Development of Fiber Optic Ph Meter Based on Colorimetric Principle a L Chaudhari & a D Shali Gram

Indian Journal of Pu re & Applied Ph ys ics Vol. 40. Febru ary 2002, pp. 132- 136

Development of fiber optic pH meter based on colorimetric principle A L Chaudhari & A D Shali gram

Depa rt ment of Electroni c Science, University of Pune, Pune 4 1 I 007

(E-mai I: ads@electro nics.unipune.ernet. in)

Received 12 June 2001; revised 30 October 200 I ; accepted 6 December 200 I

A fi be r optic pH sensor, havi ng co nstructi on of probe based on th e co lorimetric principle is descri bed. The probe co nsists of a bun dle of fibers wi th the central ti ber as receiving fiber and the outer ring of fibers as transmi tting fibers. The LED' s of different co lours are used as sou rce and ph otod iode as detector. The probe is tested with pH buffer so lution prepared by th e usual meth od with un iversa l indi cator to get co loured so lutions depending upon va lue of p H. The changes in the vicinit y of the sens itive tip cause a variation in attenuation of specific renected visible radiation bands . Initial res ul ts and performance specifications using ex tern ally add ed un iversal indica tor are descri bed.

1 Introduction miniaturizati on, electrical insul ati on, and immunity from electromagneti c interference. A non-electri cal The pH is a number that exactl y describes the pH sensor has attractive features for many degree of acidity or basicity of a solution. The pH of biological and medical settings. a solution is import ant in many di fferent areas such as bi ochemi stry, agri culture, food science, chemi cal In this paper, the development of fi ber optic pH research and engineering, envi ronmental research meter is described. The primary phenome non used and polluti on contro l. Protecting our waterways is the detecti on of change in colour of th e solu tion requires constant monitoring of industrial effluent under test mixed w ith uni versal indicator. A wastewater, as also from washers in mining compact fiber optic sensor probe capabl e of operatio n. C hemi cal pl ants often have alkaline illuminating th e solution with different coloured wastewater. The pH measurement is used to guide li ght sources (LED 's) is developed . The li ght from the proper neutrali zati on of these pl ants as well as to emitting fi ber passes through soluti on, gets re fl ected monitor the fin al effluent qua li ty. at the refl ector and transverses back through the solution again to get coll ected by receiving fiber, Electrometric and colorimetri c methods are two 1 whi ch guides it to the detector. T he responses classical methods fo r pH determination • In the first obtain ed by the probe from the di ffe rent standard case the traditional in struments used to measure pH pH solutions are studied. Cali bration of the probe is of soluti on, is the glass electrode. T he colorimetric also carried out. The entire system is therefore meth od is based on absorption or fl uorescence using significantly simpler than most fl uorescence-based indicators that reveal th e acid or base c haracter of sensors, as no monochromator or optical filters are solu tion th rough th e change of colour. Opti cal fiber employed and both th e li ght source and detector sensors designed to-date are largely based on consists only of solid-state electronic components. monitoring the absorption change of several T his paper describes the working principle, immobi li zed ind icators (bromothymol blue' , phenol 3 constructio n detail s, electronic circu it s and rcd , etc.) or change in fluorescence (e.g. intensity, mechanical assembli es of th e pH meter. Result s decay ti me) of fluorometric indicators (e.g. acrid ine obtai ned are presented and d iscussed. and 2-napt h o l ~). The development of optical fiber based pH sensors has been a subject of interest over 2 Measurement Principle the years, particularly fo r biomedical appl ications'. Colorimetry deals wi th the mea ure ment of Fiber-opti cs offers great advantages over coloured in tensity. The colour of a substance is due conventional potentiometric sensor: possibility of CHAUDHARl & SHALIGRAM: FIBER OPTIC pH METER 133

to the absorbance of li ght waves of certain thus obtained for the different p H soluti ons a wavelengths. The absorption of li ght by solution simplified detection mechani sm is deve loped. results in excitation of e lectrons in its molecule. ILLUMINATION fiber ring

