Depends
On chemistry
5. Electrochemistry
Ion Selective Electrodes
Anodic Stripping Voltammetry
15000
10000
Amplitude
5000
0
0
0.2
0.4
0.6
0.8
-5000
-10000
-15000
Time (s)
Sample
Sample
Prep
Instrument
Instrument
Out put
Abraham Lincoln was born Feb. 12, 1808.
Signal (Data)
1
1.2
 phase
-
β phase
-
-
+
+
-
+
-
-
+
Mobility, u, is greater, in this
example, for negative than
positive species
+
+
+
-
-
+
Liquid Junction
Concentration gradient
β phase
 phase
-
Net charge develops due to
Difference in mobilities
-
+
+
+
-
+
+
+
+
-
-
Q
V 
C
+
Nov 18 1863 at Gettysburg: “Four score and seven years ago our fathers brought forth
Henderson Equation for a liquid junction diffusion potential
concentration
z i ui
i z Ci ,  Ci ,  RT 2.303
i
Ej 
log
 zi ui Ci,  Ci,  F
i
 z uC 
 z Ca 
i
i
i,
i
i i,
i
i
Z, charge on ion
mobility
J, Junction potential
Also develop potential across a membrane
Potential is due to both a diffusion gradient and
to interfacial equilbrium.
on this continent a new nation, conceived in Liberty, and dedicated to the proposition
Consider a membrane in which only a single ion can move within the membrane
β phase
Membrane
-
-
+
-
+
 phase
Bzb+
Aza+
+
Bzb+
+
-
+
-
-
Aza+
-
+
-
Interfacial regions where work is required to bring ions to
The surface
That all men are created equal. Now we are engaged in a great civil war, testing whether
  A RT 2.303
aA
Emembrane 

log 
zAF
zAF
aA
This term represents the work it takes to move
an ion of charge a in a potential field at the
This term represents
interface
The diffusion gradient between
The two phases
  A RT 2.303
1 RT 2.303
Emembrane 

log  
log aA
zAF
zAF
aA
zAF
V
β phase
reference
Fix this activity
Emembrane  cons tan t A 
 phase
RT 2.303
log aA
zAF
Represents the work at the
interface
And the composition of the
inner compartment
that nation, or any nation, so conceived and so dedicated, can long endure. We are met.
RT 2.303
Emembrane  cons tan t A 
log aA
zAF
Membrane potential equation
Often have to worry about the contribution to
the potential from competitive ions
on a great battle-field of that war. We have come to dedicate a portion of that field, as a
ISE Equation
E  const A 
E  const B 
RT 2.303
log( a A )
zAF
RT 2.303
log( aB )
zB F
1
RT 2.303
E  const B 
log( aB ) z B
F
1
RT 2.303
E  const A 
log( a A ) z A
F
1
1
RT 2.303
RT 2.303
zA
E  const A 
log( a A )
 const B 
log( aB ) z B
F
F
1
1
RT 2.303
RT 2.303
log( a A ) z A 
log( aB ) z B  const B  const A
F
F
1
RT 2.303
(a A ) z A
log
 const B  const A
1
zB
F
(aB )
1
(a A )
1
(aB )
zA
zB
 10
Difference in the
Interfacial work
required
F


 RT 2.303constB  constA 


zA
1
1


zA 
z
(
a
)

(
a
)
 A

 B B 10



F


 RT 2.303constB  constA 





zA
final resting place for those who here gave their lives that that nation might live. It is
zA
1
1


zA 
z
(
a
)

(
a
)
 A

 B B 10



Selectivity coefficient, converts the impact
Of the activity of the competitor
to the corresponding activity
Of the analyte

