5.Electrochemistry

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Depends On chemistry
5. Electrochemistry Ion Selective
Electrodes Anodic Stripping Voltammetry
Instrument
Sample Prep
Instrument Out put
Sample
Signal (Data)
Abraham Lincoln was born Feb. 12, 1808.
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? phase
ß phase
-
-
-
Mobility, u, is greater, in this example, for
negative than positive species



-
-
-




-
-

Liquid Junction
Concentration gradient
Net charge develops due to Difference in
mobilities
? phase
ß phase
-
-
-



-
-




-
-
-

Nov 18 1863 at Gettysburg Four score and seven
years ago our fathers brought forth
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Henderson Equation for a liquid junction
diffusion potential
concentration
mobility
Z, charge on ion
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
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Consider a membrane in which only a single ion
can move within the membrane
ß phase
-
-
? phase

Aza

-
-


-
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
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This term represents the work it takes to move an
ion of charge a in a potential field at the
interface
This term represents The diffusion gradient
between The two phases
Fix this activity
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.
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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
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ISE Equation
Difference in the Interfacial work required
final resting place for those who here gave their
lives that that nation might live. It is
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Selectivity coefficient, converts the impact Of
the activity of the competitor to the
corresponding activity Of the analyte
Difference in work at the interface Between the
two competing ions
altogether fitting and proper that we should do
this.
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If you have a competing ion, you can convert its
activity to that of the analyte by The
selectivity coefficient and sum it up
Nikolskii- Eisenman Equation

• 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 dedicatewe
can not consecratewe can not
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Activities
concentration
Activity coefficient
Charge on the ion
Ionic strength
hallowthis ground. The brave men, living and
dead, who struggled here, have
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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
consecrated it, far above our poor power to add
or detract. The world will little note, nor
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Temperature Effects
Suppose you calibrate at 50oF but make A
measurement of -140mV at 80oF
You would conclude the concentration Is
log(conc.) -5.26.3x10-6
The real log conc. From the 80oF calibration
curve would be
Log(conc) -4.8 1.6x10-5
That is an error Of a factor of 2!
50oF
80oF
long remember what we say here, but it can never
forget what they did here. It is for us
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Selectivity impact is concentration dependent
Pb2
Manufacturers data
Lets calculate the selectivity coefficient for
H vs Pb2 to check the manufacturers data
H
the living, rather, to be dedicated here to the
unfinished work which they who fought here
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Electrode is 1/0.0665 Or 15 more responsive To
lead than to protons
have thus far so nobly advanced. It is rather for
us to be here dedicated to the great task
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Equation does not give model lines that Match the
manufacturers reported data
remaining before usthat from these honored dead
we take increased devotion to that
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Calculate a value for z from the slopes
cause for which they gave the last full measure
of devotionthat we here highly resolve
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Using the manufacturers data and the equation
pH 1
pH 2
pH 3
pH 4
Looks like a good match!
that these dead shall not have died in vainthat
this nation, under God, shall have a
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2009 experimental Results
new birth of freedomand that government of the
people, by the people, for the people,
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2009 ISE results
Blank -235 -218 -216
-223/-10.44 LODsignal -2233(10.44)-192
LODconc3(10.44)/251.25ppm Total concentration
used 1.13 ppm!!
Experimental conditions
shall not perish from the earth.
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Modeled behavior using PbTotal
6x10-5 ConstantPb -115 KPbH 3545 ZPb
2 ZH1 Allowed Pb2 to change With pH
Data, 2009
Both terms contribute! So you have Pb2 and H
decreasing With pH making a complicated plot
0.06
10-6
10-4
Mar 4, 1865, 2nd Inaugural address, Fellow
CountrymenOn 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
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Types of Ion Selective Electrodes

1. Crystalline
2. Polymeric Membrane
3. Gas sensing
4. 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 warseeking to
dissolve the Union and divide
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V
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
Pb2
S2-
Ag
S2-
S2-
H
S2-
SO42-
S2-
Pb2
S2-
Fixed internal Ag activity
Pb2
S2-
Pb2
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.
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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- Satd 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
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Other common ion selective electrodes? pH
electrode
Exchange reaction at the surface
http//grandinetti.org/Research/Applications/Glass
Studies/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
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http//www.rsc.org/ej/CP/2002/b201721k/
Physical Chemistry Chemical PhysicsDOI 10.1039/b201721k                  
Alkali ion migration mechanisms in silicate glasses probed by molecular dynamics simulations                  
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
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Link to PVC structure
www.3dchem.com/molecules.asp?ID327
Types of Ion Selective Electrodes

