Nexpensive Field Portable Programmable Potentiostat Engineering Essay

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Potentiostat is an electronic instrument used for the electrochemical characterization of the redox species and in determination if the thermodynamic and kinetic parameters of electron transfer events. The Potentiostat applies the potential waveforms between the working and the reference electrode and measures the resultant current at the counter electrode. The cyclic voltammetry technique is applied for the potentiodynamic electrochemical measurements. In CV, the Potentiostat the working electrode is linearly ramped v/s the current. When the CV attains the potential that is set, the potential ramp of the working electrode is inverted. The current versus applied voltage is plotted to give the cyclic voltammogram trace. In the waveform, the forward scan produces the current peak of the analyte and the reverse scan produces the peak current of the catalyte.The current peak indicates the concentration of the chemical and provided other useful information. The Potentiostat circuit is designed in this project using a PIC microcontroller which is programmed to generate the potential waveform and determination of the current peaks and their location in the CV .The results are stored in the calibration table of the PIC. The amplifier circuitry is used for the amplification of the signals. An RC equivalent circuit of the electrochemical cell is designed which is used to test the Potentiostat circuit design. [8][9].

CHAPTER 1: INTRODUCTION

A Potentiostat is a device (a type of feedback amplifier) which is used to maintain the working electrode at a desired potential with respect to the reference electrode .It's also called patch clamp amplifier. [1].

Applications of the Potentiostat are

1. As precision ammeter also known as Zero Resistance Ammeter

2. Potentiostat as controlled current source i.e., Potentiostat can also be used as galvanostat.

3. In measuring the polarisation resistance

4. Redox potential measurements

5. Potentiostat can be used as precision controlled voltage sources.

6. Potentiostat can be used as HiFidelity Amplifier (Hi-Fi Amp). [1]

CHAPTER 1.1: OVERVIEW

The basic Potentiostat setup is a shown below:

BASIC POTENTIOSTAT SETUP [1]

A controlled voltage input is given to the Potentiostat circuit. This voltage generates a sufficient current through the counter electrode that is required to obtain the desired potential difference between the reference and the working electrode. The voltage control such as ramp/voltage generator can be applied through an external source or can be produced by an internal source of the Potentiostat. [1]

In this project we use a varying potential, hence external controlled voltage source is sufficient. The external sources are used where time dependent signals are required as inputs. [1]

There are two types of electrochemical processes that are used for quantitative measurements namely, [2] [3]

1. Potentiometry

In the Potentiometry, the potential of the solution is measured with no current flow. Applicationof potentiometric includes determination of pH solutions, end point titrations etc. [2] [3]

2. Electrolytic method

In this method an external energy source is applied to initiate an electrochemical reaction. If the external energy source that is applied is potential then the current that is resulting is the analytical signal and if the current is adapted then the analytical signal is the potential that is obtained. [2][3].

The approach which uses the applied potential is termed as voltammetry method and those which use current are termed as galvanostatic methods. [2][3]

In the project the voltammetric method is applied, ie, the external applied driving force is the applied potential. [2][3]

CHAPTER 1.2: VOLTAMMETRY

It is a type of electrolytic method applied in the analytical chemistry and other processes. In this method, the measurement of the current as the potential values is used to obtain information about the analyte. [4][5]

The common Voltammetry techniques are:

Polorograph

Cyclic voltammetry

Anodic stripping voltammetry

Differential -pulse polorography (DPP). [6]

In this project a Potentiostat is built that is used to analyse the redox active species. The cyclic voltammetry scans are used for the characterization of the redox species.

The redox species are the one which undergo redox reactions which occurs due to the transfer of electrons among thespecies. The redox reaction specifies a combination of reduction (gain of electrons) and oxidation (loss of electrons) reactions wherein all the chemical reactions the oxidation number of the atoms are charged. The redox reactions are the under structure of theelectrochemical cells. [7].

