41
2. The ‘salinity’ control switch be set to the salinity
value of the sample under test. Sufficient time
be allowed for the probe to stabilise.
3. The DO value be read and recorded.
3.2.3 Free Carbon Dioxide (FCO 2 )
<10 mg/l of FCO 2 is said to be, generally, present
in surface waters. However, ground waters may
contain greater amount of FCO 2 .
FCO 2 may be estimated by:
(a) Nomographic method
(b) Titrimetric method
The nomographic method gives a closer estimation of FCO 2 when the pH and alkalinity are
determined immediately after sampling. The pH
be determined with an electrometric pH meter
calibrated with standard buffer solution in the pH
range of 7.0–8.0. The total alkalinity be determined titrimetrically.
The titrimetric method of estimation of FCO 2
be done using phenolphthalein as an indicator.
The titrimetric method is discussed below in
detail.
3.2.3.1 Titrimetric Method
of Determination of FCO 2
Principle
FCO 2 reacts with Na 2 CO 3 or NaOH to form
NaHCO 3 . Completion of the reaction is indicated
by the development of pink colour at the end
point (pH 8.3). In short, FCO 2 is estimated by
neutralising the dissolved CO 2 with 0.022 N
aqueous sodium hydroxide solution using phenolphthalein as an indicator.
Sampling and Storage
Surface water sample may be collected by dipping a
collecting bottle with double lid just below the water
surface; and after the bottle is filled in, its inner lid be
closed while the mouth of the bottle is still under
water. Deep water sample may be collected with the
help of a water sampler bottle. More precisely, sample water be collected by rubber tubing discharging
at the bottom of a 100- ml graduated cylinder or
Nelson’s tube. The sample water be allowed to overflow several times. The tube be withdrawn while the
sample water is flowing. The measuring cylinder be
flicked (jerked) to throw off the excess sample water
above the 100-ml mark. Some loss of FCO 2 may be
expected in storage and on transit of the sample,
even with a careful collection technique. Hence,
field determination immediately after sampling is
desirable. Alternatively, the sampling bottle be completely filled with the sample water and be preserved
at a temperature lower than the room temperature
and FCO 2 be estimated as soon as possible.
Reagents
1. Preparation of 0.1 N stock NaOH solution: It is
prepared by dissolving about 4 g of AR grade
NaOH pellets in 1 l of DW. It be standardised
against 0.1 N H 2 SO 4 using phenolphthalein as an
indicator. 100 ml of this 0.1 N solution diluted to
440 ml with DW gives N/44(0.022 N)NaOH.
Alternatively, 0.4735 g (accurately) of NaOH
(96 % pure) dissolved in 500 ml of DW gives
0.022 N NaOH.
Procedure
1. Sample water is collected following the above
procedure and 100 ml be taken in a conical flask.
2. Ten drops of phenolphthalein indicator solution be added.
3. If the sample turns pink, it indicates that FCO 2
is absent.
4. If the sample remains colourless, it be titrated
rapidly with 0.022 N NaOH solution (taken in
a 50-ml burette), stirring gently with a glass
rod until a definite pink colour is produced
which persists for 30 s when viewed through
the depth of the sample.
NB: For checking loss of FCO 2 during titration, a second sample be taken. After adding ten
drops of phenolphthalein indicator, the full
amount of the titrant used in the first titration be
rapidly poured in. If the sample turns pink, no
FCO 2 is said to be lost. If it remains colourless,
additional amount of the titrant be added until
pink colour appears. The result of the second
titration be considered.
3.2 Chemical
2. The ‘salinity’ control switch be set to the salinity
value of the sample under test. Sufficient time
be allowed for the probe to stabilise.
3. The DO value be read and recorded.
3.2.3 Free Carbon Dioxide (FCO 2 )
<10 mg/l of FCO 2 is said to be, generally, present
in surface waters. However, ground waters may
contain greater amount of FCO 2 .
FCO 2 may be estimated by:
(a) Nomographic method
(b) Titrimetric method
The nomographic method gives a closer estimation of FCO 2 when the pH and alkalinity are
determined immediately after sampling. The pH
be determined with an electrometric pH meter
calibrated with standard buffer solution in the pH
range of 7.0–8.0. The total alkalinity be determined titrimetrically.
The titrimetric method of estimation of FCO 2
be done using phenolphthalein as an indicator.
The titrimetric method is discussed below in
detail.
3.2.3.1 Titrimetric Method
of Determination of FCO 2
Principle
FCO 2 reacts with Na 2 CO 3 or NaOH to form
NaHCO 3 . Completion of the reaction is indicated
by the development of pink colour at the end
point (pH 8.3). In short, FCO 2 is estimated by
neutralising the dissolved CO 2 with 0.022 N
aqueous sodium hydroxide solution using phenolphthalein as an indicator.
Sampling and Storage
Surface water sample may be collected by dipping a
collecting bottle with double lid just below the water
surface; and after the bottle is filled in, its inner lid be
closed while the mouth of the bottle is still under
water. Deep water sample may be collected with the
help of a water sampler bottle. More precisely, sample water be collected by rubber tubing discharging
at the bottom of a 100- ml graduated cylinder or
Nelson’s tube. The sample water be allowed to overflow several times. The tube be withdrawn while the
sample water is flowing. The measuring cylinder be
flicked (jerked) to throw off the excess sample water
above the 100-ml mark. Some loss of FCO 2 may be
expected in storage and on transit of the sample,
even with a careful collection technique. Hence,
field determination immediately after sampling is
desirable. Alternatively, the sampling bottle be completely filled with the sample water and be preserved
at a temperature lower than the room temperature
and FCO 2 be estimated as soon as possible.
Reagents
1. Preparation of 0.1 N stock NaOH solution: It is
prepared by dissolving about 4 g of AR grade
NaOH pellets in 1 l of DW. It be standardised
against 0.1 N H 2 SO 4 using phenolphthalein as an
indicator. 100 ml of this 0.1 N solution diluted to
440 ml with DW gives N/44(0.022 N)NaOH.
Alternatively, 0.4735 g (accurately) of NaOH
(96 % pure) dissolved in 500 ml of DW gives
0.022 N NaOH.
Procedure
1. Sample water is collected following the above
procedure and 100 ml be taken in a conical flask.
2. Ten drops of phenolphthalein indicator solution be added.
3. If the sample turns pink, it indicates that FCO 2
is absent.
4. If the sample remains colourless, it be titrated
rapidly with 0.022 N NaOH solution (taken in
a 50-ml burette), stirring gently with a glass
rod until a definite pink colour is produced
which persists for 30 s when viewed through
the depth of the sample.
NB: For checking loss of FCO 2 during titration, a second sample be taken. After adding ten
drops of phenolphthalein indicator, the full
amount of the titrant used in the first titration be
rapidly poured in. If the sample turns pink, no
FCO 2 is said to be lost. If it remains colourless,
additional amount of the titrant be added until
pink colour appears. The result of the second
titration be considered.
3.2 Chemical