Performance of a Constant Phase Element (CPE) sensor to detect adulteration in cow- with Siuli Das 1, Mulinti Sivaramakrishna 1, Manideepa Dey 1, Bhaswati Goswami 1, and Karabi Biswas 2 1 Department of Instrumentation and Electronics Engineering,JU, Kolkata, India 2 Department of Electrical Engineering, IIT Kharagpur, India Email- karabi@ee.iitkgp.ernet.in, bg@iee. jusl.ac.in Abstract--In this paper, the performance of a constant phase element (CPE) sensor has been studied for detecting adulteration of cow- with. Addition of in increases its volume and makes it acidic but the percentage of lactose content does not change significantly. Hence, addition of neutralizers like NaOH will balances the ph value of the adulterated and increases the shelf life. Sometimes cheap muriatic acid (used for cleaning purpose) is added to the to prepare. Both muriatic acid and NaOH causes serious health hazards. In this work, the CPE sensor has been used to detect the adulteration of cow- with which contains muriatic acid and the neutralizer, NaOH. The sensor performances are observed in pure (ph- 6.74), adulterated with 6%, 12% and 18% of (ph value of is 6.2). The ph value of adulterated with 18% is 6.44. Finally NaOH is added to bring back the ph value to 6.74. At every stages of adulteration the CPE sensor is used and the results show that the sensor can successfully detect them. Finally, an electrical equivalent circuit of the sensor dipped-in the adulterated is proposed through complex non-linear least square (CNLS) method to facilitate the design of suitable signal conditioning circuit. I. INTRODUCTION Milk adulteration is a problem of deep concern in dairy industry. Various ingredients are added to to increase its shelf life keeping the chemical property of the same. Among them addition of (the by-product after making cottage cheese from ) requires special attention as the uniqueness of this adulteration is that it does not change the lactose content of the but increases the acidity. And to neutralize the acidity, a small amount of alkaline solution (e.g. NaOH) is added so that the consistency and shelf life of the is increased. Moreover, some business men for greater profit uses cheap muriatic acid to prepare which have serious health hazard and this necessitate detection of adulterated with. The measurement of electrical conductivity of has been used [2-4] to detect the concentration of water in and to analyze the fat and protein content of and dairy products. The electrical admittance spectroscopy has been proposed in [5-7] to check the quality control of. But these techniques have high operational costs and also the testing methodology is time consuming In the present work, the performance of a low cost dip-in stick type CPE sensor [1] which is biocompatible, robust and can easily be used by the community has been studied here. The paper is organized as follows: Section-II, describes the background of development of CPE sensor to detect adulteration. Section-III gives the experiential procedure and the observations. Section-IV gives the electrical equivalent circuit of the CPE sensor dipped in adulterated. Discussion is given in Section-V and Section VI is conclusion. II. BACKGROUND The impedance Z, of an electrical element can be written as: Z = Qs -α where Q is the coefficient and α is the fractional exponent of the element which is a real number. Thus magnitude Z = Qω α, and phase angle θ = - απ/2, where θ is expressed in radians. The property of this element is that the phase angle depends on the value of α and is independent of frequency, i.e., a constant phase angle. It has been reported in [8, 9] that if α lies between -1 and 1 the element is fractional order element or called as constant phase element. The phase angle θ of the constant phase element depends on various parameters like: θcpe = f ( A, t, σ ) Here A is the area of contact of the electrodes with the polarizing medium, t is the thickness of the insulation on the electrodes and σ represents ionic concentration of the polarizing medium. The ionic concentration of pure will change with the change of adulteration. In this paper, the phase angle change with the change of the ionic concentration is explored to differentiate the pure with different adulteration of. For that purpose, a CPE reported in [1] is fabricated and used for detecting the adulteration of. A III. EXPERIMENTS AND OBSRVATIONS Fabrication of sensor: The CPE is fabricated [1] by providing thin layer of PMMA (Poly-methyl-meth-acrylate) coating on all the sides of 978-1-4244-5335-1/9/$26. 29 IEEE 745 IEEE SENSORS 29 Conference
