Download Lecture 7 Non- potentiometric methods of analysis

Lecture 7
Non- potentiometric methods of analysis
Dr. Rasha Hanafi
Dr. Rasha Hanafi, GUC
PHCM662, Lecture 7, SS2016
1
Learning outcomes
By the end of this session, the student should be able to:
1.Identify fundamentals of electrolysis.
2.Determine voltage changes when current flows.
3.Estimate overvoltage due to different types of polarization.
4. Describe controlled potential electrolysis with three electrode
system.
5. Use electrogravimetric measurement for quantitative analysis.
6. Use coulometry and coulometric titrations for quantitative
analysis
Dr. Rasha Hanafi, GUC
PHCM662, Lecture 7, SS2016
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I. Potentiometric versus non-potentiometric
methods of analysis
Galvanic cell
Dr. Rasha Hanafi, GUC
Electrolytic cell
PHCM662, Lecture 7, SS2016
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I. Potentiometric vs. non-potentiometric methods of analysis
Potentiometric methods
1. No external work (potential) is applied.
2. Spontaneous chemical reaction.
3. The two half-cells are set up in different
containers, being connected through the
salt bridge
4. No current passes due to the presence of
a high resistance in the potentiometer,
hence no change in conc. of analytes
happen, so potentiometry is a nondestructive method of analysis.
5. By convention, left position represents
always the anode. the anode is negative
and cathode is the positive electrode
6. The thermodynamic potential Ecell is
calculated from Nernst equation to be
used in quantitative calculations.
Dr. Rasha Hanafi, GUC
Non- potentiometric methods
1.
2.
3.
4.
5.
6.
7.
External work is applied: an external battery is
needed. Current is measured by an ammeter.
The chemical reaction is not spontaneous and
is forced to occur.
Both the electrodes are placed in a same
container in the solution of molten electrolyte
Consumption of analytes happen (analysis is
destructive).
The moles of e- flowing through the cell are
“It/F” (I:current, t:time and F: Faraday constant
96485 C/mol).
Positions are not important : connections of
the poles of the battery control which will be
the anode. the anode is positive and cathode is
the negative electrode
The potential of the cell can not be simply
calculated from Nernst equation: current flow
affects the potential measured.
PHCM662, Lecture 7, SS2016
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II. Fundamentals of electrolysis
Suppose we dip Cu and Pt electrodes into an aqueous solution of
Cu2+ and pass electric current through to deposit Cu2+ as Cu metal
at the cathode which necessarily liberates O2 at the anode.
cathode
Cu 2  2e   Cu ( s)
Anode
H 2O  12 O2 ( g )  2 H   2e 
Net reaction H 2O  Cu 2  Cu ( s)  12 O2 ( g )  2 H 
The electrode at which the reaction
of interest occurs is called the
working electrode (Cu-electrode),
while the other electrode is called
the counter/ auxiliary electrode.
Dr. Rasha Hanafi, GUC
cathode
PHCM662, Lecture 7, SS2016
anode
CuSO4(aq)
Two-electrodes cell
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II. Fundamentals of electrolysis, cont.
• Note that a negative potential is applied to the cathode where reduction occurs and a
positive potential is applied to the anode where oxidation occurs.
• For the aforementioned cell:
Ecell = Ecathode  Eanode
If the cell contains 0.2 M Cu2+ and 1.0 M H+ and liberates O2 at a pressure of 1.0 bar, the
thermodynamic or equilibrium cell potential will be E= 0.911 V (calculated using Nernst
equation for the cell reaction as written). Thus the reaction has a negative potential which
means that the reaction is not spontaneous.
• If we apply a voltage slightly greater than 0.911 V between the electrodes, we will provide
just enough free energy to force the reaction.
• If higher current is needed (higher reaction rate), an extra voltage (overvoltage) is needed
(Faraday 1st law of electrolysis: The mass of a substance altered at an electrode during
electrolysis is directly proportional to the quantity of electricity transferred at that electrode.
Dr. Rasha Hanafi, GUC
PHCM662, Lecture 7, SS2016
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III. Voltage Changes when current flows
When current passes (at a fixed applied potential, battery of fixed
voltage) through an electrochemical cell, the measured cell
potential departs from that derived from thermodynamic
calculations (using Nernst equation)!!