The voltammetry techniques constitutes

1. Applying the potential (E) to the electrode and observing the resultant current (I) that runs through the electrochemical cell. The potential applied is varied in accordance to the time (t) and hence the technique is depicted as a function of E, I and t. [10]

Application of the voltammetry technique include quantitative analysis of the inorganic and organic substances ,oxidation and reduction of the material takes place which in turn results in the movement of the new substances towards the surface of the electrode and generates a current. The result of the applied potential and the resultant current behaviour can be described by several laws [10]

In a reversible electrochemical reaction that can be described by the equation: o+nÄ“↔R [10]

The potential E when applied, constraints the concentration of the oxidation and the reduction at the electrode surface (i.e., coo and crop) to the ratio in accordance with the Nernst equation which is given by, [7]

E= Eo- RT/nFlncro/coo Where,

R= Molar gas constant i.e., 8.3144 Jmol-1K-1

T=Absolute temperature (K)

n=Number of the electrons that is transferred

F=Faraday's constant (96.485C/Equiv)

Eo=reduction potential for the redox reaction. [7]

The electrode potential waveform ramp versus time is as shown below. This is known as scan rate (V/s). [11]

Potential waveform [11]

This potential is measured between the working and the reference electrode. This data is then used to plot the waveform of the potential (E) applied v/s current obtained [11]. The plot is as shown below.

The duck curve (I-E curve) [12]

Where, Epc and Epa are the peak potentials at the cathode and anode and, ipaand ipc are the current peaks of the anode and cathode respectively.

The forward scan provides the peak current of the analyte that can be oxidized through a range of scanned potential. This current increases as the potential reaches the reduced potential of the analyte, but decreases due to the depletion in the concentration of the analyte when close to the electrode surface. The Redox species is said to be reversible if the redox system is in equilibrium throughout the scan. If the redox species is reversible then the applied potential when reversed will reach the potential that re-oxidizes the product that is from the first reduction reaction and thus produces a current which is of reverse polarity that is obtained from the forward scan. The current peak at both anodic and cathodic sweep usually has similar shapes. This provides information about the electrochemical reaction and redox potential. [11][12].

The peak current ip is directly proportional to the concentration C and can be defined by Randles-Sevcik equation,

ip = 0.4463 n F A C (n F v D / R T)1/2[12]

where,

n = number of electrons transferred/molecule

A = electrode surface area (cm2)

C = concentration (mol cm-3)

D = diffusion coefficient (cm2 s-1) [12]

The parameters defined below are used for the characterization of the CV of a reversible process [12].The best method to obtain the peak current is by the extrapolation of the baseline current.

 The current peak ratio  ipa/ipc = 1 is similar for all scans

Difference in the peak potential DEp (= Epc - Epa) = 58/n mV for all the scan rates at 25 oC.

the  function ip/n1/2 (n = scan rate) is independent of n .[12]

CHAPTER 2: BLOCK DIAGRAM OF THE POTENTIOSTAT CIRCUIT

The Block diagram of the Potentiostat is as shown below:

BLOCK DIAGRAM OF POTENTIOSTAT [13]

The PIC microcontroller is used of for the generation of the voltage waveform, data analysis and data acquisition. The PIC is programmed in the assemble language as it is the more efficient and powerful coding technique. A 10 bit counter is used in the PIC microcontroller for the voltage waveform generation.

The 10 Bit counter value is fed into external D/A converter through the SPI (serial programming interface) port of the microcontroller. The voltage waveform is level shifted using the amplifier circuit and is applied to the electrochemical cell between the working and the reference electrode of the cell.

The current generated from the cell is then converted into voltage and is then applied to the level shifting circuit which is used to achieve the required input voltage level that is essential for the analog input port of the PIC.The current readings are noted down for each incrementing value of the voltage.[13][9]

CHAPTER 2.1 THE BASIC POTENTIOSTAT CIRCUIT:

THE BASIC POTENTIOSTAT [9]

The basic Potentiostat circuit for the three electrode cell with three opamps (operational amplifiers) is as shown in the figure above.

Input Triangular voltage is given through opamp A and through the referenceelectrode. The output of the opamp A is connected to the counter electrode. The feedback of the opamp B is connected to the inverting input terminal of the opampA.This feedback applied decreases the differences between inverting and non-inverting input of opamp A and as a result the reference electrode assumes the as input triangular voltage of opampA.Since the potential difference between the reference and the working electrode is zero, the potential across the counter electrode is as same as that of the applied voltage. The input is isolated from the output due to which the infinite current can be obtained since the input does not limit the output, thus satisfying the requirement for the counter electrode. The opamp c acts as a current to voltage converter which converts the current generated at the counter electrode into voltage that is measured using a PIC circuitry. The Opamp B has infinite input resistance and is connected to the reference electrode as a result there is very little flow of current through the reference electrode.[9][14].

