+ 3+ 2+ 0

Oxidation Reduction



Recall from before… ???

???

1,69 V

129

from Wiki

7+ 6+ 4+ 3+ 2+ 0

Oxidation Reduction




???

… anyway, why are

these bottom E0

values not on the

Latimer diagram?

1,69 V

130

from Wiki

7+ 6+ 4+ 3+ 2+ 0

Oxidation Reduction




???

… anyway, why are

these bottom E0

values not on the

Latimer diagram?

… because they

are at basic/alkaline

standard state with

~1 M OH–!

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1,69 V

131

from Wiki

7+ 6+ 4+ 3+ 2+ 0

Oxidation Reduction




???

What would this E0

value be when at

acidic standard

state?

132Two diagrams of empirical standard potentials…


???

𝐸 = 𝐸𝑎𝑐𝑖𝑑0 −

0.05916 V

𝑛log

𝑀𝑛𝑂21 𝐻2𝑂

2

𝑀𝑛𝑂42− 1

𝐻+ 4= 𝐸𝑎𝑐𝑖𝑑

0 −0.05916 V

2log

1 1

1 1 10−14 4 = 𝐸𝑎𝑐𝑖𝑑0 − 0.02958 V 56

What would this E0

value be when at

acidic standard

state?

1,69 V

from Wiki

7+ 6+ 4+ 3+ 2+ 0

Oxidation Reduction

???



???


0.05916 V

𝑛log


2

𝑀𝑛𝑂42− 1


0 −0.05916 V

2log

1 1

1 1 10−14 4 = 𝐸𝑎𝑐𝑖𝑑0 − 0.02958 V 56

What would this E0

value be when at

acidic standard

state?

1,69 V

from Wiki

7+ 6+ 4+ 3+ 2+ 0

Oxidation Reduction

???

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???


0.05916 V

𝑛log


2

𝑀𝑛𝑂42− 1


0 −0.05916 V

2log

1 1

1 1 10−14 4 = 𝐸𝑎𝑐𝑖𝑑0 − 0.02958 V 56

What would this E0

value be when at

acidic standard

state?

1,69 V

from Wiki

7+ 6+ 4+ 3+ 2+ 0

Oxidation Reduction

???



???

1,69 V

from Wiki

7+ 6+ 4+ 3+ 2+ 0

Oxidation Reduction

???


0.05916 V

𝑛log


2

𝑀𝑛𝑂42− 1


0 −0.05916 V

2log

1 1

1 1 10−14 4 = 𝐸𝑎𝑐𝑖𝑑0 − 0.02958 V 56

𝐸 = 𝐸𝑎𝑐𝑖𝑑0 − 1.65648 V = 0.60 V

What would this E0

value be when at

acidic standard

state?

1,69 V

136

from Wiki

7+ 6+ 4+ 3+ 2+ 0

Oxidation Reduction



???

???

𝐸 = 𝐸𝑎𝑐𝑖𝑑0 − 1.65648 V = 0.60 V

𝐸𝑆𝐻𝐸0 = 2,25648 V

SWEET!


0.05916 V

𝑛log


2

𝑀𝑛𝑂42− 1


0 −0.05916 V

2log

1 1

1 1 10−14 4 = 𝐸𝑎𝑐𝑖𝑑0 − 0.02958 V 56

What would this E0

value be when at

acidic standard

state?

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1,69 V

137

from Wiki

7+ 6+ 4+ 3+ 2+ 0

Oxidation Reduction



???

???

𝐸 = 𝐸𝑎𝑐𝑖𝑑0 − 1.65648 V = 0.60 V

𝐸𝑆𝐻𝐸0 = 2,25648 V

SWEET!


0.05916 V

𝑛log


2

𝑀𝑛𝑂42− 1


0 −0.05916 V

2log

1 1

1 1 10−14 4 = 𝐸𝑎𝑐𝑖𝑑0 − 0.02958 V 56

… but then why did the

CRC not list this? …

What would this E0

value be when at

acidic standard

state?

138

from Wiki

Marcel Pourbaix

(1904–1998)http://corrosion-doctors.org/Biographies/PourbaixBio.htm

… Second one (not truly standard potentials)…

Pourbaix, Atlas of electrochemical equilibria in aqueous solutions, 1974

Chemist



A Pourbaix diagram is a map of the predominant equilibrium species of an aqueous electrochemical system; it is useful for identifying which materials/species are present/stable

… mostly based on thermochemical data

139



from Wiki

Marcel Pourbaix



Chemist

1,69 V

from Wiki

7+ 6+ 4+ 3+ 2+ 0

Oxidation

???




… because in acid, the reaction does

not occur!