k ab  10

a A  (aB )
zA
zB
a A  (aB )
F


 RT 2.303constB  constA 





zA
zB

10

F


 RT 2.303constB  constA 


F


 RT 2.303constB  constA 








zA
zA
zA
k AB
altogether fitting and proper that we should do this.
Difference in work at the interface
Between the two competing ions
If you have a competing ion, you can convert it’s activity to that of the analyte by
The selectivity coefficient and sum it up
Nikolskii- Eisenman Equation
E  const A 
zA
RT 2.303
log( a A  k AB aB zB )
zAF
EmV  const A 
zA
59.16
log( a A  k AB aB zB )
zA
Key Points
• Activities
• Temperature
• Difference in interfacial work controls
the selectivity coefficient
4. Also means response is SLOW
But, in a larger sense, we can not dedicate—we can not consecrate—we can not
Activities
ai  f i Ci
concentration
Activity coefficient
 051
. z2 I
log f z 
1 I
Ionic strength
I
1
2
Charge on the ion
2
C
z
 ii
hallow—this ground. The brave men, living and dead, who struggled here, have
If activity was directly proportional to conc. the ISE response would be linear with
Concentration.
If there is no ionic strength buffer, increasing the concentration changes the ionic
Strength, which changes the activity coefficient, which drives the activity away from
The concentration, and, therefore, the ISE response is non-linear with concentration
2
mV proportional
proportional to
to log
log aaPb(NO3)2
Pb(NO3)2
1
0
-7
-6
-5
-4
-3
-2
-1
0
1
2
-1
-2
-3
-4
-5
-6
-7
Log [Pb(NO3)2]
consecrated it, far above our poor power to add or detract. The world will little note, nor
Temperature Effects
-90.00
mV
-7
-6
-5
-4
-3
-2
-1
0
Suppose you calibrate at 50oF but make
A measurement of -140mV at 80oF
-100.00
You would conclude the concentration
Is log(conc.) = -5.2=6.3x10-6
-110.00
The real log conc. From the
80oF calibration curve would
be
-120.00
-130.00
Log(conc) = -4.8 = 1.6x10-5
That is an error
Of a factor of 2!
-140.00
50oF
-150.00
80oF
log [Pb2+]
long remember what we say here, but it can never forget what they did here. It is for us
Selectivity – impact is concentration dependent
0
E  const A 
-50
zA
59.16
log( a A  k AB aB zB )
zA
Pb2+
Manufacturer’s data
-100
mV
-150
-200
Let’s calculate the
selectivity coefficient for
H+ vs Pb2+ to check the
manufacturer’s
data
H+
-250
-300
-350
-10
-8
-6
-4
-2
0
log [conc]
the living, rather, to be dedicated here to the unfinished work which they who fought here
pH or pM
0

k ab  10

10
F


 RT 2.303constB  constA 



k ab  10

8



6
4
2
0
zA
-50
y = -31.143x - 74.333
R2 = 0.9994
-100
2
 1

 59.16  109.33 74.333



  0.0665

-150
mV
12
-200
Electrode is 1/0.0665
Or 15 more responsive
To lead than to protons
-250
y = -46.5x - 109.33
R2 = 0.9981
-300
-350
zA
59.16
E  const A 
log( a A  k AB aB zB )
zA
2
E  74.333  29.58 log( aPb  0.0665aH1 )
have thus far so nobly advanced. It is rather for us to be here dedicated to the great task
2
E  74.333  29.58 log( aPb  0.0665aH1 )
0
8
7
6
5
4
3
2
1
Equation does not give model lines that
Match the manufacturer’s reported data
0
-50
pH 1
-150
mV
-100
-200
pH 2
Data from manufacturer's manual
-250
pH 3
pH >= 4
-300
pM
remaining before us—that from these honored dead we take increased devotion to that
zA
RT 2.303
E  const A 
log( a A  k AB aB zB )
zAF
Calculate a value for z from the slopes
RT 2.303 0.05916V