copper cladded printed circuit board. The PMMA coating is provided by inserting the probe in PMMA-chloroform solution. The length, width and coating thickness of the fabricated CPE are 12mm, 6mm and 45μm respectively. The CPE is then characterized by measuring the impedance (Z), phase angle (θ), resistance (R) and reactance (X) for the frequency from 4 Hz to 4 MHz. The characterization is done by a LCR Meter (TEGAM Model No: 355) in Z, θ mode, excited with a sinusoidal signal of 1V peak to peak. This measurement is performed by dipping 1cm length of the CPE into standard solutions of ph 4., ph 7. and ph 9.2. The change of phase angle with the change of frequency is plotted in Fig.1 and Fig.2 where it remains constant. The phase angle remains almost constant in the frequency range of 1 khz to 1 khz called as the lower frequency zone and in the frequency range of 3 khz to 4 MHz called as the high frequency zone. But the value of phase angle is different when the CPE is dipped into solutions of different ionic concentration namely ph 4., ph 9.2 and ph 7.. The readings are repeated five times and the average value of phase angle in low frequency zone is tabulated in Table 1 and Table 2 indicates those in high frequency zone. TABLE 1-5 -25-3 -35-4 -45-5 CONSTANT PHASE ANGLE OF CPE IN LOW FREQUENCY ZONE When dipped in ph solution ph 4. ph 9.2 ph 7. 1 6 11 Constant Phase Angle 14 o 24 o - 25 o 34 o -35 o ph4. ph9.2 ph7. Fig. 1. The behavior of average value of phase angle of CPE when dipped in ph 4., ph 9.2 and ph 7. at low frequency zone. TABLE 2 CONSTANT PHASE ANGLE OF CPE IN HIGH FREQUENCY ZONE When dipped in ph Constant phase Angle solution -3-4 -5-6 -7-8 -9 ph 4. ph 9.2 ph 7. 78 o -79 o 8 o 72 o -73 o 3 13 23 33 43 ph4. ph9.2 ph7. Fig. 2. The behavior of average value of phase angle of CPE when dipped in ph 4., ph 9.2 and ph 7. at high frequency zone. It may be inferred from Table 1 and 2 and also from Fig. 1 and 2 that the CPE exhibits different phase angle when dipped into different standard ph solutions. B Preparation of : 25 ml of raw cow is boiled for more than 2minutes and then 1.5 ml of muriatic acid is added to it. Light green color is separated out from the white colored solid part of the. The is filtered out its ph value is measured. The ph value of is 6.2 and that of raw is 6.74. Then 5 ml of raw cow is adulterated with different percentages of. The readings of Z, θ, R and X are recorded by the same LCR meter for total range of frequencies between 4 Hz to 5 MHz after dipping 1 cm of the length of the CPE in pure. The recordings are repeated 5 times. It is observed that the phase angle remains almost constant only in the lower frequency zone, that is, 1 khz to 1 khz. The change of phase angle in this zone of frequency is plotted in Fig. 3 and tabulated in Table 3. 746
TABLE 3 CONSTANT PHASE ANGLE OF CPE FOR PURE MILK Milk -5-25 I Set II Set III Set IV Set V Set Phase Angle 17 o - 18 o 16 o - 17 o 17 o 17 o 16 o - 17 o 1 3 5 7 9 11 I set II set III set IV set V set Fig. 3. The phase angle behavior of pure in the frequency range 1 khz to 1 khz. TABLE 4 CONSTANT PHASE ANGLE OF CPE AND ph VALUE IN PURE MILK AND ADULTERATED MILK When dipped in Constant phase ph Value Angle 17 o 6.74 6% adulterated 16 o 6.56 12% adulterated 13 o 6.49 18% adulterated 12 o 6.44 The readings of Z, θ, R and X are recorded by the similar procedure for total range of frequencies between 4 Hz to 5 MHz similarly after dipping 1 cm of the length of the CPE in adulterated with 6%, 12% and 18%. The recordings are repeated 5 times. It is observed here also that the phase angle remains almost constant only in the lower frequency zone, that is, 1 khz to 1 khz. The corresponding ph values of the solutions are also noted down. The phase angle in the frequency zone 1 khz to 1 khz, where it remains almost constant, are tabulated in Table 1 for pure and different percentages () of adulterated. The measured ph values are also given in Table 4. From the Table 4, there is an indication that with the increase of -11-12 -13-14 -16-17 -18-19 1 2 3 4 5 6 7 8 9 1 pure 6% of 12% of 18% of Fig. 4. Phase angle behavior in pure (average) and different percentages of adulterated () -16-17 -18-19 1 2 3 4 5 6 7 8 9 1 After neutralizing with NaOH Fig. 5. Phase angle behavior of the sensor in pure and adulterated (18% ) after neutralizing by NaOH. Each graph shows the average for five sets of readings. percentage of adulteration with, the phase