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140

from Wiki

RHE

E(O2,H+/H2O)

Marcel Pourbaix



Anyway, … standard state is here, at ~1 M H+ (pH = 0) SHE… but if written under alkaline conditions, ~1 M OH– is standard state (pH 14)

Chemist


Why don’t I like this? … Even though EVERYONE plots it this way



141

(1) What is the electrocatalyst for O2 evolution through water oxidation?(2) At what pH values is a solid electrocatalyst for H2 evolution stable?

Marcel Pourbaix



Chemist


from Wiki

RHE

E(O2,H+/H2O)



142

(1) What is the electrocatalyst for O2 evolution through water oxidation? MnO2

(2) At what pH values is a solid electrocatalyst for H2 evolution stable?

Marcel Pourbaix



Chemist


from Wiki

RHE

E(O2,H+/H2O)



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143

(1) What is the electrocatalyst for O2 evolution through water oxidation? MnO2

(2) At what pH values is a solid electrocatalyst for H2 evolution stable? pH 7.5 – 13

Marcel Pourbaix



Chemist


from Wiki

RHE

E(O2,H+/H2O)



144

How else could we write this? … “60 mV/2 log”! (at room temp.)

𝐸 = 𝐸0 −𝑅𝑇

𝑛𝐹ln𝑄Nernst Equation:

145

{Facile



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146

{Facile



Recall

• Ecell does not require “n”

• ΔG does require “n” (-nFEcell)

Pt(s)|Hg(l) Pt(s)|Hg2Cl2(s)+ Cl–(aq)

147

Cu SCE

Cu2+(aq) Cu(s)

– –

Common Inert Electrodes: Platinum, Carbon, Gold

Common Reactive Electrodes: Copper, Zinc, Cadmium, Lead, Silver

high impedance to measure potential

NOT The Daniell Cell

148Write and explain the line notation for the redox reaction

between Cu/Cu2+ and an SCE electrode, where Cu2+ is CuSO4

(0.1 M), and KCl (1 M) is present in all cells.

Cu SCE

– –

Eo(Cu2+/Cu) = +0.1 V vs. SCE

Cu2+(aq) Cu(s) Pt(s)|Hg(l) Pt(s)|Hg2Cl2(s)+ Cl–(aq)

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152

Cu SCE

– –


(a) What is Ecell in this case (1 M KCl, 0.1 M CuSO4)?

(b) What is Ecell if [KCl] = 0.1 M?



153(a) What is Ecell in this case (1 M KCl, 0.1 M CuSO4)?



𝐸cell = 𝐸𝑜 −𝑅𝑇

𝑛𝐹ln

𝑎𝐶𝑢𝑎𝐻𝑔2𝐶𝑙2𝑎𝐶𝑢2+𝑎𝐻𝑔𝑎𝐶𝑙−2


(a)





𝑛𝐹ln



(a)

𝐸cell ≈ 𝐸𝑜 −0.0592 V

𝑛log

1

𝐶𝑢2+ 𝐶𝑙− 2

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𝑛𝐹ln

𝑎𝐶𝑢𝑎𝐻𝑔2𝐶𝑙2𝑎𝐶𝑢2+𝑎𝐻𝑔𝑎𝐶𝑙−

2


(a)


𝑛log

1


𝐸cell = +0.1 V −0.0592 V

2log

1

0.1

𝐸cell = +0.1 V − 0.0296 V = +0.0704 V





𝑛𝐹ln



(a)


𝑛log

1


𝐸cell = +0.1 V −0.0592 V

2log

1

0.1

𝐸cell = +0.1 V − 0.0296 V = +0.0704 V

𝐸cell = +0.1 V −0.0592 V

2log

1

(0.1)3

𝐸cell = +0.1 V − 0.0888 V = +0.0112 V

(b)

* Remember, there is no such thing as a half-cell reaction, unless you’re working with Trasatti

157

Quick quiz: Do the following make sense?

The grams (or grammage) of my material was 0.1 g.

The liters (or literrage) of my beaker was 0.1 L.

The m/s (or m/s-age) of that baseball was 10 m/s…

In general, IUPAC will be our standard guide for this course…

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158





Then I prefer that you don’t say:

“The voltage of my cell is 0.1 V.” Let’s call it a potential…

In general, IUPAC will be our standard guide for this course…

159In general, IUPAC will be our standard guide for this course…

http://goldbook.iupac.org/V06635.html

… and IUPAC prefers it too!







The kinetic process was graphed as an M–s curve.

The kinetics were followed as the concentration versus s…



http://goldbook.iupac.org/V06635.html

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The kinetic process was graphed as an M–s curve.