 slope A
zAF
zA
pH or pM
0
12
10
8
6
4
2
0
-50
59.16mV
 31.143mV ; z Pb2  '  1.89
z Pb2  '
y = -31.143x - 74.333
R2 = 0.9994
-100
mV
-150
-200
59.16mV
 46.5mV ; z H  '  1.29
zH  '
-250
y = -46.5x - 109.33
R2 = 0.9981
-300
-350
cause for which they gave the last full measure of devotion—that we here highly resolve
zA
59.16
E  const A 
log( a A  k AB aB zB )
zA
0
7
6
5
4
3
2
1
Using the manufacturer’s data and the equation
1.899
59.16
E  74.333 
log( aPb2  0.0665aH  1.29 )
1.899
pH 1
0
-50
-100
-150
pH 2
mV
8
-200
pH 3
-250
pH 4
-300
pM
Looks like a good match!
that these dead shall not have died in vain—that this nation, under God, shall have a
2009 experimental Results
Experimental Alpha and Theoretical Alpha Plots
12
1.1
10
Alpha Plots
1
0.9
Alpha Fraction
Alpha Fraction
0.8
8
0.7
0.6
0.5
0.4
0.3
6
0.2
0.1
0
0
4
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
pH
2
0
0
2
4
6
8
10
12
14
16
pH
new birth of freedom—and that government of the people, by the people, for the people,
2009 ISE results
Blank: -235
-218
-216
-223+/-10.44
-180
-5.1
-5
-4.9
-4.8
-4.7
-4.6
-4.5
-4.4
-4.3
-4.2
LODsignal = -223+3(10.44)=-192
LODconc=3(10.44)/25=1.25ppm
-185
y = 21.255x - 100.72
R2 = 0.6245
-190
-195
-200
-205
-210
Experimental
conditions
shall not perish from the earth.”
Total concentration used:
1.13 ppm!!
0.00E+00
0
2
4
6
8
10
-1.00E+02
-2.00E+02
mV
-3.00E+02
-4.00E+02
-5.00E+02
Modeled behavior using
-5
PbTotal
12 = 6x1014
16
ConstantPb = -115
KPbH = 3545
ZPb = 2
ZH=1
Allowed Pb2+ to change
With pH
Data, 2009
Both terms contribute! So you have Pb2+ and H+ decreasing
With pH making a complicated plot
-6.00E+02
-7.00E+02
z Pb
59.16
E  cons tan t Pb 
log( aPb2  k PbH aH  zH )
z Pb
-8.00E+02
pH
~0.06
~10-6
~10-4
Mar 4, 1865, 2nd Inaugural address, “Fellow Countrymen:……On the occasion corresponding to this four years ago
all thoughts were anxiously directed to an impending civil war. All dreaded it, all sought to avert it. While the
Types of Ion Selective Electrodes
1.
2.
3.
4.
Crystalline
Polymeric Membrane
Gas sensing
Field Effect transitors
inaugural address was being delivered from this place, devoted altogether to saving the Union without war,
insurgent agents were in the city seeking to destroy it without war—seeking to dissolve the Union and divide
Silver moves in membrane down a
concentration gradient
Charge develops across the membrane
leading to a voltage difference
The interfacial concentration of silver in
the bulk solution is controlled by
solubility
V
Pb2+
S2-
S2-
Ag+
S2-
Fixed internal Ag+ activity
H+
S2SO42-
Pb2+
S2S2- Pb2+
S2Pb2+
Ag+
Pb2+
S2-
Ag+
Cl-
effects by negotiation. Both parties deprecated war, but one of them would make war rather than let the nation
survive, and the other would accept war rather than let it perish. And the war came.
Similar crystalline commercial electrodes:
Analyte ion
Conc. Range, pM
Interferences
Br-
0 to 4.3
CN-, I-, S2-
Cd2+
1 to 7
Fe2+, Pb2+, Hg2+, Ag+,
Cu2+
Cl-
0 to 3.3
CN-, I-, Br-, S2-, OH-, NH3
Cu2+
1 to 8
Hg2+, Ag+, Cd2+
CN-
2 to 6
S2-, I-
F-
Sat’d to 6
OH-
I-
0 to 7.3
CN-
Pb2+
1 to 6
Hg2+, Ag+, Cu2+
Ag+/S2-
0 to 7
Hg2+
SCN-
0 to 5.3
I-, Br-,CN-, S2-
One-eighth of the whole population were colored slaves, not distributed generally over the Union, but localized in
the southern part of it. These slaves constituted a peculiar and powerful interest. All knew that this interest was
Other common ion selective electrodes?
pH electrode
Exchange reaction at the surface:



Na  H membrane
,outeredge  H  Namembrane,outeredge
http://grandinetti.org/Research/Applications/GlassStudies/assets/GlassStructure.gif
somehow the cause of the war. To strengthen, perpetuate, and extend this interest was the object for which the
insurgents would rend the Union even by war, while the Government claimed no right to do more than to restrict
Physical Chemistry Chemical Physics
DOI: 10.1039/b201721k
Alkali ion migration mechanisms in silicate glasses
probed by molecular dynamics simulations
http://www.rsc.org/ej/CP/2002/b201721k/
A. N. Cormack, J. Du and T. R. Zeitler
School of Ceramic Engineering and Materials Science, New York State College of
Ceramics, Alfred University, 2 Pine St., Alfred, NY 14802, USA
Received 15th February 2002, Accepted 9th May 2002
First published on the web 13th June 2002
Image of Na “hopping” through glass
To different coordinate sites with oxygen
the territorial enlargement of it. Neither party expected for the war the magnitude or the duration which it has
already attained. Neither anticipated that the cause of the conflict might cease with or even before the conflict
Link to PVC structure
www.3dchem.com/molecules.asp?ID=327
Types of Ion Selective Electrodes
1.
2.
3.
4.
Crystalline
Polymeric Membrane
Gas sensing
Field Effect transitors
V
May have greater selectivity
May have faster response (rise) time
Pb2+
β phase
reference
Pb2+
O
O
O
O
O
O
Pb2+
 phase
PVC
Create a “plastic” membrane
with an ionophore
Whose job is to ferry the ion across the membrane
itself should cease. Each looked for an easier triumph, and a result less fundamental and astounding. Both read
the same Bible and pray to the same God, and each invokes His aid against the other. It may seem strange that
From the Critical Reviews
any men should dare to ask a just God's assistance in wringing their bread from the sweat of other men's faces,
but let us judge not, that we be not judged. The prayers of both could not be answered. That of neither has been
Clips from 3 recent articles follow
Authors are promoting
a.
b.
c.
d.
Sensitivity (slope)
Selectivity (selectivity coefficients)
Response Time
Linear Range
answered fully. The Almighty has His own purposes. 'Woe unto the world because of offenses; for it must needs
be that offenses come, but woe to that man by whom the offense cometh.' If we shall suppose that American
Response to Pb2+
Response to Pb2+
slavery is one of those offenses which, in the providence of God, must needs come, but which, having continued
through His appointed time, He now wills to remove, and that He gives to both North and South this terrible war
Next works on right ionophore to plastic composition
Linear Range
sensitivity
Selectivity
As the woe due to those by whom the offense came, shall we discern therein any departure from those divine
attributes which the believers in a living God always ascribe to Him? Fondly do we hope, fervently do we pray,
Response Time
Selectivity
that this mighty scourge of war may speedily pass away. Yet, if God wills that it continue until all the wealth piled
by the bondsman's two hundred and fifty years of unrequited toil shall be sunk, and until every drop of blood
Selectivity
Linear Range
sensitivity
Detection Limit
LODsignal  xblank  bconcLOD 
LODsignal  xblank  3sblank
xblank  bconcLOD   xblank  3sblank
bconcLOD   3sblank
concLOD   3sblank
b
They haven’t reported the
sblank so we cannot
Confirm their LODconc.
Response Time
Key concepts
Selectivity – depends upon mobility and interfacial equilibria
therefore often slow response
may be complicated by solution chemistry
Potential depends upon activity therefore dependent upon ionic strength, I
