Case Study: Cyclic Voltametric Measurement

Glasgow Caledonian University MSc Applied Instrumentation &Control
School of Engineering, Science & Design Measurements Systems

CASE STUDY 1

The design of an ac Cyclic Voltammetric Measurement System for the in –
situ measurement of dissolved oxygen in sediment on the seabed. The
measurement strategy should be based on linear ramp cyclic voltammetry.

PART A
i. Key features of microelectrode.
Microelectrodes are electrodes that have at least one dimension in the range 5-50
μm. Moreover, their small size, low cost, low power requirements make them
absolutely suitable for a wide range of electrochemical applications. Also, because
of the minimization of the voltage drop and the fact that they rapidly reach steady
state diffusion conditions microelectrodes are widely used in resistive solutions
without any additional electrolyte.

ii. Principal of operation of the microelectrode sensor

Figure 1: Electrolysis diagram
When electrolysis occurs at the microelectrode, the analyte at the surface of the
electrode is consumed and a concentration gradient is formed between the
electrode surface and the bulk solution. This reaction takes place in three basic
steps:
1. Mass transport of the species O to the electrode surface.

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2. Oxidation of species O to product R with an associated transfer of
electrons at the electrode.
3. Mass transport of the product R from the electrode surface to the bulk
solution.
The rate of this reaction depends on the slower step. Since step 2 can be controlled
by the applied voltage, the slowest step becomes the mass transport of the species
to or from the electrode surface; hence the magnitude of the measured current will
be dependent on the mass transport process.

The mass transport is mainly taking place by diffusion and Fick’s laws can be
used to relate the flux and concentrations of a substance as a function of time and
position.

Figure 2: Concentration versus distance for a mictroelectrode

The electrocell can be modelled by the RC circuit shown in figure 3.

Rct microelectrode
Rsol

Reference Cdl
electrode

Figure 3: Equivalent electrocell model

2


iii. Typical linear ramp cyclic voltammetry
A typical linear ramp voltammetric response for a microelectrode under diffusion
was obtained in the lab. The acquired in LabView data were stored in a
spreadsheet and illustrated in figure 3.
For this voltammogram the electrolytes is aerated water, the period is 50s, the
amplitude is 2V while 1000 samples are taken. The diffusion limited current is the
current between the the ‘plato’ positions of the current.

Voltammogram, aerated water

1.00E-08
5.00E-09
0.00E+00
-1.5 -1 -0.5
-5.00E-09 0 0.5 1 1.5
Current (A)

-1.00E-08
Diffusion limited current

-1.50E-08
-2.00E-08
-2.50E-08
-3.00E-08
Applied Voltage (V)

Figure 4: Measured Voltammogram

There is rapid increase of the current as the voltage approaches -1V. This fact is
associated with the evolution of hydrogen gas. A small peak close to 0 Volt is

because of the presence of Ag + ions. The release of oxygen is responsible for the
diffusion current.
Measuring the produced current with an inlaid disc microelectrode the
concentration of dissolved oxygen in the solution can be calculated from the
diffusion limited current at an inlaid disc formula:
i = 4nFaDC ∞

Equation 1

3


where n =number of electrons involved in the reaction, F=Faraday constant,

cm 2
D=diffusion coefficient ( ), C = concentration of bulk solution, a= radius of
sec
the electrode (cm).
It is obvious that the concentration is proportional to the produced current.
Solving equation 2 for C, the concentration of dissolved oxygen can be calculated.

iv. Simulation results
The simulation results from exercise 1 are presented in the table below.

Scan Rate (V/s) Icdl
0.001 2.80E-12
0.056 1.57E-10
0.116 3.25E-10
0.176 4.92E-10
0.2 5.60E-10
0.236 6.60E-10
0.296 8.28E-10
0.356 9.96E-10
0.416 1.16E-09
0.476 1.33E-09
Table 1
Capacitance is a measure of the amount of electric charge stored (or separated) for
a given electric potential. The slope of the characteristic of the current versus the
scan rate will give the value of the double layer capacitance because of the
following equation
dq dV dV , where q= the electric charge,
q = CV ⇒ =C ⇒ icdl = C icdl = the
dt dt dt

dV
double layer current, V= electric potential, = Scan rate.
dt

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Icd Vs Scan Rate

1.40E-09
y = 2.792E-09x + 8.293E-13
1.20E-09
1.00E-09
Current (A)