CHAPTER 2.2: SCHEMATIC OF PIC CONTROLLED POTENTIOSTAT

The Potentiostat circuit is designed using a PIC which controls the operation of the circuit. The circuit diagram of the Potentiostat circuit with the PIC microcontroller is as shown below

PIC controlled Potentiostat circuit[9]

The components used in the circuit design are explained below:

PIC18F452

It is a 8 bit microcontroller from the microchip PIC architecture consisting of 2 CCP function,10 bit A/D converter ,synchronous serial port that can be used as 2 I2C (inter integrated circuit)or as SPI (serial peripheral interface),timers, C complier friendly development environment.[15]

MAX5354:

The MAX5354 contains an output voltage DAC (digital to analog converter) that can be easily detected using a 3 wire serial interface. Each integrated chip consists of a 16 bit shift register and a double buffered input consisting of a DAC and input register. [16]

The DAC behaves as an R-2R ladder network that converts the digital input data into an equivalent analog output data which is proportional to the applied reference voltage. [16]

CTX 144:

The oscillator is 8/14 pin compatible with an I/p voltage = ±5DC, ±.5V.

It provides 4 MHz clock to the Potentiostat system which is internally divided by 4 from the microcontroller to achieve 1MHz clock. [17]

DTC 90:

It's the most versatile switch used for printed switch boards.

It is used for resetting the PIC externally.

PT5061:

It is ±5 V input dual output integrated switching regulator with adjustable output voltage. It s is used in the system that required power for the analog interface circuitry that includes op-amps, D/A and A/D converter. It provides ±12V that is essential for the op-amp circuitry. [18]

LM148 and LM741:

They consist of 4 independent high gain operational amplifier of low power. This amplifier package and the op-amp chip are used for signal processing of the signals occurring before and after the electrode cell implementation. [19].

Chapter 2.3: OPERATION OF CIRCUIT:

The operation of the circuit is classified into 2 sections

Applying the triangular voltage waveform to the Potentiostat circuit through the PIC and measuring the resulting current

Analysis of the resulting current from the electrochemical cell.[7]

CHAPTER 2.3.1: PIC AND AMPLIFIER CIRCUITRY:

The PIC generates the triangular voltage waveform that is given to the electrochemical cell. The 10 bit up down counter is used to generate the waveform. Regular interrupts are produced using the timer capture module to update the 10 bit up down counter value, update D/A and take the A/D measurements. The output of the digital -to analog (D/A) converter is changed from unipolar to bi-polar by using an op-amp level shifting circuit. The circuit andoutput waveformof the D/A converter and the level shifting circuit is shown below. [7]

D/A converter and level shifting amplifier [7]

To prevent the loading of previous opamp stages from the current resulting from the electrochemical cell, a buffer is applied between the level shifting voltage and the counter electrode. Thereference electrode is connected to the non-inverting part of the voltage follower circuit. This circuit is used to achieve constant potential at the reference electrode thus limiting the drawing current. The working electrode is connected to the current-to-voltage converter's inverting terminal due to which the electrode is grounded and produces the voltage that is equivalent to the current produced by the electrochemical cell at the O/P of the voltage connecter .this voltage is level shifted and amplified to the voltage level that can be accepted by the PIC and is given to the analog I/P of the PIC microcontroller. [7]

CHAPTER 2.3.2: ELECTROCHEMICAL CELL:

AnRC equivalent circuit for the electrochemical cell is implemented instead of designing an electrochemical cell.

The different I versus E curvesis obtained when the concentration of the solution is varied. This is achieved through the RC circuit by varying the value of the capacitance in the circuit. Once we obtain the I versus E graph the peak value of the currents is obtained for both anodic and cathodic sweeps and the calibration curve (concentration of analyte versus response) is plotted. [9].

Considering the physiochemical process involved in the interface of the electrolyte and the metal electrodes, an RC network circuit is obtained. The circuit consists of 4 sections. [20]

The section which transfers (electrons) charges between metal and electrolyte which is resistive.

The double layered capacitance which is parallel to the first section.

The charge stored in this capacitance can be varied by applying external voltage .This differential capacitance is given as,

C= dE/dT

Where, dE/dT is the change in the potential with respect to time.

