38
The liberated iodine is said to be equivalent to
the original DO content of the sample. The amount
of liberated iodine is estimated by titration with
standard solution of sodium thiosulphate using
starch as indicator:
I
S O
S O
I
2
2 3
4
6
2
2
+
®
+
Interference
The Alsterburg azide modification of Winkler
method, in which, sodium azide is used, is applicable to most natural waters. Interference by
higher concentrations of Fe (III) is reduced by
this Alsterburg modification with azide playing
the pivotal role.
Sampling Procedure
Collection of water sample be done very carefully
so that the sample does not mix with air nor get
agitated. Samples from any depth in streams, lakes,
wetlands or reservoirs need special precautions to
eliminate changes in pressure and temperature.
Surface water samples be collected in narrowmouthed glass stoppered BOD bottles of 300-ml
capacity with tapered ground glass pointed stoppers and flared mouths. The bottle be overflown 2
or 3 times its volume and the stopper replaced
without air bubble being allowed to enter.
Samples from depths of >2.0 m are collected by
Kemmerer type of sampler. The bottle is filled by
overflowing for nearly 10 s but without turbulence or formation of bubbles.
Sample Preservation
DO in water be determined immediately after
collection of sample. However, samples which
are supposed to have low DO in them may be
stored by adding manganous sulphate–alkaline
iodide–sodium azide and sulphuric acid, shook
properly, and be kept away from strong
sunlight.
Procedure for DO Determination
Reagents Required
(a) Manganous Sulphate: It is prepared by dissolving 364 g of MnSO 4 .H 2 O in DW. Then it
is filtered and diluted to 1 l. This solution is
not to produce colour with starch when added
to acidified solution of KI. Alternatively,
18.2 g of MnSO 4 .H 2 O is dissolved in DW,
filtered and made up to 50 ml.
(b) Alkaline Iodide–Sodium Azide Solution: It is
prepared by dissolving 500 g of NaOH or 700 g
of KOH and 150 g of KI or 135 g of NaI in DW.
This be diluted to 1 lit with DW. Next, 10 g of
NaN 3 (Sodium azide) is dissolved in 40 ml of
DW; and this be added to 950 ml of the solution
prepared above. Alternatively, if less quantity
of the reagent is required, it is prepared by dissolving 25 g of NaOH and 7.5 g of KI in DW.
This be diluted to 50 ml with DW. Next, 1 g of
NaN 3 is dissolved in 4 ml of DW; and this be
added to 45 ml of the solution prepared above.
(c) Starch Indicator: To 0.25 g of starch powder,
a few ml of cold water is added and is rubbed
to a paste. This is added, with constant stirring, to 1 l of boiling water, so slowly that
boiling never ceases. Boiling be continued
for an additional minute or two and then
allowed to cool. It is preserved in a glass
bottle. A fresh solution be prepared as soon
as mould appears or when a strong colour is
not given by very dilute iodine solution.
(d) Concentrated Sulphuric Acid: To be of AR
grade
(e) Stock Sodium Thiosulphate (Hypo) Solution
(0.1 N): It is prepared by dissolving 24.82 g
of Na 2 S 2 O 3 .5H 2 O in boiled and cooled DW. It
is then diluted to 1 l.
(f) Standard Sodium Thiosulphate Titrant
(0.025 N): It is prepared by diluting 250 ml of
stock solution to one litre of boiled and cooled
DW. Alternatively, 1 l of 0.025 N hypo solution
is prepared by dissolving 6.2047 g (approx.
6.3 g) of sodium thiosulphate (AR grade) in 1 l
of DW. This standard sodium thiosulphate
solution (which is a secondary standard) is to
be standardised with a primary standard (say,
potassium dichromate solution) before every
set of titration for DO estimation, for which,
any standard book of chemistry dealing with
volumetric analysis could be consulted.
Procedure for Estimation of DO
1. To the 300 ml/250 ml of sample collected in
300 ml/250 ml BOD bottle is added 2 ml of
MnSO 4 solution followed by 2 ml of alkaline
iodide–sodium azide reagent.