8.00E-10
Icdl
6.00E-10
4.00E-10 Linear (Icdl)
2.00E-10
0.00E+00
0 0.1 0.2 0.3 0.4 0.5
Scan Rate (V/s)

Figure 5: The slope i=f(dV/dt) delivers the value for the double layer capacitance
Therefore, the double layer capacitance is calculated to be 2.79nF.
v. Hysteresis
Hysteresis occurs due to the charging current associated with the capacitor at the
microelectrode, shown in the equivalent model in figure 3. The magnitude of the
capacitance is proportional to the area of the electrode.
To reduce the magnitude of hysteresis, i) the size of the microelectrode should be
as small as possible, ii) the scan rate should be slow.

PART B

vi. Block Diagram

Figure 6: AC linear ramp cyclic voltammetric measurement system with phase sensitive
detection

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vii. Design of microelectrode probe for submersible lander.
In order to properly design a microelectrode system, a clear understanding for the
needs and constraints of the application must be understood. The primary issues
that must be considered are: the electrical, mechanical, chemical, biological,
thermal, magnetic, and optical performance specifications in the context of the
intended function and application. The number of electrodes, spacing/density,
position in tissue, duration of use, and extent of use are all important design
criteria that are driven by the application.

With a low electrode count, simple individual microwires or microprobes with
external discrete amplifiers are sufficient. However, as the electrode count
increases, the number of wires becomes increasingly inconvenient and greatly
burdens the system integration and reduces system reliability. The integration of
mixed-signal circuitry has the advantage of reducing the number of wires
necessary to connect to the device, positioning the amplifier nearer to the signal
source to lower the noise level, but has the disadvantage of substantially
increasing the cost and design complexity.

The primary mechanical performance criteria involve the shape and elasticity of
the microelectrode array and the supporting structure. Supporting structures can
vary from thin and flexible large-area membranes to thick planar substrates and a
variety of penetrating microprobes. Also, the shape of the microelectrode is
critical.

The chemical performance criteria involve the general chemical compatibility /
stability of the materials in the device. When microelectrodes are coated with an
ion-selective membrane, the potential between the solution and the electrode can
be related to the concentration of the ionic species of interest (potentiometry).
Similarly, cyclic voltammetry can be used to relate the magnitude of current
flowing at different potentials to ionic concentration. Therefore, another chemical
performance criteria would be the sensitivity of the electrode to desirable ionic
species and the lack of sensitivity to undesirable ionic species.

6


The thermal specification of most interest is the heat dissipation of the
microelectrode system. Although this is essentially negligible for conventional
microwires, the integration of a substantial amount of circuitry can increase
concerns. In particular, the circuitry needed for the transmission of information
via wireless telemetry often requires the most power and is the least efficient
circuit block. Carefully designing the thermal characteristics of electrically
powered systems to distribute heat uniformly can reduce the problems caused by
hot spots.

From a magnetic and electromagnetic perspective, the two greatest concerns lie
with the use of microelectrode systems in magnetic resonance imaging (MRI)
systems and the transmission /reception of wireless telemetry and inductive
power.
Microelectrode systems typically consist of the following materials: electrodes
are made of Pt, Ir, Ag/AgCl. Electrical interconnects can be made of Al, Au,
polysilicon. The electrical insulation is usually made of SiO 2, Si3N4 or polyamide.
The mechanical structure is Si, glass, Al2O3, PtIr. The materials used in the
electrode and supporting structure possess mechanical properties that drive the
mechanical design. Small holes ( d= nm) were made on the probe’s surface for to
allow the liquid come in touch with the electrodes.

Figure 7: The designed probe

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The probe illustrated in figure 7 is designed to be put on a submersible lander on
the seabed. The stainless steel cover protects from corrosion, shielding, gives
mechanical strength while it has small holes in the range of a few nm to allow
accurate measurements to the microelectrode. The microelectrode itself is not a
new invention; it is made of platinum, glass, copper. The closer the amplifier will
be to the sensor the best for our measurements. Therefore the amplifier was put
inside the probe. A necessary reference microelectrode was put inside the sensor
to provide accurate measurements. When the lander reaches the seabed the
probe’s shaft enters the land for 3-5 μm which is the height of the cone in the
bottom part of the probe. Four or more probe can be put on the lander to provide
higher accuracy about the concentration of oxygen in sediment on the seabed.
viii. Microelectrode diameter .
dV 2
The scan rate for the given time, rate is = = = 0.01V / sec . The double
dt 200

layer current should not be larger than 20pA and C dl = 550 µFcm −2 .
In scan rate.xls file the diameter of the electrode is set to 10 µm . The resistance of
the solution is Rsol =1.00E+06 Ohm, Rct= 1.00E+09 Ohm, and the current is
4.32E-12A which is less than 20pA.