1. Crystalline
2. Polymeric Membrane
3. Gas sensing
4. Field Effect transitors

May have greater selectivity
May have faster response (rise) time
Pb2
Pb2
PVC
Pb2
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
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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
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• Clips from 3 recent articles follow
• Authors are promoting
• 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
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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
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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,
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Selectivity
Response Time
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
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Selectivity
Linear Range
Response Time
sensitivity
Detection Limit
They havent reported the sblank so we
cannot Confirm their LODconc.
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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
Instrument
Sample Prep
Instrument Out put
Sample
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'.
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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.
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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 Eo at 25oC, V vs NHE Solubility Metal in Hg Wt()
-0.7626 5.6
-0.444
-0.4025 5
-0.257 0.0021
-0.1375 1.3
-0.1205 1.2
0
0.268
0.340 0.008
0.7991
NHE Normal Hydrogen Electrode
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

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Hold voltage negative of 0.268 V vs NHE

• Reduce Mercury to form a film (small volume
  container) of thickness, l

V
e
Hg deposition time
Reference electrode (comparison point)
Counter Electrode (sacrificial reactions)
Working electrode where current is measured
e
e
Hg
Hg
l
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Hold voltage between negative of 0.268 V and
positive of -0.1205 V vs NHE

1. Remove Hg2Cl2 solution and replace with analyte
   solution holding potential such that no current
   flows

V
e
Reference electrode (comparison point)
Counter Electrode (sacrificial reactions)
Working electrode where current is measured
quiet time allows Solution to revert to Bulk
solution like conditions
Hg
Pb2 Sn2
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Hold voltage between negative For example, -0.8
V vs NHE

1. Step potential negative to reduce metals into the
   liquid Hg

V
e
Analyte deposition time, td
Reference electrode (comparison point)
Counter Electrode (sacrificial reactions)
Working electrode where current is measured
e
e
Hg
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Hold voltage between negative For example, -0.8
V vs NHE

1. Step potential negative to reduce metals into the
   liquid Hg

V
e
Analyte deposition time, td
Reference electrode (comparison point)
Counter Electrode (sacrificial reactions)
Working electrode where current is measured
Initially we remove all the Pb2 and Sn2 near
the Hg film . If we dont stir then we set up a
concentration gradient and deposition will be
determined by diffusion to the surface
e
Deposition current And time of deposition
control the Amount deposited
e
Hg
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Hold voltage between negative For example, -0.8
V vs NHE

1. Step potential negative to reduce metals into the
   liquid Hg

V
e
Analyte deposition time, td
Reference electrode (comparison point)
Counter Electrode (sacrificial reactions)
Working electrode where current is measured
e
Hanging mercury drop electrode spherically
shaped
Notice that we have transferred Moles from a
large volume to A small volume
pre-concentration!
e
Hg
Concentration in the drop depends On deposition
time
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Scan voltage between -0.8 and 0 V vs NHE
4. Scan potential negative to remove metals from
the liquid Hg (oxidize back into aqueous phase)
The rate at which you scan, v, affects the shape
and peak current of the oxidation
V
e
Reference electrode (comparison point)
Counter Electrode (sacrificial reactions)
Working electrode where current is measured
e
Mercury Film Electrode
Area of electrode
Hg film thickness
e
e transferred
Scan rate
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Deposition time, td
Change to analyte containing solution
Change potential with time to oxidize
Set potential to reduce the metals
Scan rate ? dV/dt
Potential range to deposit Mercury thin liquid
film
AND to NOT reduce the analytes
Do not keep Scanning to these Potentials, why?
Starting Potential soln contains only salts and
Hg2Cl2
Set potential to NOT oxidize the deposited Hg
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2001 Student Data
Raw Data
FFT data
120 microL Pb Std. addition
800 microL Pb Std addition
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2001 Student Data
Standard series (FFT filtered)
Allows you to get a very Nice calibration curve!
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2001 Student Data
Cd2
Pb2
Difference in peak potential allows Both metals
to be measured simultaneously! Good selectivity!
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To make this method more acceptable to commercial
testing need to Get rid of the mercury.
JChem Ed 2007
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8 mmol Cu2/L
8 mmol Pb2/L
blank
8 mmol/L Zn2
8 mmol Cd2/L
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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!!
Measures Pb2 and Cu2 Selective against Cd2 And
Zn2
Increased sensitivity