Potential depends temperature!
Signal depends on physics of membrane transport
Which ALSO depends upon chemistry of lead
15000
10000
Amplitude
5000
0
0
0.2
0.4
0.6
0.8
1
-5000
-10000
-15000
Time (s)
Sample
Sample
Prep
Instrument
Instrument
Out put
Signal (Data)
drawn with the lash shall be paid by another drawn with the sword, as was said three thousand years ago, so still
it must be said 'the judgments of the Lord are true and righteous altogether'.
1.2
5. Electrochemistry
Ion Selective Electrodes
Anodic Stripping Voltammetry
Abraham Lincoln was born Feb. 12, 1808.
With malice toward none; with charity for all; with firmness in the right, as God gives us to see the right, let us
strive on to finish the work we are in; to bind up the nation's wounds; to care for him who shall have borne the
battle, and for his widow, and his orphan – to do all which may achieve and cherish a just and lasting peace,
among ourselves, and with all nations.”
Electrons “roll” away from negative
If the applied potential is more positive than -0.76 then Zn metal
Will oxidize (electrons will flow from Zn metal into an acceptor in soln.)
Reaction
Zn 2  2e  Zn
Eo at 25oC, V vs NHE
Solubility Metal in Hg Wt(%)
-0.7626
5.6
Fe 2  2e  Fe
-0.444
Cd 2  2e  Cd
-0.4025
Ni 2  2e  Ni
-0.257
Sn 2  2e  Sn
Pb2  2e  Pb Hg 
-0.1375
1.3
-0.1205
1.2
2 H   2e  H 2  g 
0
Hg 2Cl2( s )  2e  2 Hg (  )  2Cl2
0.268
Cu 2  2e  Cu
0.340
Ag   e  Ag
NHE =
Normal
Hydrogen
Electrode
5
0.0021
0.008
0.7991
If the applied potential is more negative than 0.799 then silver ion
Will be reduced (electrons will flow from electrode to aqueous ion)
1. Mercury is stable as a liquid at potentials less than 0.268 V vs NHE
2. Lead, tin, nickel, cadmium, iron, and zinc metals are soluble in liquid mercury
ℓ
Hg
e
Hg+
Hg 2Cl2( s )  2e  2 Hg (  )  2Cl2
t (s)
-1
-0.8
Hg deposition time
-0.6
-0.4
V vs NHE
Reference electrode (comparison point)
e
Counter Electrode (sacrificial reactions)
Working electrode where current is measured
1. Reduce Mercury to
form a film (small
volume container)
of thickness, ℓ
Hold voltage negative of
0.268 V vs NHE
V
e
-0.2
0
0
0.2
0.4
0.6
50
100
150
200
250
300
Hg
Pb2+ Sn2+
t (s)
-1
-0.8
-0.6
-0.4
V vs NHE
Reference electrode (comparison point)
Counter Electrode (sacrificial reactions)
Working electrode where current is measured
2. Remove Hg2Cl2
solution and
replace with
analyte solution;
holding potential
such that no
current flows
Hold voltage between negative of
0.268 V and positive of -0.1205 V vs NHE
V
e
-0.2
0
50
100
150
200
250
300
0
0.2
0.4
0.6
“quiet time” allows
Solution to revert to
Bulk solution like conditions
Hg
e
Sn 2  2e  Sn
Pb2  2e  Pb Hg 
t (s)
-1
-0.8
-0.6
-0.4
V vs NHE
Reference electrode (comparison point)
e
Counter Electrode (sacrificial reactions)
Working electrode where current is measured
3. Step potential
negative to reduce
metals into the
liquid Hg
Hold voltage between negative
For example, -0.8 V vs NHE
V
e
Analyte deposition time, td
-0.2
0
0
0.2
0.4
0.6
50
100
150
200
250
300
Hg
e
Sn 2  2e  Sn
Pb2  2e  Pb Hg 
t (s)
-1
-0.8
-0.6
-0.4
V vs NHE
Reference electrode (comparison point)
If we don’t stir then we
set up a concentration
gradient and deposition
will be determined by
diffusion to the surface
e
Counter Electrode (sacrificial reactions)
Initially we remove all
the Pb2+ and Sn2+ near
the Hg film ….
Working electrode where current is measured
3. Step potential
negative to reduce
metals into the
liquid Hg
Hold voltage between negative
For example, -0.8 V vs NHE
V
e
Analyte deposition time, td
-0.2
0
50
100
150
200
250
300
0
0.2
0.4
0.6
i t   
d
d
nelectrons  
Coulombs   N moles 