ix. Low noise current to voltage converter

Figure 8: I/V converter

Using the 741 IC and the above diagram a low noise current to voltage is
achieved. The input impedance and the voltage gain are of the main
characteristics to select an operational amplifier. The low input impedance and the
infinite resistance between the inputs of the 741 amplifier in figure 7 drive the
current throught the resistance Rf and finally the current is converted to voltage.

8


For further noise reduction it is proposed the connection of a lock-in or a
differential amplifier at the output of the I/V converter.

x. Setting the phase at a lock-in amplifier.

Figure 9: Lock-in amplifier

A lock-in amplifier is an ac amplifier which provides a dc output proportional to
the measured ac signal. The control switch is controlled by a reference signal
between a gain of -1 and 1.

Assuming that the reference is a sum of cosines, using Fourier series it can be
written as v r = (4Vr / π )[ sin ω r t − sin(3ω r t ) / 3 + sin(5ω r t ) / 5 − sin(7ω r t ) / 7 + ...] ,

and the signal is e s = E s sin(ωt + φ ) , then:

9


es vr = (2 E sVr / π)([cos(ωs t +ωr t ) +φ] + cos[(ωs t −ωr t ) +φ]
−[cos(ωs t + 3ωr t ) +φ] / 3 + [cos(ωs t − 3ωr t ) +φ] / 3
+ [cos(ωs t + 5ωr t ) +φ] / 5 + [cos(ωs t + 5ωr t ) +φ] / 5
−...)

If ω s = ω r then the low pass filter will remove all the terms except one, hence:
e s v r = (2 E sVr / π ) cos φ

Equation 2

Equation 2 shows that the maximum dc output will come when the phase
difference φ between the signal and the reference will be zero.

For an in-phase squarewave the expected output is shown in figure 9.

Figure 9: dc output = 0.637, phase difference=0 degrees

Figure 10: dc output = 0, phase difference= 90 degrees

The use of square wave reference signal guarantees 27% larger output than using
a sinusoidal signal.

10


CASE STUDY 2

The investigation of an Earth Resistivity Measurement System.

1. The measurement System

Figure 3: Employed measurement system
The measurement system that used was based on the four point probe
Schlumberger configuration and it is shown in figure 1. The circuit diagram of
a voltage controlled current source is shown in figure 2. The circuit shown
here has a nearly infinite output resistance, since the output current is equal to
the current through resistor R, which is i= Vin/R = 0.6/3000 = 0.0002A. The

11


output of the op-amp changes its voltage so that this current i flows, whatever
the load resistance.

Vin = 0.6V

Rload
Vload

R=3KOhm

Figure 4: V/I converter
Drawing a graph Vload versus Rload from the provided data, given in table 1,
in figure 3 it is obvious that the current is independent of the load resistance.
R(Ohms) V (Volts) I load(A)

1000 0.2 0.0002

2000 0.4 0.0002

3000 0.6 0.0002

4000 0.8 0.0002

5000 1 0.0002

6000 1.2 0.0002

Table 2

Vload VS Rload

1.4
1.2
Voltage (Volts)

1
0.8
V (Volts)
0.6
0.4
0.2
0
0 2000 4000 6000 8000
Resistance (Ohms)

Figure 5: Constant current output

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2. Measurement Strategy
Using the measuring system shown in figure 1 the two electrodes are driven by a
constant current source, and the potential difference is measured by the two inner
electrodes.
The constant source will ensure the current flowing throughout medium is
independent of contact impedance and use of high input impedance
instrumentation amplifiers will ensure that no current will flow throughout the
voltage measuring electrodes.

