DIFFERENT TYPES OF pH MEASUREMENT TECHNIQUE
Abstract: -
pH is a measure of how acidic or
basic a substance is. In our everyday routine, we encounter and drink many
liquids with different pH. Water is a neutral substance. Soda and coffee are
often acidic. 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. An acidic substance is
anything that will give up a proton. A basic substance will accept a proton.
There are several methods of measurement the pH that are-
method
of pH indicator, which is A number of organic substances are known which show
distinctly different colours below and above a small pH range. Thus
phenolphthalein, the common acid base indicator, is colourless below a pH of
8.3 and distinctly pink above 10.0. Over the pH range 8 to 10 it changes colour
gradually through different shades of pink. By 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. By the
Quinhydrogen-electrode method a rapid and easy determination of pH is possible.
Antimony-Antimony Oxide Electrode also operates as an electrode reversible to
hydrogen ion concentration. It consists of a rod of antimony placed in a
solution having hydrogen ions. The antimony surface is cleaned and polished
with fine emery paper. The glass electrode method
uses two electrodes, a glass electrode and reference electrode, to determine
the pH of a solution by measuring the voltage (potential) between them.
This method is the one most commonly used for pH
measurement, since the potential quickly reaches equilibrium and shows good
reproducibility, and because the method can be used on various types of
solutions, with oxidizing or reducing substances having very little impact on
the result.
Introduction: -
pH is
a measure of how acidic or basic a substance is. In our everyday routine, we
encounter and drink many liquids with different pH. Water is a neutral
substance. Soda and coffee are often acidic. 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. An acidic substance is anything that will give up a proton. A basic
substance will accept a proton
The pH
formula is:
pH =
-log ([H+])
The pH scale starts from the number
0 and ends at the number 14. These numbers allow the classification of
substances based on their pH; the most acidic substances will be close to 0,
while the most basic or alkaline substances will be close to 14. The lower the
pH, the more H+ ions will be present and the stronger the acid. The most basic
or alkaline substances will have a classification between 7 and 14.
pH = −log ([H+])
The formula for pH is shown above.
pH is defined as the negative log base 10 of the hydronium concentration.
The pH is a logarithmic measure of
the concentration of hydrogen ions in a solution. Because pH is on a log scale
that means that increasing the pH by 1 corresponds to multiplying the
concentration of H+ ions by 10! So even though the difference between pH 6 and
pH 7 might sound small, it’s actually quite sizeable. For the pH equation, the
concentration of hydrogen ions is always a molar concentration, that is, moles
of H+ per liter. If you know the concentration of hydrogen ions, then
calculating the pH is just plugging in to the pH equation. Sometimes, a problem
will tell you that an acid completely dissociates into ions in solution. In
this, knowing the hydrogen ion concentration is straight forward.
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.
1.
Method of pH Indicator.
A number of organic substances are
known which show distinctly different colours below and above a small pH range.
Thus phenolphthalein, the common acid base indicator, is colourless below a pH
of 8.3 and distinctly pink above 10.0. Over the pH range 8 to 10 it changes
colour gradually through different shades of pink. Methyl orange shows a
distinct red colour below pH 3.1 and a yellow colour above a pH of 4.4. The use
of these two indicators could easily tell if the pH of a solution is below 3.1,
between 3.1 and 4.4, between 4.3 and 10.0 and above 10.0.
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 |
To reduce the labour involved in
the choice of appropriate indicator, several companies have marketed proprietary
mixtures of indicators under trade names as Universal Indicators. From the
colour developed with the universal indicator, one can easily fix the
approximate value of the pH of a solution. Then a more appropriate indicator
can be chosen such that approximate pH value of a solution under test falls
within the working range of this indicator. By comparing the colour developed
on mixing the indicator with the solution with already prepared standard colour
plates or charts, the pH of the solution under test can be fixed with
remarkable accuracy. For quickness and convenience, now we have Indicator
Papers made from universal indicator as well as from short range indicators.
Advantages
of pH indicators: -
The
advantages of pH indicators are as follows-
·
This
pH determination technique is quick, inexpensive and easy.
·
They
quickly determine the nature of the sample, whether it is acidic or basic.
·
Saving
time and money produces quick results.
·
It's
quick and easy to use than pH meter.
·
pH
indicators are very easy to use, just add a few drops to the sample.
