39
2. The bottles are to be stoppered carefully and
immediately to exclude air bubbles.
3. The contents are mixed by inverting the bottle
at least 15 times. When the brown ppt. settles
leaving a clear supernatant above the manganese hydroxide floc, it is shaken again and the
floc is allowed to settle for the second time.
4. After at least 2 min, settling is supposed to
have produced minimum 100 ml of clear
supernatant. Now, the stopper is removed carefully; and immediately 2 ml of concentrated
sulphuric acid is added in such a way that it
flows down the neck of the bottle. The bottle is
re-stoppered and inverted gently, until dissolution of the above floc is complete. A brown
colour solution is formed in the bottle.
5. This brown-coloured solution is titrated
against standard 0.025 N sodium thiosulphate
solution (popularly called ‘hypo’) taken in a
50-ml burette. The brown colour goes on fading until a straw-coloured solution is obtained
near the end point.
6. Now, 2–3 drops of starch indicator solution is
added. An intense blue-coloured solution is
obtained.
7. The titration is continued and hypo solution is
added drop by drop from the burette. The blue
colour goes on fading, until the solution
becomes completely colourless at the end point.
8. The burette reading is noted.
9. The titre value is the amount of DO present in
mg/lit in the sample water.
The data may be tabulated in the format given
below:
No. of
obs.
IR of
burette
FR of
burette
Difference
DO in
mg/lit
Average value of DO (mg/lit):
IR = Initial reading
FR = Final reading
3.2.2.2 Electrometric (Electronic)
Method of DO Estimation
by Dissolved Oxygen Meter
The dissolved oxygen (DO) meter is generally a
battery-operated portable instrument for estimation of DO and determination of temperature of
the sample water at the same time. DO is measured in ppm (mg/lit) over a range of 0–20 ppm
with 0.1 ppm resolution and temperature is indicated in degrees Celsius from – 5 to +45 °C. The
DO measurement is automatically temperature
compensated for solubility of oxygen in water
and permeability of the probe membrane. Salinity
compensation is manual. Measurements are displayed by a 3-digit LCD.
Principle
The probe supplied with the unit (instrument) is
a special Clark-type membrane covered polarographic sensor. A thermistor probe is also supplied for the automatic temperature compensation
and measurement. A thin, permeable membrane
stretched over the sensor isolates the sensor elements from the environment but allows oxygen
to enter. When a polarising voltage is supplied
across the sensor, the oxygen that has passed
through the membrane reacts at the cathode,
causing a current to flow.
Panel Features in Brief
1. The following principal switches:
Power, salinity, zero, set zero, mode, DO
meas/CAL, and temperature
2. LCD display
Power Supply
This unit is battery operated. Hence, it does
not require any mains outlet. It uses one 9-V
small battery, e.g. Eveready 216 and Hi-Watt
6F22. Correct polarities are ensured while inserting fresh batteries.
Operating Steps
Probe Preparation
The DO probe consists of five main parts, namely,
main stem, electrode cover, sleeve, membrane
and bakelite nut.
The main stem is made of Perspex with gold
electrode at the lower end and silver ferrule fully
surrounding the gold. The upper end carries a
connecting cable and a connector. The main stem
has threads, so that electrode cover along with
membrane assembly could be fitted to it by means
of bakelite nut.
The electrode cover is a thin cylinder which
fits into the lower half of the main stem.
A Teflon, oxygen-permeable membrane is
3.2 Chemical
2. The bottles are to be stoppered carefully and
immediately to exclude air bubbles.
3. The contents are mixed by inverting the bottle
at least 15 times. When the brown ppt. settles
leaving a clear supernatant above the manganese hydroxide floc, it is shaken again and the
floc is allowed to settle for the second time.
4. After at least 2 min, settling is supposed to
have produced minimum 100 ml of clear
supernatant. Now, the stopper is removed carefully; and immediately 2 ml of concentrated
sulphuric acid is added in such a way that it
flows down the neck of the bottle. The bottle is
re-stoppered and inverted gently, until dissolution of the above floc is complete. A brown
colour solution is formed in the bottle.
5. This brown-coloured solution is titrated
against standard 0.025 N sodium thiosulphate
solution (popularly called ‘hypo’) taken in a
50-ml burette. The brown colour goes on fading until a straw-coloured solution is obtained
near the end point.
6. Now, 2–3 drops of starch indicator solution is
added. An intense blue-coloured solution is
obtained.
7. The titration is continued and hypo solution is
added drop by drop from the burette. The blue
colour goes on fading, until the solution
becomes completely colourless at the end point.
8. The burette reading is noted.
9. The titre value is the amount of DO present in
mg/lit in the sample water.
The data may be tabulated in the format given
below:
No. of
obs.
IR of
burette
FR of
burette
Difference
DO in
mg/lit
Average value of DO (mg/lit):
IR = Initial reading
FR = Final reading
3.2.2.2 Electrometric (Electronic)
Method of DO Estimation
by Dissolved Oxygen Meter
The dissolved oxygen (DO) meter is generally a
battery-operated portable instrument for estimation of DO and determination of temperature of
the sample water at the same time. DO is measured in ppm (mg/lit) over a range of 0–20 ppm
with 0.1 ppm resolution and temperature is indicated in degrees Celsius from – 5 to +45 °C. The
DO measurement is automatically temperature
compensated for solubility of oxygen in water
and permeability of the probe membrane. Salinity
compensation is manual. Measurements are displayed by a 3-digit LCD.
Principle
The probe supplied with the unit (instrument) is
a special Clark-type membrane covered polarographic sensor. A thermistor probe is also supplied for the automatic temperature compensation
and measurement. A thin, permeable membrane
stretched over the sensor isolates the sensor elements from the environment but allows oxygen
to enter. When a polarising voltage is supplied
across the sensor, the oxygen that has passed
through the membrane reacts at the cathode,
causing a current to flow.
Panel Features in Brief
1. The following principal switches:
Power, salinity, zero, set zero, mode, DO
meas/CAL, and temperature
2. LCD display
Power Supply
This unit is battery operated. Hence, it does
not require any mains outlet. It uses one 9-V
small battery, e.g. Eveready 216 and Hi-Watt
6F22. Correct polarities are ensured while inserting fresh batteries.
Operating Steps
Probe Preparation
The DO probe consists of five main parts, namely,
main stem, electrode cover, sleeve, membrane
and bakelite nut.
The main stem is made of Perspex with gold
electrode at the lower end and silver ferrule fully
surrounding the gold. The upper end carries a
connecting cable and a connector. The main stem
has threads, so that electrode cover along with
membrane assembly could be fitted to it by means
of bakelite nut.
The electrode cover is a thin cylinder which
fits into the lower half of the main stem.
A Teflon, oxygen-permeable membrane is
3.2 Chemical