 t   moles
mole   moleelectron   s  d
Deposition current
And time of deposition control the
Amount deposited
Hg
e
Sn 2  2e  Sn
Pb2  2e  Pb Hg 
t (s)
-1
-0.8
-0.6
-0.4
V vs NHE
Reference electrode (comparison point)
e
Counter Electrode (sacrificial reactions)
Notice that we have
transferred
Moles from a large
volume to
A small volume =
pre-concentration!
Working electrode where current is measured
3. Step potential
negative to reduce
metals into the
liquid Hg
Hold voltage between negative
For example, -0.8 V vs NHE
V
e
Analyte deposition time, td
-0.2
0
50
100
150
200
250
300
0
0.2
0.4
0.6
Hanging mercury drop electrode
spherically shaped
C
*
M
id t d

 4 3
nF    ro
 3
Concentration in the drop depends
On deposition time
Scan voltage between -0.8 and 0 V vs NHE
The rate at which you
scan, v, affects the shape
and peak current of the
oxidation
V
e
e
PbHg   Pb2  2e
Sn  Sn 2  2e
t (s)
-1
-0.8
-0.6
-0.4
V vs NHE
Reference electrode (comparison point)
e
Counter Electrode (sacrificial reactions)
Working electrode where current is measured
4. Scan potential
negative to remove
metals from the
liquid Hg (oxidize
back into aqueous
phase)
-0.2
0
50
100
150
200
250
300
0
0.2
0.4
0.6
Mercury Film Electrode
Area of electrode
Hg film thickness
2
2
n F v AC
ip 
2.7 RT
*
M
# e transferred
Scan rate
t (s)
-1
Deposition time, td
Change to analyte containing solution;
-0.8
Change potential with time to oxidize
Set potential to reduce the metals
Scan rate = ν= dV/dt
-0.6
Cd 2  2e  Cd
V vs NHE
-0.4
Sn 2  2e  Sn
Pb2  2e  Pb Hg 
-0.2
0
50
100
150
200
250
300
0
0.2
0.4
0.6
Potential range to deposit Mercury thin liquid film
AND to NOT reduce the analytes
Hg 2Cl2( s )  2e  2 Hg (  )  2Cl2
not
keep
StartingSet
Potential;
soln
only
salts
and Hg2Cl
potential
to contains
NOTDo
oxidize
the
deposited
Hg2
Scanning to these
Potentials, why?
Raw Data
2001 Student Data
120uL Lead
FFT data
0.25
0.25
0.2
0.2
0.15
0.15
0.1
0.1
Current
0.05
0
0.05
120uL Lead
0
-0.05
-0.8
-0.7
-0.6
-0.5
-0.4
-0.3
-0.2
-0.1
-0.1
0
Series1
Series2
-0.05
-0.15
-0.1
-0.2
-0.15
-0.25
-1
-0.9
-0.8
-0.7
-0.6
-0.5
-0.4
-0.3
-0.2
-0.1
0
-0.2
Time
-0.25
120 microL Pb Std. addition
800uL Lead
0.4
0.4
0.2
0.2
0
0
-0.8
-0.7
-0.6
-0.5
-0.4
-0.3
-0.2
-0.1
0
Current
-0.2
-0.2
800uL Lead
Series1
Series2
-0.4
-0.4
-0.6
-0.6
-0.8
-0.8
-1
-1
-0.9
-0.8
-0.7
-0.6
-0.5
-0.4
-0.3
-0.2
-0.1
0
Time
n 2 F 2 v AC M*
ip 
2.7 RT
-1
800 microL Pb Std addition
2001 Student Data
0.2
2
n F v AC
ip 
2.7 RT
0
-0.7
-0.6
-0.5
-0.4
-0.3
-0.2
-0.1
0
*
M
-0.2
Series1
Series2
Series3
Series4
Series5
Series6
-0.4
-0.6
1
-0.8
y = 0.3445x - 0.0879
R20.988 =
0.8
-1
Standard series (FFT filtered)
Allows you to get a very
Nice calibration curve!
0.6
FT cleaned up mA
-0.8
2
Series1
Linear (Series1)
0.4
0.2
0
-0.5
0
0.5
1
1.5
-0.2
ppm Pb
2
2.5
3
n 2 F 2 v AC M*
ip 
2.7 RT
2001 Student Data
Cd solution
0.4
0.2
0
absorbance
-0.2
-0.4
Cd solution
-0.6
-0.8
Cd2+
-1
Pb2+
-1
-0.9
-0.8
-1.2
-0.7
-0.6
-0.5
time
-0.4
-0.3
-0.2
-0.1
0
Difference in peak potential allows
Both metals to be measured simultaneously
Good selectivity!
To make this method more acceptable to commercial testing need to
Get rid of the mercury.
JChem Ed 2007
8 mmol Cu2+/L
8 mmol Pb2+/L
blank
8 mmol Cd2+/L
8 mmol/L Zn2+
Note the selectivity in potential and the discrimination between various
metals
8 mmol/L of
Pb2+, Cu2+, Cd2+ and Zn2+
Au particle modified
Au electrode
blank
Ordinary Au electrode
Note scale
Change!!
Increased sensitivity
Measures Pb2+ and Cu2+
Selective against Cd2+
And Zn2+;