The kinetics were followed as the concentration versus s…

Then I also prefer that you don’t say:

“The cell’s behavior is shown as the I–V curve.” Let’s call

it an I–E curve, or best yet, a J–E curve.



162Electrochemistry:

conventions… oh, conventions!

http://upload.wikimedia.org/wikipedia/commons/thu

mb/c/cc/Map_of_USA_TX.svg/2000px-

Map_of_USA_TX.svg.png

163Electrochemistry:

conventions… oh, conventions!

Handbook of Electrochemistry, Zoski (ed.), Elsevier, 2007

WE WILL USE THIS ONE…

… which is like you’ve learned in every

math class you’ve ever taken…

… and so yay!

http://upload.wikimedia.org/wikipedia/commons/thumb/c/cc/Map_of_USA_TX.svg/2000px-Map_of_USA_TX.svg.png

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164

And FINALLY… A review of Chapter “0”

● Cool applications

● Redox half-reactions

● Balancing electrochemical equations

● History of electrochemistry

● IUPAC terminology and Ecell = Ered – Eox

● Nernst equation and Common reference electrodes

● Standard and Absolute potentials

● Latimer and Pourbaix diagrams

● Calculating Ecell under non-standard state conditions

● Conventions

165

Measurements in

Electrochemistry and Mass

Transfer Processes

Chapters 1, 15, and 4

166

Q: What’s in this set of lectures?

A: B&F Chapters 1, 15 & 4 main concepts:

● Section 1.1: Redox reactions

● Chapter 15: Electrochemical instrumentation

● Section 1.2: Charging interfaces

● Section 1.3: Overview of electrochemical experiments

● Section 1.4: Mass transfer and Semi-empirical treatment of

electrochemical observations

● Chapter 4: Mass transfer

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167

Looking forward… Section 1.1 (and some of Chapter 15)

● 2-electrode versus 3-electrode measurements

● Reference electrodes

● Potentiostats

● Compliance voltage/current

● J–E and I–E curves

● Kinetic overpotential

● Faradaic reactions

168Although we would like to measure electrochemical observables

(e.g. the current, voltage, etc.) associated with a single “working”

electrode (WE), we cannot.

We must always couple our working electrode to a second electrode in

order to make a measurement. These two electrodes comprise an

electrochemical “cell.”

Power

Supply

Red line resists current flow

169These schematics introduce some terms that we must define:

electrometer – A device for measuring a potential difference (Ecell). An

ideal voltmeter has infinite input impedance (i.e. it

draws no current). (impedance is “complex resistance”)

ammeter – A device for measuring a current. An ideal ammeter

has zero input impedance (i.e. it imposes no potential

drop).

Power

Supply


http://chemwiki.ucdavis.edu/Analytical_Chemistry/E

lectrochemistry/Electrochemistry_2%3A_Galvanic_c

ells_and_Electrodes

-0.76 V

http://chemwiki.ucdavis.edu/Analytical_Chemistry/Electrochemistry/Electrochemistry_2:_Galvanic_cells_and_Electrodes

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170Experiments:

95% of the measurements that you will perform have a problem

Power

Supply


… Oftentimes, most of us wish

to control the potential of this

“working” electrode...

171

Power

Supply

… while not affecting the

potential of the second

(reference) electrode that is

used to “complete the circuit.”


Experiments:

95% of the measurements that you will perform have a problem

… Oftentimes, most of us wish

to control the potential of this

“working” electrode...

172

E, V vs. ???0 0.2 0.4 0.6 0.8 1.0-0.2-0.4-0.6-0.8-1.0

WE

RE

ΔEoc = 0.0 V = EWE – ERE

… for example, let’s say both electrodes are platinum…

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173

E, V vs. ???0 0.2 0.4 0.6 0.8 1.0-0.2-0.4-0.6-0.8-1.0

WE

RE


… and at “open circuit,” no potential bias is applied between them…

(disconnect the wire!)

… and by the way, we don’t know this potential…

174

E, V vs. ???0 0.2 0.4 0.6 0.8 1.0-0.2-0.4-0.6-0.8-1.0

WE

RE


… and at “open circuit,” no potential bias is applied between them…

(disconnect the wire!)

… and by the way, we don’t know this potential…

… and it is not well-defined because we cannot answer the question:

What is the half-reaction that defines it?