This is due to phenomena such as
1. Ohmic resistance.
2. Polarization effects :
A. Concentration polarization .
B. Charge-transfer polarization.
Dr. Rasha Hanafi, GUC
PHCM662, Lecture 7, SS2016
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III. 1. Ohmic resistance
 It is the potential needed to overcome the resistance of the ions to
move toward the anode and the cathode i.e., to overcome the electric
resistance (R) of the solution in the electrochemical cell when current
is flowing.
Eohmic = I R
The resistance, R, depends on the kinds and
concentrations of ions in solution
Ecell = Ecathode  Eanode  I R, if no current passes I=0 IR=0
 Thus, we have to increase the potential to operate an electrolytic cell
(make it more powerful = more negative).
Dr. Rasha Hanafi, GUC
PHCM662, Lecture 7, SS2016
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III. 2. Sources of polarization
For the half-cell shown in figure, the overall electrode reaction is:
Ox + ne
Bulk
Ex: Cu2+ + 2e
Red
Bulk
For current to pass continuously across the
surface of the electrode, Ox in the bulk of
solution should diffuse to the electrode
region in what is known as mass transfer
step followed by electron transfer step
when Ox reaches the electrode surface.
The electrochemical reaction is completed
when the formed Red diffuses back to the
bulk of solution. If one of these steps is
slow, it will limit the overall rate of
reaction and thus reduces the magnitude
of current.
Dr. Rasha Hanafi, GUC
Cu
Steps of electrochemical reaction
Electrode surface
Mass transfer
Ox
ne
Solution bulk
Ox
Electron
transfer
Red
PHCM662, Lecture 7, SS2016
Red
Mass transfer
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III. 2. A. Concentration polarization
 It arises when the transport of reactive species to the electrode surface is
insufficient (slow mass transfer) to maintain the current needed by the
equation:
Ecell = Ecathode  Eanode  I R
 It is observed when concentrations of the electroactive species are not
the same at the surface of the electrode as in the bulk solution
 In case of Cu2+:
Ecathode  0.34 
0.0591
log [Cu 2 ]surf .
2
where [Cu2+]surf. is the concentration of Cu2+ at the electrode surface
 If the reduction of Cu2+ occurs more rapidly (fast electron transfer) than
the diffusion of Cu2+ ions from bulk to the electrode surface (slow mass
transfer), then, [Cu2+]surf. will decrease: A concentration polarization is
observed.
 Consequently, a concentration overvoltage, c, should be applied in order
to increase the rate of mass transfer from the bulk of solution to the
electrode surface. Ecell = Ecathode  Eanode  I R  c
Dr. Rasha Hanafi, GUC
PHCM662, Lecture 7, SS2016
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III. 2. A. Concentration polarization, cont.
 In order to know how concentration polarization can be prevented or induced as
required, it is important to investigate the mechanisms by which ions are transported
from the bulk of solution to the electrode surface.
 An electroactive species has three ways to reach the surface of an electrode:
1. Diffusion through a concentration gradient. Whenever a concentration
difference develops between two regions of solution, as it does when a species
is reduced at a cathode surface (or oxidized at an anode surface), ions move
from the more concentrated region to the more dilute as a result of diffusion.
2. Migration, the process by which ions move under the effect of an electrostatic
field where they are attracted or repelled by a charged surface.
3. Convection, which is the movement of bulk fluid by mechanical means such as
stirring or agitation.
 Concentration polarization is observed when diffusion, migration and
convection are insufficient to transport the reactant to or from electrode
surface at a rate demanded by the theoretical current.
Dr. Rasha Hanafi, GUC
PHCM662, Lecture 7, SS2016
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III. 2. B. Activation or charge transfer polarization
It arises when the rate of the oxidation or
reduction reaction at one or both electrodes is
not rapid enough (slow charge transfer) to
yield currents demanded by the theory.
After application of external
potential
So, an activation overvoltage, a, is needed to
activate the reactant to pass across the
electrode surface and to facilitate the charge
transfer step.