3 Lentic Water (Physico-chemical Characteristics of Water)
The liberated iodine is said to be equivalent to
the original DO content of the sample. The amount
of liberated iodine is estimated by titration with
standard solution of sodium thiosulphate using
starch as indicator:
I
S O
S O
I
2
2 3
4
6
2
2
+
®
+
Interference
The Alsterburg azide modification of Winkler
method, in which, sodium azide is used, is applicable to most natural waters. Interference by
higher concentrations of Fe (III) is reduced by
this Alsterburg modification with azide playing
the pivotal role.
Sampling Procedure
Collection of water sample be done very carefully
so that the sample does not mix with air nor get
agitated. Samples from any depth in streams, lakes,
wetlands or reservoirs need special precautions to
eliminate changes in pressure and temperature.
Surface water samples be collected in narrowmouthed glass stoppered BOD bottles of 300-ml
capacity with tapered ground glass pointed stoppers and flared mouths. The bottle be overflown 2
or 3 times its volume and the stopper replaced
without air bubble being allowed to enter.
Samples from depths of >2.0 m are collected by
Kemmerer type of sampler. The bottle is filled by
overflowing for nearly 10 s but without turbulence or formation of bubbles.
Sample Preservation
DO in water be determined immediately after
collection of sample. However, samples which
are supposed to have low DO in them may be
stored by adding manganous sulphate–alkaline
iodide–sodium azide and sulphuric acid, shook
properly, and be kept away from strong
sunlight.
Procedure for DO Determination
Reagents Required
(a) Manganous Sulphate: It is prepared by dissolving 364 g of MnSO 4 .H 2 O in DW. Then it
is filtered and diluted to 1 l. This solution is
not to produce colour with starch when added
to acidified solution of KI. Alternatively,
18.2 g of MnSO 4 .H 2 O is dissolved in DW,
filtered and made up to 50 ml.
(b) Alkaline Iodide–Sodium Azide Solution: It is
prepared by dissolving 500 g of NaOH or 700 g
of KOH and 150 g of KI or 135 g of NaI in DW.
This be diluted to 1 lit with DW. Next, 10 g of
NaN 3 (Sodium azide) is dissolved in 40 ml of
DW; and this be added to 950 ml of the solution
prepared above. Alternatively, if less quantity
of the reagent is required, it is prepared by dissolving 25 g of NaOH and 7.5 g of KI in DW.
This be diluted to 50 ml with DW. Next, 1 g of
NaN 3 is dissolved in 4 ml of DW; and this be
added to 45 ml of the solution prepared above.
(c) Starch Indicator: To 0.25 g of starch powder,
a few ml of cold water is added and is rubbed
to a paste. This is added, with constant stirring, to 1 l of boiling water, so slowly that
boiling never ceases. Boiling be continued
for an additional minute or two and then
allowed to cool. It is preserved in a glass
bottle. A fresh solution be prepared as soon
as mould appears or when a strong colour is
not given by very dilute iodine solution.
(d) Concentrated Sulphuric Acid: To be of AR
grade
(e) Stock Sodium Thiosulphate (Hypo) Solution
(0.1 N): It is prepared by dissolving 24.82 g
of Na 2 S 2 O 3 .5H 2 O in boiled and cooled DW. It
is then diluted to 1 l.
(f) Standard Sodium Thiosulphate Titrant
(0.025 N): It is prepared by diluting 250 ml of
stock solution to one litre of boiled and cooled
DW. Alternatively, 1 l of 0.025 N hypo solution
is prepared by dissolving 6.2047 g (approx.
6.3 g) of sodium thiosulphate (AR grade) in 1 l
of DW. This standard sodium thiosulphate
solution (which is a secondary standard) is to
be standardised with a primary standard (say,
potassium dichromate solution) before every
set of titration for DO estimation, for which,
any standard book of chemistry dealing with
volumetric analysis could be consulted.
Procedure for Estimation of DO
1. To the 300 ml/250 ml of sample collected in
300 ml/250 ml BOD bottle is added 2 ml of
MnSO 4 solution followed by 2 ml of alkaline
iodide–sodium azide reagent.
3 Lentic Water (Physico-chemical Characteristics of Water)