175

E, V vs. ???0 0.2 0.4 0.6 0.8 1.0-0.2-0.4-0.6-0.8-1.0

WE

RE

ΔEapp = +0.8 V

… now, if we apply +0.8 V to the WE (reconnect the wire)…the potential of both electrodes likely changes, and not likely symmetrically…

ΔE = 0.8 V

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176

E, V vs. ???0 0.2 0.4 0.6 0.8 1.0-0.2-0.4-0.6-0.8-1.0

WE

RE

ΔEapp = +0.8 V

… even worse, we don’t now the potential of either electrode…

ΔE = 0.8 V

… and we don’t know this potential!… we don’t know this potential…

177

E, V vs. ???0 0.2 0.4 0.6 0.8 1.0-0.2-0.4-0.6-0.8-1.0

WE

RE

ΔEapp = +1.2 V

… you get the picture!

ΔE = 1.2 V

178

In principle, this problem can be solved by using

a second electrode that is an (ideal) reference

electrode… (ideally) non-polarizable:

E, V vs. RE0.5 1.0 1.5 2.0-0.5-1.0-1.5-2.0

J, A

1E-3

2E-3

3E-3

-3E-3

-2E-3

-1E-3

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179

ΔEoc ≠ 0.0 V (likely)

… so get rid of the Pt reference electrode, and substitute in an SCE…

… which has a Pt wire in it…

E, V vs. SHE0 0.2 0.4 0.6 0.8 1.0-0.2-0.4-0.6-0.8-1.0

RE = saturated calomel electrode (SCE)

WE

180


E, V vs. SHE0 0.2 0.4 0.6 0.8 1.0-0.2-0.4-0.6-0.8-1.0


WE

… where we still don’t know this potential because we cannot answer:




181


E, V vs. SHE0 0.2 0.4 0.6 0.8 1.0-0.2-0.4-0.6-0.8-1.0


WE

… but, where we know this potential because we can answer:


Pt(s)|Hg(l)|Cl–(aq) Pt(s)|Hg2Cl2(s)





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182


E, V vs. SHE0 0.2 0.4 0.6 0.8 1.0-0.2-0.4-0.6-0.8-1.0


E0SCE = +0.241 V vs. SHE

WE



… the SCE has a defined potential of +0.241 V vs. SHE…

183

ΔEapp = 0.0 V (ammeter ≠ 0 A)

E, V vs. SHE0 0.2 0.4 0.6 0.8 1.0-0.2-0.4-0.6-0.8-1.0

WE


E0SCE = +0.241 V vs. SHE

Power

Supply

Current must flow!




… and it “does not” “move” (much, usually)…

184

ΔEapp = +0.4 V


… and it “does not” “move” (much, usually)…

E, V vs. SHE0 0.2 0.4 0.6 0.8 1.0-0.2-0.4-0.6-0.8-1.0

WE


+0.241 V +0.641 V

ΔE = 0.4 V

… how did we calculate that (meaning +0.641 V)?

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185

ΔEapp = +0.4 V

… how did we calculate that (meaning +0.641 V)?

E, V vs. SHE0 0.2 0.4 0.6 0.8 1.0-0.2-0.4-0.6-0.8-1.0

WE


+0.241 V +0.641 V

ΔE = 0.4 V

ΔE = EWE – ERE

EWE = +0.4 V + +0.241 V = +0.641 V

186

ΔEapp = -0.7 V

… you get the picture!...

… but let’s learn some more about reference electrodes…

E, V vs. SHE0 0.2 0.4 0.6 0.8 1.0-0.2-0.4-0.6-0.8-1.0

WE


+0.241 V-0.459 V

ΔE = -0.7 V

ΔE = EWE – ERE

EWE = -0.7 V + +0.241 V = -0.459 V

187

http://www.gamry.com/Products/RefElec_SCE.htm

… here is what a commercial SCE looks like:

Some major companies that have excellent

additional information on their websites

AMETEK (PAR, Solartron), BASi, Bio-Logic,

CH Instruments, Gamry, Metrohm, Pine

Vycor frit

heat shrink tubing

plastic caps

copper wire

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188

1. It has a well-defined and invariant potential. That

is, no matter how much current we draw from this

electrode, its potential must not vary.

2. It has zero impedance. That is, it imposes no

resistive load on our cell.

3. It does not “contaminate” our solution. That is, it

is not a source of undesired ions in our

electrochemical cell.

Specifically, we would really like to have a reference

electrode that has the following attributes.

189

Vycor frit

heat shrink tubing

4 – 6 mm (o.d.) glass

tubing shaped like

an “h”

white Epotec epoxy or

TorrSeal, heat gunned...

copper wire

platinum wire

plastic caps

… but no such thing exists.

The closest approximation: the saturated calomel electrode (SCE)

190

mercury

calomel*

calomel* - a paste

containing liquid

mercury, Hg2Cl2 and

some sat’d aq. KCl

filling solution:

aqueous saturated KCl

Vycor frit

heat shrink tubing

4 – 6 mm (o.d.) glass

tubing shaped like

an “h”

copper wire

plastic caps

… but no such thing exists.