The path in which the current flows depends on how the electrical resistivity is
distributed within the region. Hence, the potential difference between two pairs of
adjacent electrodes contains information about the distribution of electrical
resistivity.
The lock-in amplifier is used to improve the signal to noise ratio. Actually, the
lock-in amplifier is a phase sensitive detector with additional element which
allows:
• Amplification and filteration of input signal
• Shifting of phase of input signal with respect to signal.
• Amplification of the output.
The main advantages are that it responds to the frequency of interest, and the
reference frequency can be chosen to minimise the effect of 1/f and to avoid
strong interfering noise signals.

3. Archaeological application
The provided data are illustrated in columns 1,2 of the following table.
π ( L2 − x 2 ) 2 ∆V
Resistivity was calculated using the formula: ρ=
2l ( L2 − x 2 ) I
meters*Volts/Ampere, where L=3.25m, 2l=0.2m, I=0.002A, x=L -LHSposition
Postion from LHS
(m) distance x from the centre (m) Resistivity
0.5 2.75 545.987137
1 2.25 2128.743182
1.5 1.75 3242.469482
2 1.25 11542.92394

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2.5 0.75 42358.55263
3 0.25 16299.32189
3.5 -0.25 13039.45751
4 -0.75 7059.758772
4.5 -1.25 5771.461968
5 -1.75 5187.951171
5.5 -2.25 4561.592533
6 -2.75 4289.898934
Table 3

The graph of the resistivity versus position is shown in figure 4.

resistivity Vs distance

45000
40000
35000
30000
Resistivity

25000
resistivity
20000
15000
10000
5000
0
-3 -2.5 -2 -1.5 -1 -0.5 0 0.5 1 1.5 2 2.5 3
Distance x (m)

Figure 6: The large variation in resistivity in the region 0.5-1m indicates change in the layer

The observed wave at a distance of 0.7 meters is a good indicator of changes in
composition, layer thickness or contaminant levels. Electrical resistivity of stones,
rocks, and hydrocarbons are about thousand times higher than that of soil’s.
Hopefully, an object of great archaeological significance is hidden at this place.

4. Vertical electrical sounding (VES)
To achieve vertical electrical sounding the electrodes should be maintained at the
same relative spacing. Also, the position of the electrodes should be expanded
over a central fixed point as shown in figure 5.

14


Electrodes

Ground
Figure 5: Vertical Electrical sounding measurement system

CASE STUDY 3

Optical fibre Techniques for Chemical Measurements

1. A sensor for N02.

The fraction of a parallel beam of light absorbed by a sample is related to the

concentration of the absorbing species by the Beer-Lambert Law :

Io
log10 = Ecl
I
Equation 3
where Io= intensity of incident light, I=intensity of transmitting light,
E=molecular extinction coefficient, c= concentration in gm moles/L,

Io
l=paththlength of sample. The quantity log10 is known as absorbance of the
I
sample.
When using a sampling cell with mirrored curve faces the pathlength becomes
longer and the intensity ratio Io/I becomes bigger according to equation 1. In other
words the absorbance becomes higher.

15


2. A ph sensor using fluorescence intensity measurements and the
evanescent wave.
The light sources are characterised by the spectral power distribution, the
luminescence, the stability of light source, the way of control, the cost, and are
mainly distinguished in coherent and incoherent ones. In this investigation
coherent light source was used. In particular, a nitrogen dye laser was used
because of the narrow line width and the high intensity it provides. Also, it is
highly directional, and it has stable pulse output.
Of the key parameters that influence the distance of the sensing is the
concentration of the indicator. Actually, the strongest the measured signal the best
for the measurements. For example, an increase in concentration of the distal end
indicator would produce a decrease in signal due to absorption in the thicker distal
sensing zone.
3. A sensor for dissolved oxygen.

The digital curve method for oxygen sensing with a nitrogen laser or a high
voltage photomultiplier gives not very fast response due to the repetition rate and
the process for the calculation of the time constant tau. To decrease the repetition
rate, using the same method it is proposed the use of a LED. However, the
calculation time would be the same.
Changing the amplifier used in the digital curve method with a phase shift
detector and using a modulated pulse, the frequency domain fluorometry
technique can be used to measure the lifetime of the sample. In this method the
excitation light is modulated sinusoidally at frequency f, and the phase shift of the
fluorescence relative to the excitation is measured as illustrated in the figure
below.

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Figure 1: Frequency modulation technique

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Case Study: Cyclic Voltametric Measurement

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