angle decreases. is slightly acidic in nature with ph value 6.74. Addition of makes the more acidic and hence its ph value has a decreasing trend. The plot of the average of the phase angle of the CPE of the 5 sets of readings when it is dipped in pure and different percentages of adulterated is shown in Fig. 4. It is clear from the plotting that as the adulteration is increased phase angle also decreases from 17 o (pure ) to 12 o (up to 18% of adulteration). The adulterated with 18% of is neutralized with NaOH to bring back the ph value of adulterated to that of the pure ph value i.e., 6.74. After addition of NaOH, the phase angle of adulterated (18% of ) increases from 12 o to 17 o, which is the constant phase angle of pure. The plot is shown in Fig. 5. 747
-11-12 -13-14 -16-17 -18-19 6 12 18 2 khz 3 khz 4 khz 5 khz 6 khz Poly. (5 khz) Impedance kohm 9 8 7 6 5 4 3 2 1 6 12 18 2 khz 3 khz 4 khz 5 khz 6 khz Poly. (3 khz) % of adulteration % of adulteration Fig. 6. Phase angle versus % of adulteration Fig. 7. Impedance versus % of adulteration The temperature and humidity of the room has been recorded and it is almost kept constant throughout the experiment. It is observed that at the frequency of 5 khz the phase angle of CPE, when it is dipped in pure, has almost same value for all five sets of readings or in other words the deviation from the mean is minimum. So if the measurement is done at 5 khz then there is a remarkable change in the values of the phase angle of CPE when it is dipped in pure, adulterated with 6%, 12%, 18% and then addition of NaOH as a neutralizer. But the ph measurement is not sufficient to detect the adulteration of if there is addition of and then addition of NaOH. C Relation between the percentage of adulteration in and Phase angle The change of phase angle of CPE with percentage of adulteration of is plotted in Fig. 6 at some specific frequency for the purpose of its detection from pure where pure is specified as % of adulteration. The frequencies chosen are 2 khz, 3 khz, 4 khz, 5 khz and 6 khz and it is observed that all the plots are of same shape and are changing almost linearly. The variation of the phase angle follows the curve of equation 6 2 θ = 9 1 x +.266x 17.687 where θ = phase angle in degrees x = Percentage of adulteration The equation is obtained by the curve fit method. So from the equation it is clear that if phase angle is known, the percentage of adulteration can be calculated easily. D Relation between the percentage of adulteration in and Impedance The attempt has been taken to find out the relation between the percentage of adulteration in and impedance. In Fig.7 the measured impedance Z of CPE is plotted with respect to percentage of adulteration of at frequencies 2 khz, 3 khz, 4 khz, 5 khz and 6 khz.the equation obtained by using polynomial curve fitting is given below: 2 y =.4x.475x + 4.9378 where y = impedance in kohms x = Percentage of adulteration IV. ELECTRICAL EQUIVALENT CIRCUIT OF CPE The equivalent circuit model of the electrode-electrolyte interface is proposed here. Many researchers have studied the interface layer behavior of CPE dipped in different polarizing medium and proposed several electrical equivalent circuit models [11, 12]. In the present work LEVM [13] computer program for complex nonlinear least square data fitting is used to model the probes. Using this LEVM software, modeling of the probe with finite element as well as distributed elements is possible. Also the software gives the best fit for the complex impedance data. The measured impedance Z data when the CPE is dipped in pure for the lower frequency zone, i.e., 1 khz to 1 khz where the phase angle remains constant are fed to the LEVM software. The best fit equivalent circuit model of the probe is given in Fig. 8. In a similar procedure the equivalent circuit models of the same CPE are obtained with the help of LEVM software when it is dipped in different percentages like 6%, 12% and 18% of. In every case, the equivalent circuit remains same but only the values of the parameters are different. The values of R 1, R D, C P, Q and α, when CPE is dipped in different solutions having different ionic concentration are tabulated in Table 5. 748
