Acidic

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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 [1].
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 [2]
pH = − log [H+]
(1)
where pH = puissance d’ hydrogen [3]
or
power of hydrogen [4],
[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 [5]. 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:
[1]
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.
[2]
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.
[3]
Temperature Compensation Circuit
In fact, the temperature being a vital parameter needs to be kept at a constant level of
25C 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 25C. To
maintain the ambient temperature at a constant level of 25C 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 25C although the ambient temperature goes beyond this
figure. In fact, the Temperature Compensation Circuit has been constructed with VR1
along side the IC CA3140E.
[4]
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.
.
[5] 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.
[6]
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 25C . 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 25C. 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 [6].
Table 2 . The data relating [H+] and pH of the solution of HCl and NaOH along with pure
water at 25C
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
1010
10
0.001 M
NaOH
Alkaline
1011
11
0.01 M
NaOH
Alkaline
1012
12
0.1 M
NaOH
Alkaline
1013
13
1.0 M
NaOH
Alkaline
1014
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 25C
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 25C.
Indeed, to maintain the ambient temperature at a constant level of 25C 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 25C 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.
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