The diffusion layer impedance that is in series with the 2 sections.

Electrolyte's bulk resistance which is in series with the third section.[20]

The equivalent circuit for the electrochemical cell is shown below:

RC EQUIVALENT NETWORK OF THE ELECTROCHEMICAL CELL [9]

Where,

Rc = counter resistance (in this project we set the resistance value = 13.5K).

Ruc= uncompensated resistance (13.5K)

Cd= Double layer capacitance (4.7µF).

CHAPTER 3: PROGRAMMING OF POTENTIOSTAT:

CONTROL FLOW:

The main program provides the control flow for the entire program. There are 4 ports A, B, C and D in the PIC.The Port A pin is used for A/D conversions, Port B for the interrupts and port C for the SPI communication. Hence the Port C is configured as output. The configuration settings are done for the A/D, timer and SPI and the interrupt is enabled. Once the count reaches 2048, the interrupt handler disables the interrupt and the control passes from the main program to the sub program to calculate the current peaks. Once this is done the PIC goes into sleep mode which saves power. [9].

The program to generate the voltage waveform and obtain the peak current is written in assembly language and is loaded into the PIC using MBLAB IDE debugger .

Main Program:

1. Configure the ports of the PIC as input port and output ports.

2. Enable interrupts: The high priority interrupts are enabled first and later the low priority interrupts are enabled. The high level priority interrupt is located at 0X008.When the interrupt is received, the PC (program counter) is loaded with interrupt subprogram service value. [9]

3. Configure timer, ADC, SPI for A/D conversion and serial data communication.

4. Enable ADC interrupt, load the values and wait till the interrupt is disabled.

5. Calculate the slope and peak at both anodic and cathodic directions.

6. Close ADC, Timer, SPI and the disable all the interrupts. [9]

Subprograms:

1.Interrupt subprogram (ADC handle):

This updates the values in the 10 bit up down counter which generates the triangular voltage .The values are padded and with control information and is sent to the SPI.waveform.It also takes the average of the 8 current vales and stores in 256 byte array. Once the array is loaded fully with the values, the interrupt is disabled by the ADC. [9]

2.Calculation of the slope:

This subprogram calculates the current peak in both anodic and cathodic directions and also the knee of the curve.The cathodic and anodic peak is then calculated with respect to the left slope of the knee in both the directions. [9]

The calibration data in terms of the y intercept and the slope is stored in the PIC.The y axis and the slope determines the concentration with respect to the peak value and the corresponding 8 bit current value is generated. [9]

CHAPTER 4.PEAK AND PEAK CURRENT DETERMINATION:

The analog-to-digital measurements are periodically taken from the special event mode of the TC mode (timer capture mode) by setting the go/done bit of the A/D converter which starts the analog-digital measurements. Once this is done an interrupt of high level is applied. [9]

The stages involved in the interrupt subroutine are as shown below. [9]

A/D values read from the conversion register

Continuous 8 values averaged

( to eliminate noise)

Store the data in an array of length=256

Disable interrupts after 2048 counts

Once the interrupt is disabled the peak current of the anodic and cathodic sweeps are obtained.

The slope of the left side and the knee is calculated. Then the peak of both anodic and cathodic sweeps with respect to the slope is calculated. The calibration data of the curve is stored in the PIC in terms of slope and y-intercept of the best fit line. This slope and the y-intercept are used to calculate the concentration that corresponds to the peak current values and the corresponding 8 bit data value of the current is generated. [9]

CHAPTER 4.1: BASELINE CURRENT EXTRAPOLATION:

The point of maximum curvature is used to determine the baseline current for both anodic and cathodic sweeps. A line is made to pass through all the points to the left side of the point of maximum curvature. [9]

CHAPTER 4.1.1: L METHOD FOR THE DETERMINATION OF KNEE OF THE CURVE OR POINT OF MAXIMUM CURVATURE

The novel method (L method) finds the edge/boundary between the pair of lines that fits the cure closely. This method considers all points of data together at same time and is used where the curve includes sharp jumps. [22]

The linearity of the region to the right and left of the line is taken into consideration in the L method. If a line is drawn fitting all the points on the left hand side and another line is drawn on the right hand side, the area that is obtained in between these two lines will be same as the area of the knee. To find these 2 lines, a pair of line that fits the curve well (i.e., that covers maximum points) is obtained. [22]

