Design and Development of A Digital pH Meter Md. Sirajul Islam Electronics Laboratory, Scientific Information Unit, Atomic Energy Research Establishment, Bangladesh Atomic Energy Commission, Ganakbari, Ashulia, Savar, Dhaka-1349, Bangladesh Abstract The pH Meter is an essential instrument which is used to measure the pH of the aqueous solution. Actually, with the measurement of pH of the aqueous solution, one would be quite able to know the [H] of the solution. In this paper, design and fabrication of a Digital pH Meter using state-of-the-art technology has been reported. The Digital pH Meter having the Temperature Compensation Circuit and the pH Calibration Circuit with integrated circuits could be operated with input bias current of 10-12 A and on a power of 220 V, 50 Hz. The pH of the aqueous solution varies from 0 - 14 depending on the concentrations of the solution. For the solution of any strong acid (HCl), concentrations of the solution covering the range from 1 M to 0.000001 M have been taken into account. These solutions yield 0 - 6 pH. For the solution of any strong alkali (NaOH), concentrations of the solution covering the range from 0.000001 M to 1 M have been taken into consideration. These solutions give 8 - 14 pH. The pure water being the neutral solution yields the pH value of 7. Keywords: puissance d’ hydrogen, hydrogen ion concentration, aqueous solution, temperature compensation circuit, digital panel meter. 1. Introduction The acidity or alkalinity of the aqueous solution is determined in terms of the hydrogen ion concentration and hydroxyl ion concentration denoted by [H] and [OH] respectively. In particular, [H] of the aqueous solution can be measured in terms of its pH. In order to measure the pH of the aqueous solution, Digital pH Meter is of paramount importance. In context with the increasing demand of the pH Meter an attempt has been made to design a Digital pH Meter using the electronic components available in the local market. Essentially, pH is a measure of the concentration of hydrogen ions in the solution and is effectively a measure of acidity of the solution. Absolute measurement of pH and monitoring of its rate of change are important in chemistry. However, the Digital pH Meter is widely used in many areas viz. nuclear science, food industries, drink industries, pharmaceutical industries etc. -----------------------------------------------------------------------Corresponding Author : E-Mail : [email protected] The Digital pH Meter is found to be more useful compared to the Analog pH Meter. Followings are the reasons: (i) design methodology of the Digital pH Meter is relatively easier than the Analog pH Meter, (II) reading of pH is accurately obtained in the case of Digital pH Meter whereas in the case of Analog pH Meter the pH reading is not so accurately obtained and (iii) outlook of the Digital pH Meter is nice compared to the Analog pH Meter. The concept of pH appears in the literature in a very useful way . 2. 2.1 Materials and Method Theory of pH Before we design the Digital pH Meter, the theory of pH should be taken into consideration. However, the pH of the solution as introduced by S. P. L. Sörensen (1868-1939), a Danish chemist, working in the Carlsberg Laboratory in 1909 has been defined as  pH = − log [H+] (1) where pH = puissance d’ hydrogen  or power of hydrogen , [H+] = concentration of hydrogen ion (mol/L). On the eve of designing the Digital pH Meter, its block diagram has been thought to be of enormous importance. The block diagram of the Digital pH Meter is exhibited in Fig.1. INPUT PREAMPLIFIER SUMMING AMPLIFIER pH ELECTRODE DIGITAL PANEL METER OUTPUT pH TEMPERATURE COMPENSATION CIRCUIT pH CALIBRATION CIRCUIT Fig.1. Block diagram of the Digital pH Meter 2.2 Design Methodology The Digital pH Meter is considered to be composed of five sections viz. (i) Pre-Amplifier, (ii) Temperature Compensation Circuit, (iii) Summing Amplifier, (iv) pH Calibration Circuit and (v) Digital Panel Meter . The schematic diagram of the Digital pH Meter is shown in Fig.2. Fig.2. Schematic diagram of the Digital pH Meter In order to design the Digital pH Meter the following methodologies can be explicitly dealt with:  Sensing Element : pH Electrode Indeed, the pH electrode acts as the sensing element. The pH electrode while immersed in the aqueous solution, generates an electrical signal of very small amplitude. In the present work, the combination type pH electrode has been used. In general, the combination type pH electrode consists of two half cells viz. [i] pH glass cell and [ii] reference cell. The internal half cell consists of a high resistance glass tube which has a pH sensitive glass membrane at the lower end. There is an internal reference system within this tube which remains constant. This cell is referred to as the glass half cell. The reference cell is housed concentrically between the outer body of the electrode and the glass half cell. This reference cell comprises a reference element (silver/ silver chloride or a calomel type) and an electrolyte solution which seeps through a porous liquid junction to make the necessary electrical connection with the sample. The cell generates a small voltage which provides the glass half cell with a stable reference voltage against which all measurements can be carried out.  