36
3.2.1 Hydrogen Ion
Concentration (pH)
pH is the negative logarithm of hydrogen ion
activity. pH of most natural waters falls in the
range of 4.0–9.0. However, majority of the water
are slightly basic because of the presence of carbonates and bicarbonates. A departure from the
normal pH could be due to influx of acidic or
alkaline wastes, mainly from the industries. pH
adjustments of effluents as well as water treatment plants are a common practice. Whereas
acidity and alkalinity are measures of total resistance to pH change or buffering capacity of a
sample, pH represents the free hydrogen ion
activity not bound by carbonate or other bases.
Hydrogen ion concentration, expressed as pH,
may be determined by the following methods:
(a) Colorimetric method
(b) Electronic method
3.2.1.1 Colorimetric Method
Colorimetrically, pH is determined on the spot
with the help of a Hellige Comparator using
BDH (Qualigens) indicator solution and colour
discs of the range 2.0–10.5.
However, the colorimetric method is said to
suffer from interferences due to colour, turbidity,
salinity, colloidal matter, oxidants and reductants.
Indicators themselves may alter pH of the poorly
buffered liquids unless preadjusted to nearly the
same pH as the sample. Hence, colorimetric
method is suitable only for rough estimation.
3.2.1.2 Electronic Method
This method involves operation of an electronic
pH meter using a standard ‘glass electrode’ along
with a ‘reference calomel electrode’ which produces a change of 59.1 milli volt (mv) per pH
unit at 25 °C. Glass is an ion-exchange material
with preference for H
+
ions which are adsorbed
on one surface and their positive charge is transmitted through the glass by Na
+
ion displacement. The H
+
ions are desorbed on the other side.
Therefore, glass is a membrane specific for H
+
ions. Glass electrode consists of a glass bulb with
dilute HCl in it and is connected to the internal
circuit through AgCl.
Materials Required
1. Electronic pH meter with temperature
compensation
2. Glass electrode
3. Reference electrode
4. Magnetic stirrer
Standard Solutions (Reagents)
(a) Standard buffer prepared by dissolving
required amount of substances in distilled
water (DW) having conductivity <2.0 microsiemens at 25 °C.
NB: The buffer solutions be preserved in
polythene or Pyrex bottles and the solutions
be replaced every month.
(b) Saturated potassium hydrogen tartrate solution to be prepared by dissolving finely crystalline 5–10 g of potassium hydrogen tartrate
in 100–300 ml of DW at 25 °C in a glass
stoppered bottle. The clear solution be
decanted and preserved by adding a thymol
crystal.
(c) Saturated KCl solution to be prepared by dissolving AR grade KCl in DW.
Procedure
1. The pH meter is connected to 230 V AC having good earthing and switched on. It is kept
as such for few minutes to warm up.
2. Before use, both the glass and reference electrodes be kept dipped in DW for at least 24 h.
NB: The tips of the electrodes be kept
immersed in water when not in use.
At the time of using the electrodes, they are
rinsed with DW and wiped with tissue paper and
never with hand.
pH Measurement
1. The electrodes are connected to the pH meter.
2. Buffer solutions of pH 7.0 and 4.0 or 9.2 are
prepared by grinding and dissolving standard
buffer tablets (of respective pH) in DW.
3. The electrodes are washed with DW and dried
with tissue paper.
3 Lentic Water (Physico-chemical Characteristics of Water)
3.2.1 Hydrogen Ion
Concentration (pH)
pH is the negative logarithm of hydrogen ion
activity. pH of most natural waters falls in the
range of 4.0–9.0. However, majority of the water
are slightly basic because of the presence of carbonates and bicarbonates. A departure from the
normal pH could be due to influx of acidic or
alkaline wastes, mainly from the industries. pH
adjustments of effluents as well as water treatment plants are a common practice. Whereas
acidity and alkalinity are measures of total resistance to pH change or buffering capacity of a
sample, pH represents the free hydrogen ion
activity not bound by carbonate or other bases.
Hydrogen ion concentration, expressed as pH,
may be determined by the following methods:
(a) Colorimetric method
(b) Electronic method
3.2.1.1 Colorimetric Method
Colorimetrically, pH is determined on the spot
with the help of a Hellige Comparator using
BDH (Qualigens) indicator solution and colour
discs of the range 2.0–10.5.
However, the colorimetric method is said to
suffer from interferences due to colour, turbidity,
salinity, colloidal matter, oxidants and reductants.
Indicators themselves may alter pH of the poorly
buffered liquids unless preadjusted to nearly the
same pH as the sample. Hence, colorimetric
method is suitable only for rough estimation.
3.2.1.2 Electronic Method
This method involves operation of an electronic
pH meter using a standard ‘glass electrode’ along
with a ‘reference calomel electrode’ which produces a change of 59.1 milli volt (mv) per pH
unit at 25 °C. Glass is an ion-exchange material
with preference for H
+
ions which are adsorbed
on one surface and their positive charge is transmitted through the glass by Na
+
ion displacement. The H
+
ions are desorbed on the other side.
Therefore, glass is a membrane specific for H
+
ions. Glass electrode consists of a glass bulb with
dilute HCl in it and is connected to the internal
circuit through AgCl.
Materials Required
1. Electronic pH meter with temperature
compensation
2. Glass electrode
3. Reference electrode
4. Magnetic stirrer
Standard Solutions (Reagents)
(a) Standard buffer prepared by dissolving
required amount of substances in distilled
water (DW) having conductivity <2.0 microsiemens at 25 °C.
NB: The buffer solutions be preserved in
polythene or Pyrex bottles and the solutions
be replaced every month.
(b) Saturated potassium hydrogen tartrate solution to be prepared by dissolving finely crystalline 5–10 g of potassium hydrogen tartrate
in 100–300 ml of DW at 25 °C in a glass
stoppered bottle. The clear solution be
decanted and preserved by adding a thymol
crystal.
(c) Saturated KCl solution to be prepared by dissolving AR grade KCl in DW.
Procedure
1. The pH meter is connected to 230 V AC having good earthing and switched on. It is kept
as such for few minutes to warm up.
2. Before use, both the glass and reference electrodes be kept dipped in DW for at least 24 h.
NB: The tips of the electrodes be kept
immersed in water when not in use.
At the time of using the electrodes, they are
rinsed with DW and wiped with tissue paper and
never with hand.
pH Measurement
1. The electrodes are connected to the pH meter.
2. Buffer solutions of pH 7.0 and 4.0 or 9.2 are
prepared by grinding and dissolving standard
buffer tablets (of respective pH) in DW.
3. The electrodes are washed with DW and dried
with tissue paper.
3 Lentic Water (Physico-chemical Characteristics of Water)