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1. DIFFERENT pH
MEASUREMENT TECHNIQUES
BY SAYAN KUNDU

2. Introduction: -
• pH is a measure of how acidic or basic a substance is.
• pH has a range of 0-14. A pH greater than 7 means the substance is basic.
A pH less than 7 means the substance is acidic. When the pH is exactly 7
that indicates that the substance is neutral.
• pH = -log ([H+])
Measurement of pH: -
There are two methods which are generally used for determining the pH of a
solution. These are:
1. Method of pH indicator.
2. Potentiometric method.

3. 1. Method of pH Indicator.
2. Table below gives the various indicators in use and their
working range in pH units.
Indicator Colour Change
of Acid
Colour change
of Alkali
pH range
Thymol Blue Red Yellow 1.2-2.8
Methyl Yellow Red Yellow 2.9-4.0
Methyl orange Red Yellow 3.1-4.4
Methyl red Red Yellow 4.2-6.3
Bromo thymol
blue
Yellow Blue 6.0-7.6
Phenol red Yellow Red 6.8-8.4
Cresol red Yellow Red 7.2-8.8
Phenolphthalein Colourless Pink 8.3-10.0

4. Advantages of pH indicators: -
• Quick, inexpensive and easy.
• Quickly determine the nature of the sample, whether it is acidic or basic.
• Saving time and money produces quick results.
Disadvantages of pH indicators: -
• The sample solution should be colourless enough to clearly observe indicator
colour changes.
• An indicator is not functional above its pH range because the indicator does not
change colour at these pH values.
• If the substance or sample is contaminated, the colour may be incorrect.
• Measure pH with low accuracy, they only indicate the acidity or alkalinity of the
sample and not the exact pH.

5. 2. Potentiometric method.
Most accurate method of determining the pH of a solution. For finding the pH value of the
solution we shall use an electrode reversible to H+ ions. Following are some of the electrodes
which can be used
(a)Hydrogen Electrode: -
A normal hydrogen electrode can be set up by bubbling pure hydrogen gas at a pressure of one
atmosphere through a solution of an acid in which activity of hydrogen ions is unity. For
detection of the electrode potential of this electrode a plate of an inert metal like gold or platinum
is so placed that it dips partly in the acid and hydrogen gas bubbles on it at a slow rate. A
platinized platinum plate is preferred as it permits the equilibrium value of potential to be
reached quickly. When the two electrodes are coupled together to form a galvanic cell, the
electrodes which has stronger tendency to lose electrons into external circuit acquires the
negative polarity and becomes the negative pole.

6. The other electrode then becomes the positive pole of the cell. If a
hydrogen electrode is immersed in a solution (the pH of which is
to be measured), and the half cells are coupled with a normal
hydrogen electrode by means of a saturated KCl bridge in order to
eliminate the liquid junction potential, the E.M.F. of the resulting
cell
Pt | H₂, H+ (a=1) || H₂, H+ (a=x) | Pt
can be calculated potentiometrically. The E.M.F. of the
concentration cell at 25°C, is given by
E=E ͦ- (RT/nF) ln(C2/C1)
=0- 0.0591 log (1/[H+] )
=-0.0591 pH
Thus simply by measuring the E.M.F. of the cell we can find out
the value of pH.

7. Advantages of Hydrogen Electrode
(i)It is a fundamental electrode to which all measurements of pH are ultimately
referred.
(ii) It can be used over the entire pH range.
(iii) It gives no salt error.
Disadvantages of Hydrogen Electrode
(I)It cannot be used in presence of air, dissolved oxygen, oxidising or reducing
agents.
(ii) The platinum black coating deteriorates and hence it should be renewed
from time to time. Sometimes it is poisoned due to the presence of compounds
like alkaloids, sulphides, cyanides arsenic and antimony salts.
(iii)Difficult to maintain the pressure of the hydrogen gas at a fixed value due
to mechanical difficulties.
(iv) It is not easy to get pure hydrogen