.._-::rF2

,.,..1---1~1----"-F Fig. 3-Typical fiber bundle arrangement in probe

s

R

Fig. I - Working principle diagram ILS 1 and LS f Li ght Source I and 2. RF: receiving fiber, D: detector, TF1 and TF2: transmitting fiber I and 2, SL: solution, R; renector)

Fig. 4- Front view of R4R8-7: Rellection fibe r pro he

3 Constructional Details of pH Meter Fig. 2 shows the block diag ram of fiber opti c pH sensor based on colorimetric principle. It consists of fiber optic pH probe, c he mi cal cell , different li ght Fig. 2 - Block diagram of experimental set-up for pH sensing sources with dri ver c ircuit s, detector, and signal conditioning c ircuit and displ ay. The designing of The working principle of pH meter is illustrated fiber optic pH sensor probe is a critical job. Various in Fi g. I. The li ght sources with different colours sensin g confi gurations have bee n reported for 5 are adjusted to emit the same intensity. Now when chemical sensin g . This paper represents the typical this li ght travels through the solution from the probe fiber arrangement shown in Fig. 3 where the inner end to reflector and back, depending on th e colour fiber collech the return li ght due to reflecti on and ab orption characteristics of the solution, different outer fiber ri ng transport the incoming li ght. intensities will be received for different colours. Fig. 4 show::. front view of designed probe Thest.: can be plotted against the wavL: Ie ngth of th e R488-7: Reflection fiber probe. The multi­ source to obtain a fairly simple absorption spectrum wavelength reflect1 e probe consis ts of a bundle nf a: those di:crete wavelength:. From the spectrum 134 INDIAN 1 PURE & APPL PHYS, VOL 40, FEBRUARY 2002

seven fibers - six ILLUMINATION fibers around distilled water. The desired pH buffer solution7 are one READ fiber. Each fiber is a multi-mode plastic prepared by adding stock solution in the proportion fiber of 488 11m core diameter with a numerical shown in Table I . aperture of 0.47. The length of each fiber is 90 mm. The fiber tips were polished with zero emery paper. Table I - Proportion of pH butTer solution A 30 mm x 6.9 mm brass cylinder houses the fiber Sr. No . 0.2M Na2HP04 0. 1M Ci tri c acid Measured bundle. To the sensing tip end of fiber bundle a (ml ) (ml) pH round cut glass pl ate is press fitted in order to avoid 11 .40 28.60 3. 12 damage of polished tip due to interaction with 2 16.56 23.44 3.72 chemical under test. A circul ar brass di sc of 3 20.60 19.40 4.40 5.18 diameter 24 mm and height 15 mm holds hi gh 4 24. 18 15 .82 5 29 .1 0 10.90 6.25 bri ght LED's in a c ircular fa shion round a 6 36.34 3.66 7.31 photodiode on one sid e and the oth er side holds 7 38.90 1.10 7.90 fibers in front LED's and photodiode . One side of th e assembly is enclosed in brass cylinder of 24 mm The 0.5 ml of universal indicator (Indikrom, diameter and 55 mm length, while the oth er side is Uaali gens Fine Chemicals) with help of micro­ enc losed in aluminium cylinder of size 30 mm in burette is added to each solution and shaken well. diameter and 30 mm in length wi th DIN connector The pH of solutions is measured with the help of for electrical connections. standard pH meter of ELICO (fndia). The intensity The high brightness 5 mm LED's of different of colour of the indicator in an aqueous solution colours are used as li ght sources. LED's emits depends upon the degree of dissociati on, which is relatively narrow wavelength bands. They are also dependent on the pH value of the indicator. The amenable to direct intensi ty modulation, so that a solution of lower pH values exhibits red colour and mechanical chopper i5 not necessary. The desire to those havi ng too hi gh a pH value shows an yell ow use these devices in design of fluorescent sensors colour. Some solutions having in between pH values has therefore often been stated in the literature". The show an intermediate colour between red and laser diodes also can be used in place of LED's, yell ow. The absorption of solutions is determined as however, because of the requirement of large drive a fun ction of pH at appropriate wavelength. current, required additional heat sinks are considered to be difficult to handle. The driving r-- 5.5 -----l circuit of LED consists of a V to I converter, buffers 5 . and a subtractor. Photodiode is used as a detector. I 4.5 The signal conditioning consists of buffers, 4 -+-air 3.5 -pH=3.12 invertin g ampli fier with variable gain and a ~ 3 subtractor for zero-setting designed with opamps. > 2.5 l --....-pH=3.72 2 ·-pH=6.25 1 .5 . --pH=7.31 4 Experimental Proced';lre 1 0.5 4.1 Preparation of buffer solutions 0 ' ' ' '-•""'·'j A series of solutions of known pH between 3.12 '------0- is_ t_a _nc_e_m_ m_ ) ------and 7.90 are prepared by the procedure d iscussed below and an equal amount of commercially Fig. 5- Response of sensor with distance va ri ation avai lable universal indicator is added to each of these solutions to get coloured soluti ons. 4.2 Measurements