To sum up, Ohmic resistance, concentration and activation polarization
make electrolysis more difficult. They make the cell potential more
negative (decrease the potential of galvanic cell).
Ecell = Ecathode  Eanode  IR  c  a
As a result more voltage (overvoltage) is needed from the power supply
to drive the reaction of the cell forward.
Dr. Rasha Hanafi, GUC
PHCM662, Lecture 7, SS2016
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III. 3. Controlled potential electrolysis with 3-electrode cell
• In general we need to adjust the potential of the working electrode so that some
electroactive species react while others do not.
• Metal working electrodes are generally polarizable, which means that their
potentials easily change when small current flow.
• On the other hand, a reference electrode is said to be non-polarizable, because its
potential does not vary with the flow of current.
Thus, in order to measure and control the potential of
a polarizable working electrode, a third reference
electrode should be introduced
 The working electrode is the one at which the
reaction of interest occurs
 The reference electrode is used to measure
the potential of the working electrode
 The auxiliary electrode (the counter electrode)
is the current supporting partner of the
working electrode.
Dr. Rasha Hanafi, GUC
PHCM662, Lecture 7, SS2016
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III. 3. Controlled potential electrolysis with 3-electrode cell, cont.
• In three electrode configuration, a potential is applied between the auxiliary and
working electrodes and the potential of only the working electrode (measured with
respect to the reference electrode) is monitored.
• In controlled potential electrolysis, The voltage difference between the working
electrode and reference electrode in a three-electrode cell is adjusted by an electronic
device called potentiostat.
•If the applied potential is set at E1, the
respective potential on the working
electrode will be sufficient to deposit only
Cd2+ ions via reduction to Cd.
Current
Example: Analysis of Cd2+ and Pb2+ mixture
•If the potential is adjusted at more
negative value of E2, both cations will be
deposited simultaneously.
Dr. Rasha Hanafi, GUC
PHCM662, Lecture 7, SS2016
Pb2+
Cd2+
Pb
Cd
E1
E2
-ve
potential
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III. 3. Control of the applied voltage
Observed current-voltage relationship for
• Nothing special happens at -0.911 V
electrolysis of 0.2 M CuSO4 solution
(voltage calculated from Nernst). At this
low voltage, a small residual current is
observed as a result of reduction of traces
of dissolved O2 or some Fe3+ impurities.
• Near 2.0 V, electrolysis of Cu2+ starts and
the rate of reaction (the reduction current)
increases steadily.
Cu2+(aq) + 2e-
Cu(s)
• The voltage between the two electrodes is
E = Ecathode  Eanode  IR  overpotential
Suppose we hold the applied voltage at E = 2.0 V until all Cu2+ is
reduced . As Cu2+ is used up (at the end of electrolysis), the current
decreases and both the ohmic and overpotentials decrease. Note that
During
Eanode is fairly constant because of the high concentration of solvent
(H2O) being oxidized at the anode. Since the applied voltage was held
End
constant, Ecathode eventually becomes more negative in order to keep the
integral equality in the above equation.
Dr. Rasha Hanafi, GUC
PHCM662, Lecture 7, SS2016
-2 = x -1 -0.5 ;
x=-0.5
-2 = x -1 -0
; x= -1!!
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III. 3. Control of the applied voltage, cont.
• When the applied potential is held constant, the
potential of the cathode may become negative
enough that unintended reductions may occur:
1. reduction of H+ or water to H2 gas. The gas bubbles
evolved at the cathode surface interfere with the
deposition of the solid.
2. reduction of other ions such as Co2+, Sn2+ or Ni2+.
• To prevent the cathodic evolution of H2 gas at the cathode, a cathodic
depolarizer such as NO3- can be added to the solution. It is more easily
reduced than H+ or any other interfering species.


NO3  10H   8e   NH 4  3H 2 O
• Alternatively, we can use three-electrode cell with potentiostat to control the
cathodic potential and prevent unwanted side reactions.
Dr. Rasha Hanafi, GUC
PHCM662, Lecture 7, SS2016
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Electroanalytical methods may be based on the measurement of
either:
1. Current at a fixed potential.
2. Potential at a fixed current.
Today’ s lecture
Dr. Rasha Hanafi, GUC
PHCM662, Lecture 7, SS2016
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IV. Electrogavimetric Analysis
• The analyte is quantitatively deposited on an electrode by electrolysis.