8. (b) Quinhydrone Electrode
Quinhydrone is a 1:1 molar compound of quinone and hydroquinone and in solution it provides equimolecular
quantities of these two substances.
C6H4O2.C6H4(OH)₂ → C6H4O₂ + C6H4 (OH)₂
Quinhydrone Quinone Hydroquinone
Quinone and hydroquinone and hydrogen ions form a reversible redox system.
C6H4O₂ + 2H+ + 2e → C6H4 (OH)₂
Quinone (Q) Hydroquinone (QH₂)
If an inert electrode such as platinum is immersed in this system, the electrode potential is given by:
E=E ͦ- (2.303 RT/2F) log([QH2]/[Q][H+]2)
= E ͦ- (2.303 RT/2F) log([QH2]/[Q]) + (2.303 RT/2F) log[H+]2
Now in aqueous solution of quinhydrone,
[QH2]=[Q]
log([QH2]/[Q])= log 1 =0
E= E ͦ+ 2*(2.303 RT/2F) log[H+]
= E ͦ- 0.0591 pH at 25 deg C

9. A calomel electrode often replaces the normal hydrogen
electrode. The complete cell can be represented as :
Hg | Hg₂ Cl₂ (s), KCl saturated || H* (unknown) Q. QH₂ | Pt
In this cell the oxidation takes place on the calomel electrode
while reduction takes place at the quinhydrone electrode. Using
reduction electrode potential values, we have
pH= (0.4576- Ecell)/ 0.0591
Advantages of Quinhydrone Electrode
(i) It has a low resistance.
(ii) Equilibrium is reached quickly.
(iii) Its use is not affected by dissolved oxygen.
(iv) It can be used for micro-determinations.

10. Disadvantages of Quinhydrone Electrode
(i) It can be used for determining pH values less than 8
only.
(ii) The solution to be tested gets contaminated.
(iii) It suffers from salt error defect.
(iv) It cannot be used in presence of oxidising and
reducing agents.
EG= E°G-0.0591 log [H+]
= E°G+ 0.0591 pH
where EG is the potential of the electrode and Eº is a constant for the given
glass electrode
The glass electrode consists of a glass bulb with a long neck and is made of a
special type of glass of relatively low melting point. A solution of 0.1 M HCl
saturated with quinhydrone is filled up in the bulb. A platinum wire is
inserted into the solution for making the electrical contact as shown in Fig.
(c) Glass Electrode

11. The glass bulb containing the solution is then dipped in the beaker containing the solution whose
pH is to be determined. For determining the pH of the solution, the above half-cell is joined with
a reference electrode. The E.M.F. of such a cell can be determined by a potentiometer.
Advantages of Glass Electrode
(i) It can be used for solutions having pH values 2 to 10. With some special glass, measurements can be extended
to pH values up to 14.
(ii) It is immune to poisoning.
(iii) It is simple to operate.
(iv) The equilibrium is reached quickly.
Disadvantages of Glass Electrode
(i) In this electrode, the bulb is very fragile and therefore has to be used with great care.
(ii) As the glass membrane has a very high electrical resistance, the ordinary potentiometer
cannot be
used for measuring the potential of the glass electrode. Thus, the electronic potentiometers are
used.
(iii) Standardization has to be carried out frequently.

12. Semiconductor sensor method: -
The semiconductor pH sensor, replaces a glass electrode with a semiconductor chip. This sensor,
known as an ion sensitive field effect transistor (ISFET), is not only resistant to damage but also
easily miniaturized. Miniaturization allows the use of smaller amounts of sample for measurement,
and makes it possible to perform measurements in very small spaces and on solid state surfaces.
This sensor promises useful applications in measurement in the fields of biology and medicine.
Advantages of Semiconductor Sensor
• It is used for wide variety of applications such as cheese making, pool maintenance, to grow healthier
plants by measuring soil pH, stain removal etc.
• The meters provide numerical value of the pH directly.
• pH meters are very accurate and provide exact pH value with the help of pH sensors.
• It helps in determining how much acidic or basic any substance is.
Disadvantages of Semiconductor Sensor
• They are very expensive.
• pH meters are required to be calibrated.

13. Conclusion: -
The pH is an important property, since it affects how substances interact with one another
and with our bodies. In our lakes and oceans, pH determines what creatures are able to
survive in the water. Read on to learn vocabulary associated with pH, how to use the pH
formula, how to calculate pH, and why pH is an important measurement. pH is a measure
of how basic or acidic a substance is. pH has a range of 0-14. A pH greater than 7 means
the substance is basic. A pH less than 7 means the substance is acidic. When the pH is
exactly 7 that indicates that the substance is neutral