The stock solutions of 0 .2 M Na2HP0-1 is Initially the di stance between probe and prepared by dissoiving 28.4 g of Na2HP0-1 in one reflector is varied and the output for each di stance liter of di st illed water and 0.1 M citric acids by measured. The results obtained are shown in Fig. 5. dissolving 2 1.01 6 g of citric acid in one liter of It is observeri :r.:it at the probe-to-reflector spacing & CHAUDHARI SHALIGRAM : FIBER OPTIC pH METER 135

of about 2 mm separation between curves .-----·------, corresponding to different values of pH is optimum. 2.6 It was, therefore, decided to carry out all further 2.55 experiments by keeping th e separation fixed at 2 2.5 mm. i 2.45 The experimentation is carried out with the 0 2.4 ~ 0 prepared buffer solutions. The fiber optic pH probe > 2.35 is dipped into the chemical cell filled with buffer 2.3 solution of different pH values. By using the 2.25 selection logic, LED of a parti cular emission colour 2.2 is selected. As the colour of the solution depends on 3 4 5 6 7 8 its pH va lue, and the final reading of voltmeter on pH of Buffer Solutions the colour of illumination, the reading will in turn depend upon the pH value of the solution. Illumination fiber carries the light to the solution. Fig. 7-Sensor response for blue LED Depending upon the colour of the solution and illumination, light gets absorbed and reflected. The r------c------·----- reflected light is detected by the read fiber and gives the corresponding output. The light from the illuminated fiber is reflected at the surface of mirror - 3.5 (reflector) and received by read fiber through the :! 0 ~ liquid inside the cell. The received output voltage 0 depends upon the pH of solution. > 3.3 5 Results and Discussion The response of sensor is shown in Fig. 6 in the 3.1 3.6 4.1 4.6 5.1 5.6 6.1 pH range from 3.12 to 7.90. Response thus obtained pH of Buffer Solutions shows two trends. In the first trend output is low and '------~ increases with increase in pH value for the light of wavelength < 570 m (green), while in the second Fig. 8 -Sensor response for green LED trend output is high initially and decrease with increase in pH for wavelength > 570 nm. Absorption data were subsequently re-plotted versus pH to obtain the classic sigmoid curve for solutions with added universal indicator shown in 4 Fig. 7 for blue LED and Fig. 8 for green LED. In these curves, the regions with the greatest slope are 3.5 the regions with the greatest sensitivity to the pH ! ~RED change. Badni et a!. R studied the fluorescent 0 > 3 response of an impregnated sensitive dye­ 0 -BLUE > fluorescein isothiocyemate (FJTC) usin g sol-gel 2.5 --6--GREEN -+-YaLOW method under the condition of varying pH using excitation at 488 nm using fluorescent intensity for 2 -a-ORANGE 3 4 5 6 7 8 pH range 6-8. The results obtained matches well with results shown in Figs 7 and 8. Note that sensor pH of Buffer Solutions response is linear or reasonably linear over pH range 3.72-6.25 for blue LED and linear over pH range 3.6-5.6 for green LED, while the reported linear pH Fig. 6-pH Sensor response range is 7-9 using a fiber optic sensor based on 136 INDIAN J PURE & APPL PHYS, VOL 40, FEBRUARY 2002