Consider a graph of no of data v/s evaluation graph. Let the number of data be'd'.Hence the data range is from '1-d'.Let 'p'be the partitioning of the data points. Hence the right hand side of the data points is defined to be from 1..p and the left side of the data points from p+1...d.[22]

RMSEc = 1/d {p * RMSE (Lc) + (d-p) * RMSE (Rc)} [22]

Where,

RMSEc = the mean square error in total

RMSE (Lc) =the mean square error if the left hand side of the partition in total

RMSE (Rc)= the mean square error if the right hand side of the partition in total.

Steps to find the knee of the curve using the L method is as shown below.[24]

The data points are varied from 1 to d.

Using the points on the right hand side(1 to p), a line is fit and using the points on the left (p+1 to d),another line is fit.

The RMSEc is calculated for each value of p.

The knee of the curve is obtained by the least mean square error.

The regressive line fitting is used for fitting a line to the set of data provided.

CHAPTER 4.1.2 REGRESSIVE LINE FITTING:

The best fit line(regression line) is used to obtain minimum value for the mean square error(i.e., the error between the expected and the obtained values if the data points) .[23]

If the data points are defined by {xi,yj} then the regression line s used to obtain the equation of a straight line[23]

Y= mx+c

Where, m = slope

c = y-intercept.

The values for the m and c can be obtained by using calculus and is given as,[24]

m= covariance (xy) / {variance (x) - mean(x) mean(y)}

c = a * mean(x) - mean(y)

The peak is obtained w.r.t the baseline current and the concentration is obtained from the calibration curve.[23]

CHAPTER 6 : CONCLUSIONS:

The concepts and determination of the embedded circuit design for a field portable programmable Potentiostat is studied and obtained. Further work needs to be done to implement the design on the PCB board and writing the code for the micro controller in C language to implement the design. The pilot study done for this project has helped in understanding the main concepts of the potentiostat, its applications, components that are required and their specifications to design a field portable programmable potentiostat. 

CHAPTER 6 :REFERENCES

1. http://www.bank-ic.de/encms/knowhow/1_faq.html

2. http://www.answers.com/topic/electrolysis

3. http://www.files.chem.vt.edu/

4. Kissinger, Peter; William R. Heineman (1996-01-23). Laboratory Techniques in Electroanalytical Chemistry, Second Edition, Revised and Expanded (2 ed.). CRC. ISBN 0824794451.

5. Zoski, Cynthia G. (2007-02-07). Handbook of Electrochemistry. Elsevier Science. ISBN 0444519580.

6.http://www.files.chem.vt.edu/chem-ed/echem/electrol.html

7.http://shodor.org/unchem/advanced/redox/index.htm

8.http://en.wikipedia.org/wiki/Cyclic_voltammetry

9.http://chemeducator.org/sbibs/s0011001/spapers/1110023dr.pdf

10.http://www.phattimes.com/myoglobin/chapter3.htm

11.http://www.answers.com/topic/cyclic-voltammetry

12.http://www.basinc.com/mans/EC_epsilon/Techniques/CycVolt/cv_analysis.html

13.http://www.mdpi.com/1424-8220/6/11/1679/pdf

14.http://www.prenhall.com/settle/chapters/ch37.pdf

15.http://www.microchip.com/wwwproducts/devices.aspx?ddocname=en010296

16.http://datasheets.maxim-ic.com/en/ds/MAX5354-MAX5355.pdf

17.http://www.datasheet4u.com/html/C/T/X/CTX128_Digikey.pdf.html

18.http://www.datasheetcatalog.org/datasheet/texasinstruments/pt5061.pdf

19. http://www.national.com/ds/LM/LM124.pdf

20.http://www.springerlink.com/content/r482356118113j5u/fulltext.pdf

Or

21. http://www.phattimes.com/myoglobin/chapter3.htm

22. Salvador, S.; Chan, P. Determining the Number of Clusters/Segments in Hierarchical Clustering/Segmentation Algorithms. Department of Computer Science Technical Report CS−2003-18, Florida Institute of Technology, 2003.

23. http://www.zweigmedia.com/ThirdEdSite/Summary1.html

24.

MICRO-CONTROLLER BASED POTENTIOSTAT

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