Fig. 8: Equivalent circuit of the probe when dipped in pure. increases, the constant phase angle decreases from 17 o to 13 o. This means the phase angle of the CPE is different for plain and the with some impurity. While modeling the probe in pure, adulterated and different ionizing medium it is clear from Table 5 that as the adulteration of the pure increases, ph value increases, constant phase angle increases and α value increases. But for ph 4. solution (acidic), α obtained from LEVM software is.237 and constant phase angle obtained from LCR meter is 14 o. Again pure with 12% gives constant phase angle of 13 o which is very close to the constant phase angle of ph 4. solution. But α and ph value of this adulterated are.4 and 6.49 respectively which is mismatching. TABLE 5 THE PARAMETERS OF THE EQUIVALENT CIRCUIT When dipped in Solutions R 1 kω R D kω C P nf ph 4..139 54.275 6.75 1 3.155 ph 9.2.994 46.58 1 3 73.42 6. 1 12.237 + 6% of + 12% of + 18% of After Neutrali- Zing with NaOH 3.86 63.97 1 3 1.1 1.98 1 5.485 2.61 14.15 1 3.85 1.3 1 5.44 3.41 7.42 1 3.631.94 1 5.4 5.17 1.37 1 3.254.481 1 5.26 3.18 76.9 1 3.926 2.17 1 5.498 From the above Table 5 it is observed that as the becomes more acidic the capacitance and α value are decreasing. Though all the parameters are changing, the change in these two parameters is noticeable. V. DISCUSSIONS The experiment is performed with a CPE having coating thickness 45μm which gives different constant phase angle in the frequency range 1 khz to 1 khz when dipped in the different polarizing medium. As the adulteration of Q α VI. CONCLUSION In the above study, the PMMA based CPE sensor is used to detect adulteration which is not very common. The performance of the sensor indicates that it may be possible to identify or detect the adulteration of with and then addition of NaOH to neutralize the increased acidic nature by observing the changes of the phase angle of the sensor. The CPE sensor is cheap, biocompatible, robust, low cost and its parameters can be easily varied by varying the parameters such as ionic concentration of the medium, the area of contact with the polarizing medium, thickness of coating etc. The main advantage of using this sensor is that it does not contaminate the process under test. The above adulteration testing process is very simple and the sensor used is a single stick type probe and can be easily connected to the electronic circuit. ACKNOWLEDGEMENT The authors would like to acknowledge the excellent ooperation received from the UGC-DAE, Kolkata scientists and staff. REFERENCES [1] Karabi Biswas, Siddartha Sen, and Pranab Kumar Dutta, A Constant Phase Element sensor for monitoring microbial growth, Sensors and Actuators B: Chemical, 119, pp. 186-191, 26,. [2] M.Luz, V.Alonso, and M.Arturo Minor, Design and Construction of a System for Measuring the Concentration of Water in Milk, Journal of mass spectrometry, vol. 36, pp. 13137, 26. [3] J.H.Prentice, The Conductivity of -the effect of the volume and degree of dispersion of the fat, Journal Of Dairy Research, 29, pp. 131-139, 1962. [4] C.Peris, P.Molina, N.Fernandez, M.Rodriguez, and A.Torres, Variation in somatic cell count, Califirnia mastitis test,and electrical conductivity among various fractions of ewe s, J. Dairy Science, 74(5), pp. 1553-6, May 1991. [5] M.F Mabrook, and M.C.Petty, Application of electrical admittance measurements to the quality control of, Sensors and Actuators B: Chemical, vol. 84, Issues 2-3, 22. [6] M.F Mabrook and M.C.Petty, A novel technique for the detection of added water to full fat using single frequency admittance measurements, Sensors and Actuators B: Chemical, vol. 96, Issues 1-2, 23. [7]Anwar Sadat, Pervez Mustajab, and Iqbal A. Khan, Determining the adulteration of natural with synthetic using ac conductance measurement, Journal of Food Engineering, vol, 77, Issue 3, 26. 749
[8]Roy S. On the realization of a constant-argument immitanceor fractional operator, IEEE Trans. Circuits & Syst.,vol. 14,pp. 264-274, 1967. [9]G.Carlson, and C.Halijak, Approximation of fractional capacitors(1=s)1=n by a regular Newton process, IEEE Trans. Circuits& Syst.,vol. 11,pp. 21-213, 1964. [1]Karabi Biswas, Siddhartha Sen, and Pranab Kumar Dutta, Modeling of a Capacitive Probe in a Polarizable Medium, Sensors and Actuators A: Physical, vol. 12 (1), pp. 115-122, April, 25. [11]John-hyeon Chang, Jungil Park, Youngmi Pak, and James Jungho Pak, Fitting improvement using a New Electrical Circuit Model for the Electrode Electrolyte Interface, 3 rd International IEEE EMBS Conference Neural Engineering, Kohala Coast, Hawaii, May 2-7, 27. [12]A.Geddes, Historical evolution of circuit models for the electrolyte interface, Annals of Biomedical Engineering, vol. 25, pp. 1-14, 1997. [13]J.Ross Macdonald and Solartrom Group Limited 19 997, LEVM Manual, Version 8.8. 75