optical absorption usin g~ a indicator compound 5- Acknowledgement 9 1. Deboux et al. 111 reported the linear range of pH The author wishes to thank the University 3- 10 using sensin g fiber of 200 ~rn Plastic Clad Grants Commission (UGC), WRO, Pune, for Silica (PCS) fiber with cladding removed leaving 30 financial support through Minor Research Scheme. mrn length of bare silica with meth ylene blue as The author also wishes to thank the Management, indicator in capsule in stai nless steel wi ndowed with Principal, and Head of Electroni cs and Physics, semi-permeabl e membrane (ion-selective ALPHA ASC College, Chopda (Jalgaon) and Prof M S 400 dialysis membrane). Nimase, New Arts, Commerce & Sc ience College, Ahmednagar, for their cooperation for this research ---~ work. I References

-+-- pH=3.12 Bacoi M. Baldini F. Brenci M, Confort i G, Falci al R & -a-pH=3.72 Mi gnani A G, Reprinted with the ennission from _...._ pH=4.4 OFS'86:4th International Conference on Optical Fiber Sensor, In stitute of Electronics and Communicati on -pH=6.25 En gin ee rs of Japan (Tokyo, 19 86) -pH=7.31 550 575 600 625 650 ---- pH=7.9 2 Kirkb right G I, Narayan swam i R & Welt i N A. Analyst, 109 (1984) 1025 . W avelength(nm) L______, ______j 3 John I Peterson , Seth R Goldstein , Raphael V Fitzgerald & Buckhold D K, Anal Chem, 52 ( 1980) 864. Fi g. 9 -- Absorption spectrum 4 Draxler S & Lippitsch M E, Sensor and Actuators B, I I (1993 ) 42 1. The absorption spectra. usin g the developed 5 Wolfbeis 0 S, Reprinted wit h permi ssion from OFS: sensor are shown in Fig. 9. T wo distinct peaks are Proceedings of 6th Int ernational Conf eren ce OFS'89. (Ed ) observed at 580 nm and 620 nm with shift in peaks. H J Anti ny, J P Dalkin & R Th Korsten, (Springer-Verlag, Results thus obtained are simil ar to the spectra of Berlin, Heidelberg), 1989. 11 phenol red presented by Bacci et al. , who 6 Hauser Peter C & Ta Susie S S, Analyst, 118 ( 1993) 991 . immobilized the dye on Amberlite XAD-2 polymer 7 Vogel I, A textbook of quantitative inorganic analysis beads described by Kirkbright et af. There is a clear including elementW)' in strumental analvsis (The English separati on between the magnitudes of peaks for Language Book Society & Orient Longmans, UK), 3rd acidic and basic solutions. Ed n, 196 1, p. 11 62. 8 Badni G E, Gratten K V T & Tseung A C C, Analyst, I 02 The effort described in this paper has lead to the ( 1995) I 025. development of fiber optic pH meter, which is simple in constm ction and easy to calibrate and 9 Wolthuis Roger, David McCrae, Saaski Elri c, Hartl James & Mitcheil Gordon, IEEE Trans on Biomedical maintain . With the use of commercially available Engin eerin g, 39 ( 1992) 531. universal indicator it can measure the pH in the 10 Deboux B J C, Lewis E, Scully P .J & Edward s R, CD on range of 3.72-6.25 for blue LED source quite OFS( 1983-1997):Collected papers of International rei iable. Among the prepared the proposed Conf erence on Optical Fiber Sensors, Compiled and appli cations in sugar industri es and various agro Published by SPIE, OSA. based products. II Bacci M, Baldini F & Cheggi A M, Anal Chim Acta, 207 ( 1988) 343.