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191

E0 = +0.241 V vs. SHE

the saturated calomel electrode (SCE)

the saturated mercurous sulfate electrode (MSE)

E0 = +0.236 V vs. SHE

the saturated sodium calomel electrode (SSCE)

E0 = +0.64 V vs. SHE

… but no such thing exists. (see Figure E.1 on the inside back cover of B&F)


Hg2Cl2 + 2e– 2Cl– + 2Hg0

Hg2Cl2 + 2e– 2Cl– + 2Hg0

Hg2SO4 + 2e– SO4

2– + 2Hg0

192

E0SHE = 0.0000 V vs. SHE

http://en.wikipedia.org/wiki/Standard_hydrogen_electrode

The scheme of the standard (or normal)

hydrogen electrode:

1) platinized platinum electrode (large area)

2) hydrogen blow (bubbling)

3) solution of aqueous acid with proton activity

equal to one (dimensionless)

4) means to prevent O2 interference (sealant)

5) reservoir through which the second half-

element of the electrochemical cell is

attached. This creates an ionically

conductive path to the working electrode of

interest (salt bridge).

… great. But what is an SHE (standard hydrogen electrode)?

2H+ + 2e– H2

* one rendition of an SHE

193… another common RE is the aq. Ag/AgCl electrode (KCl sat’d)!

AgCl (white)

Ag (gray), from

photodecomposition

of AgCl

https://www.youtube.com/watch?v=8e0-AbwBDYM

Moody, Oke, & Thomas, Analyst, 1969, 94, 803

Pt (s) | Hg (l) | Hg2Cl2 (s) | Cl– (sat’d, aq) | AgCl (s) | Ag (s)

E0 = +0.197 V vs. SHE

But for those of you doing

photoelectrochemistry,

beware!

AgCl + e– Cl– + Ag0

https://www.youtube.com/watch?v=8e0-AbwBDYM

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194… and three final “specialty” reference electrodes include…

E0 = +0.098 V vs. SHE

For aqueous alkaline electrolyte conditions

Mercury/Mercury Oxide (Hg/HgO, 20 wt% KOH)

E0 = +0.3 V vs. SCE (aq), which is

effectively +0.54 V vs. SHE

For non-aqueous (CH3CN) electrolyte solutions

Ag/AgNO3 (0.01 M) in CH3CN

* Used when you already have a cell with two halves of a

redox couple that will not change during your experiment

* Calibrate with Fc (ferrocene)

When a reference electrode cannot be used or is not wanted

“Quasi-reference” electrode as a Pt wire and any redox couple

B&F 2.1.7

195How would one test the accuracy of a reference electrode?

● Measure the potential of an internal standard versus this

reference electrode

(e.g. ferrocene in non-aqueous electrolyte)

● Measure the potential of this reference electrode versus several

other reference electrodes with a voltmeter

(e.g. Ag (s) | AgCl (s) | Cl– (sat’d) | AgCl (s) | Ag (s))



reference electrode





What if no matter what you do, the potential is unstable or the equipment

overloads (i.e. gives you an error; often a red light turns on)?

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reference electrode





What if no matter what you do, the potential is unstable or the equipment

overloads (i.e. gives you an error; often a red light turns on)?

● Throw the electrode away? NO WAY!

● Fix it!

● Check for (insulating) bubbles… change the frit… remake the

redox couple… something else?

… check out tidbits on troubleshooting EChem systems (B&F 15.9)

198

Specifically, we would really like to have a reference electrode

that has the following attributes:

1. It has a well-defined and invariant potential. That is, no

matter how much current we draw from this electrode, its

potential must not vary.

2. It has zero impedance. That is, it imposes no resistive

load on our cell.

3. It does not “contaminate” our solution. That is, it is not a

source of undesired ions in our electrochemical cell.

… now, as mentioned earlier, unfortunately, real reference

electrodes can do none of these things perfectly…

WE = working

electrode

RE = reference

electrode

CE = counter (or

auxiliary) electrode

“Out of sight, out

of mind” is a bad

motto!

199

http://www.porous-35.com/electrochemistry-semiconductors-10.html

… so we resort to a 3-electrode potentiostat…

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200… invented in 1937 by Hickling…

Hickling, Trans. Faraday Soc., 1942, 38, 27

lecture #3 of 17ardo/echem/uci-chem248-2019s_lectur… · 4/15/2019 2 119 7+ 6+ 4+ 3+ 2+ 0...

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