Disadvantages
of pH indicators: -
The
disadvantages of pH indicators are as follows.
·
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.
·
Acid-base
indicators show only one- or two-color changes.
·
Indicators
measure pH with low accuracy, they only indicate the acidity or alkalinity of
the sample and not the exact pH.
2. Potentiometric method.
This
is the 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 for this purpose:
(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. 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. A
simple arrangement for determining the pH H value by using hydrogen electrode
is shown in Fig. It is known that hydrogen electrodes give reproducible
results, still the use of normal hydrogen electrode as a standard electrode or
a half cell gives certain practical difficulties. It is therefore rarely used
in routine pH measurements.
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) t is
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.
(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
From
the above equation it is clear that electrode potential will change with change
in the concentration of hydrogen ions. For convenience 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
The electrode is set up by
immersing a bright platinum wire or foil into the test solution containing
excess of quinhydrone (0.5 to 1.0 per 100 ml of solution). The platinum wire
must be cleaned with chromic acid and water. For preparing quinhydrone,
dissolve 60 g of ferric alum in about 100 ml of water at about 60°C and pour it
into a warm solution containing 5g of hydroquinone in 60 ml of water.
Quinhydrone precipitates out as fine dark green needles. Cool using ice and
filter by suction. After recrystallising from water, dry the crystal’s on
filter paper.
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.
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.
(v) It is
not stable for long time, particularly above 30°C.
(vi) It
alters the equilibrium between quinone and hydroquinone.
(c)
Glass Electrode
When
a glass surface is kept in contact with a solution, a potential is established
between the glass and the solution. The value of potential is a function of H*
ion concentration of the solution and the nature of the glass electrode. This
observation was made by Haber and Klemensiewiez in 1909 and now forms the basis
of a method for determining the pH of a solution. The magnitude of the
potential difference at 25°C is given by,
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
depending upon the nature of the glass.
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. 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 may be used in the presence of strong oxidising and reducing solutions in
viscous media and in presence of proteins which interfere with operation of
other electrodes.
(ii)
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.
(iii)
It is immune to poisoning.
(iv)
It is simple to operate.
(v)
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.
(iv)
It cannot be employed in pure ethyl alcohol, acetic acid and gelatine.
(d) Antimony-antimony
Oxide Electrode.
This electrode also operates as an
electrode reversible to hydrogen ion concentration. It consists of a rod of
antimony placed in a solution having hydrogen ions. The antimony surface is
cleaned and polished with fine emery paper. Antimony takes up a thin adhering
layer of Sb₂O3, almost instantaneously when exposed to air. This
oxide is not dissolved by moderate acid concentrations.
The oxide gives an equilibrium
concentration of Sb³+ ions in solution
Sb₂O3+H₂O 2Sb³+
+ 6 0H-
According to the above equation,
the solubility product of Sb₂O3 can be written as
Ksp = [Sb³+]²
[OH-]6
From
water equilibrium,
[OH-]= kW/[H+]
Electrode
reaction, Sb3+ + 3e- = Sb0
ESb3+
, sb = E ͦ- (RT/3F) ln ([Sb]/[Sb3+])
=
E ͦ+ (RT/3F) ln Sb3+
ESb3+
, sb = E ͦ - (RT/3F) ln (Ksp1/2/[OH-]3)
=
E ͦ
+ (RT/3F) ln Ksp1/2
- (RT/3F) ln [OH-]3
ESb3+
, sb = E ͦ + (RT/6F) ln
Ksp - (RT/F) ln (Kw/[H+])
This
means that the values of E, can be used to determine the pH of the solution.
The antimony rod used is about 2-3 cm length. It is enclosed in an ebonite tube
or glass sleeve and is soldered to a copper wire lead, one end of antimony rod
protrudes out. This end is polished frequently. The electrode is stored in
water for repeated use.
Advantages of Antimony
Electrode
(i) It is
robust and can be used as a probe for viscous solutions.
(ii) It
can be used for determining pH in the range of 4 to 12.
·
(iii)
It does not contaminate the solution to be tested.
(iv) It
has a low resistance.
Disadvantages of Antimony
Electrodes
(i) It
cannot be used in presence of dissolved oxygen, oxidising agents, H₂S, heavy
metal ions, highly
acidic and
alkaline solutions.
(ii)It is
sensitive to temperature changes.
Semiconductor sensor method: -
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.
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