• The electrode is weighed before and after deposition, the increase in mass quantifies
the analyte.
 Cu2+ in solution can be quantified by reducing it to Cu(s)
on a carefully cleaned Pt gauze cathode with a large
surface area. O2 is liberated at the counter electrode
(decomposition of water).
 How do you know electrolysis is complete?
1. disappearance of color in solution (if the analyte
solution is colored).
2. to take one drop of solution and perform a qualitative
test for analyte confirming its total absence.
Dr. Rasha Hanafi, GUC
PHCM662, Lecture 7, SS2016
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V. Coulometric titrations
Coulometric titration (const. current):
2Br   Br2  2e 
(Generation of Br2 titrant)
 form of quantitative analysis based on
counting the number of moles of
electrons used in a reaction.
 Ex: cyclohexene (CYC) can be titrated with
Br2 generated by electrolytic anodic
oxidation of Br-.
 The initial solution contains an unknown
quantity of CYC and a large amount of Br-.
Generation
circuit
Detection
circuit
 The reaction is carried out at constant
current. Br2 generated at the Pt generator
anode immediately reacts with CYC.
 When CYC is consumed, the concentration
of Br2 suddenly rises, signaling the end of
the titration.
Dr. Rasha Hanafi, GUC
PHCM662, Lecture 7, SS2016
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Coulometric titration (const. current), cont.:
 The rise in Br2 concentration is detected by measuring the current between two Pt detector
electrodes (in the detection circuit). A voltage of 0.25 V applied between these two electrodes is
not enough to electrolyze any solute, so only a tiny current of <1 A flows through the
microammeter.
 At the equivalence point, CYC is consumed, [Br2] suddenly increases and the detector current flows
as a result of the reaction:
-
Detector cathode:
Br2 + 2e
Detector anode:
2Br-
2Br
Br2 + 2e-
-
 Note that both Br2 and Br must be present for the detector half-reactions to occur.
 In coulometric titration, the time needed to generate (at constant current) equivalent amount of Br2
to the analyte is measured.
Applications and Advantages of coulometry
1. Fully automated coulometers commonly generate H+, OH-, Ag+ and I2 to titrate a variety of analytes
including CO2, sulfides in food and sea water as well as H2O in proteins and purified solvents.
2. Unstable titrants that can not be stored or standardized such as Ag2+, Cu+, Mn3+ and Ti3+ can be
generated and used immediately in titration.
3. Toxic titrants (Br2) that can not be used safely in conventional titration methods can be generated
and consumed as soon they are formed in coulometric vessel.
Dr. Rasha Hanafi, GUC
PHCM662, Lecture 7, SS2016
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V. How to quantify in coulometry
Faraday’s law:
The amount of chemical reaction at an electrode (e.g., mass
of copper deposited on the cathode surface) is proportional
to the quantity of electricity passed in the circuit.
Faradaic current:
Current that passes in the circuit as a result of actual
electrolysis (oxidation and reduction at electrode surface).
q  I . t
Coulombs
Amperes
Moles of e

Also, q= n . F
seconds
( n) 
I .t
F
The coulomb is the quantity of charge that is transported in 1 sec. by a constant current of 1 A.
If a reaction requires n electrons per mole of reactant, the quantity reacting of chemical species
in time t is
I . t
Moles of substances reacted 
Mass 
Dr. Rasha Hanafi, GUC
n
F
I .t
 ( molar mass)
n F
PHCM662, Lecture 7, SS2016
Faraday’s Law
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References
1. “Principles of instrumental analysis, 5th ed. by Skoog, Holler,
Nieman” Chapter 22 and Chapter 24.
2. “Quantitative Chemical Analysis, 6th ed. Daniel Harris”
Chapter 17.
3. Lecture of “Non- potentiometric methods of analysis ” by Dr.
Raafat Aly, GUC, spring 2010.
Dr. Rasha Hanafi, GUC
PHCM662, Lecture 7, SS2016
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