Pre-Amplifier The signal-level as obtained from the output of the pH electrode is found to be very small. Therefore, the necessity has been felt to amplify this signal to an optimum level. To meet this requirement, a Pre-Amplifier is used in the present experimental set up. Hopefully, the Pre-Amplifier has been realized with IC CA3140E along with some essential components.  Temperature Compensation Circuit In fact, the temperature being a vital parameter needs to be kept at a constant level of 25C during the measurement of pH of the solution. Accordingly, the pH of the solution should be measured keeping the ambient temperature at a constant level of 25C. To maintain the ambient temperature at a constant level of 25C seems to be extremely difficult. To overcome this difficulty, the Temperature Compensation Circuit has been incorporated in the present experimental set up as if the measurement of pH of the solution is being carried out at 25C although the ambient temperature goes beyond this figure. In fact, the Temperature Compensation Circuit has been constructed with VR1 along side the IC CA3140E.  Summing Amplifier The negative signal-level as achieved from the Pre-Amplifier circuit needs to be converted into the positive signal-level. At the same time, the signal-level of the output of the Pre-Amplifier circuit is not sufficient enough. In order to meet these requirements, a Summing Amplifier has been used in the present experimental set up The Summing Amplifier has been realized with IC UA741CP along with some useful components. .  pH Calibration Circuit In order to calibrate the instrument with the known value of pH the Calibration Circuit is of paramount importance. To meet this requirement the pH Calibration Circuit has been introduced in the present design. In fact, the pH Calibration Circuit has been realized with VR5 along side the IC UA741CP.  Digital Panel Meter The output of the Summing Amplifier has been fed to the input of the Digital Panel Meter. The function of the Display Circuit is to convert the D.C. voltage into digital form. This function can be performed with the help of a Voltage-Controlled Oscillator (VCO). Thereafter, this digital signal need to be fed to a counting system consists of decade counter, decoder/driver and Light Emitting Diode (LED). It is of importance to note that The VCO, LED, decade counter, decoder/driver all are housed in a single unit. To fulfill this requirement, the DPM unit LASCAR made by Lascar Electronics Inc. 3750 West 26th Street, ERIE, PA 16606, United States of America, has been used. 2.3 List of the Components Used in the Digital pH Meter A list of the components used in the Digital pH Meter as we have designed has been presented in Table 1. Table 1. List of the components used in the Digital pH Meter SL. NO. Name of the components Value/Type of the components 1 Integrated Circuit (IC) CA 3140 E, UA 741 CP 2 Resistor 3 Variable resistor 4 Capacitor 5 Digital Panel Meter (DPM) R1 (2 k), R2 (100 R), R3 (2.2 k), R4 (33 K), R5 (1.6 k), R6 (1.2 k), R7 (100 k) VR1 (1 k), VR2 (10 k), VR3 (5 k), VR4 (100 K), V R5 (5 k), VR6 (10 k), VR7 (5 k) C1 (100 nF),C2 (1000 µF), C3 (100 nF), C4 (100 nF), C5 (100 nF), C6 (10 µF) Lascar Electronics Inc. 6 pH electrode Combination type 3. Results and Discussion Using the designed Digital pH Meter, the pH values of the solution of HCl and NaOH at different concentrations along with pure water have been measured at 25C . The results as obtained have been tabulated in Table 2. It may as well be pointed out that the functions of the Digital pH Meter we have designed Is quite satisfactory and it can be used to measure pH of the aqueous solution extending the range from 100 10−14 mol/ L. During the design of the circuits all out efforts have been made in order to obtain a good quality Digital pH Meter. In context with the increasing demand from various sectors of the country the Digital pH Meter has been designed using the electronic components available in the local market. The Digital pH Meter has been designed in a cost effective way. The Digital pH Meter as we have designed reveals state-of-the-art technology. The calibration of the Digital pH Meter is of importance. For calibration of the Digital pH Meter, buffer solutions having different values of pH (4 pH, 9 pH etc.) are normally used. During calibration of the Digital pH Meter the temperature has been kept at 25C. The calibration procedures are as follows: (i) The ”Iput” (point A) should be kept at 0V, (ii) The VR5 and VR7 needs to be kept at middle position, (iii) The “TEMP-COMP” pot (VR1) should be kept at minimum position, (iv) The switch needs to be kept at “Measure” position and then the “OFFSET NULL” (0 V) for the input of 741 IC(point C) should be made by rotating the “ASYM-POT” (VR4) using a precision Multimeter, (v) The switch now needs to be kept at “Standby” position and then the “OFFSET NULL” (0 V) for both the outputs of 3140 IC (point B) and 741 IC (point D) should be made by rotating VR2 and VR6 using a precision Multimeter, (vi) Now, the switch needs to be kept at “Measure” position and then the “ASYM-POT” should be rotated until the 741 input (point C) becomes 7.02 V. At this stage, the output of 741 (point D) will be 0.7854 V. If this voltage is not obtained then VR5 should be rotated until it becomes 0.7854 V, (vii) The DPM will now indicate 7.00. If the DPM does not indicate this value then it should be made 7.00 by rotating VR7, (viii) Using the buffer solution having the pH values of 4 and 9 etc. the Digital pH Meter can be calibrated. Now, using the Digital pH Meter, the pH of the solution of (i) HCl having the concentrations covering the range from 1 M to 0.000001 M, (ii) NaOH having the concentrations covering the range from 0.000001 M to 1 M and (iii) pure water have been measured, the data being presented in Table 2. In the present study, the solutions have been considered to be the “model solution”. This means that all the solutions provide exactly the concentrations as specified for. The theory of pH introduced by S. P. L. Sörensen in 1909 as outlined in Eq. (1) is based on weak theoretical ground. The equation of the pH theory (Eq. 1) is dimensionally unbalanced. The temperature (T) being the vital parameter does not appear in the pH theory. Moreover, the pH reflects the arbitrary meaning rather than the actual meaning. The relation between [H+] and pH has not yet been verified experimentally. Still there are the possibilities to resolve all these problems meticulously in relation to pH . Table 2 . The data relating [H+] and pH of the solution of HCl and NaOH along with pure water at 25C Type of the solution [H+] pH (mol/L) 10 0 0 1.0 M HCl Condition of the solution Acidic 0.1 M HCl Acidic 10 1 1 0.01 M HCl Acidic 10 2 2 0.001 M HCl Acidic 10 3 3 0.0001 M HCl Acidic 10 4 4 0.00001 M HCl Acidic 10 5 5 0.000001 M HCl Acidic 10 6 6 Pure H2O Neutral 10 7 7 0.000001 M NaOH Alkaline 10 8 8 0.00001 M NaOH Alkaline 10 9 9 0.0001 M NaOH Alkaline 1010 10 0.001 M NaOH Alkaline 1011 11 0.01 M NaOH Alkaline 1012 12 0.1 M NaOH Alkaline 1013 13 1.0 M NaOH Alkaline 1014 14 The technical specification of the designed Digital pH Meter is of paramount importance. Accordingly, this issue has been addressed in Table 3. Table 3 . Technical specification of the Digital pH Meter SL NO. Features Specifications 1 Size 230 × 210 × 93 mm 2 Weight 1.5 kg 3 Material Plastic 4 Measuring range 0 - 14 pH 5 Resolution 0.01 pH 6 Input impedance 1012 Ohms 7 Accuracy ± 0.01 pH 8 Type of pH electrode Combination type 9 Temperature compensation Manual 10 Input bias current 10-12 A (Approx.) 11 Power requirement 220 V, 50 Hz 12 Readout 3.5 digit 13 Price 35,000 Tk. 4. Conclusion The temperature being a vital parameter needs to be kept at a constant level of 25C during the measurement of pH of the aqueous solution. However, the pH of the solution should be measured keeping the ambient temperature at a constant level of 25C. Indeed, to maintain the ambient temperature at a constant level of 25C appears to be extremely difficult. To overcome this difficulty, the Temperature Compensation Circuit has been used in the present experimental set up as if the measurement of pH of the solution is being carried out at 25C although the ambient temperature goes beyond this figure. Acknowledgement The author is grateful to Dr. Md. Abdul Mannan Chowdhury, Professor, Department of Physics, Jahangirnagar University, Savar, Dhaka, Bangladesh, for his continued support and inspirations in this work. References 1. M. Mahbubul Huque and M. Ali Nawab, “Principles of Physical Chemistry”, Student’s Publications, Dhaka, 3rd ed., chap. 12, Bangladesh, p. 493 (1974). 2. Jerome K. Holmes and Victor S. Krimsley, Ph. D., “Introduction to General Chemistry”, The C. V. Mosby Company, United States of America, 3rd ed., chap. 13. p.258 (1976). 3. Vojtech Fried, Hendrick F. Hameka and Uldis Blukis, “Physical Chemistry”, Macmillan Publishing Co., Inc., 866 Third Avenue, New York, 10022, United States of America, chap. 24. p.883 (1977). 4. “ORION : Laboratory Products and Electrochemistry”, ORION Research Inc., 500 Cummings Center, Beverly MA,01915-9846, United States of America, chap. “pH Theory and Practice”, p.85 (1977). 5. Farhana Hafiz, Fahmida Akter, Mohammad Abu Sayid Haque, Md. Serajul Islam, Md. Abdul Mannan Chowdhury and Md. Arif Hossain, “Design and Development of Microcontroller-Based Digital pH Meter”, Technical Report. AERE/TR-12&13, June 2009, pp. 340-343 (2009). 6. Md. Sirajul Islam, “Design and Development of the Universal Scales to Unveil the Laws of Solution with Application to Physical Chemistry”, An on-going research work leading to D. Sc., University of Dhaka.