EXPERIMENT NO 7: DISSOLVED OXYGEN (DO) TEST
OBJECTIVE
TO DETERMINE THE DISSOLVED OXYGEN (DO) CONTENT IN THE WATER
SAMPLES
APPARATUS REQUIRED
The apparatus for the DO test consists of following:
Digital DO Meter: Here, we are using Lutron Dissolved Oxygen Meter of Model DO-
5519 CL- 936. It consists of input socket for DO electrode and four button to operate
the DO meter. The front panel consists of a digital display.
DO electrode: It consists of gold and silver electrode and an electrolyte tube with a ring
with DO membrane within which electrolyte can be kept. The electrode has a provision
to fit in the DO socket at DO meter.
DO membrane with ring
Readymade Electrolyte OXEL-03
BOD bottles: To collect water samples and test DO
Beakers and measuring cylinders etc.
SAMPLES AND CHEMICALS REQUIRED
Water samples to be tested: Collect the sample in narrow BOD bottle. Special
precautions are required to avoid entrainment of atmospheric oxygen into the sample.
While sampling from tap, a rubber tube attached to the tap is extended to the bottom of
the bottle. Allow water to overflow two-three times its volume and then place the
stopper over the bottle so that no air bubbles are entrained. In sampling from surface
source of water, sink the BOD bottle inverted into the source of water and collect the
water sample at least 20cm below the surface.
Distilled Water to clean electrode
THEORY
Dissolved oxygen is the content of oxygen in water. Surface water gets the dissolved oxygen
either from (i) Diffusion of oxygen from air, which is a physical phenomenon depends upon
solubility of oxygen and (ii) Photosynthesis activity in water, which is the biological
phenomena happened due to the activities of algae, tiny aquatic plants etc.
The solubility of oxygen depends upon temperature, water movement, presence of oxygen
demanding organic matters. The organic wastes cause rapid depletion of DO from water. At
higher temperature water can hold smaller amount of DO.
Saturation DO values of distilled water
Oxygen Oxygen Oxygen Oxygen Oxygen Oxygen
°C °C °C °C °C °C
Solubility Solubility Solubility Solubility Solubility Solubility
0 14.3 9 11.3 18 9.4 27 7.9 36 6.9 45 6
1 13.9 10 11.1 19 9.2 28 7.8 37 6.8 46 5.9
2 13.6 11 10.8 20 9 29 7.6 38 6.7 47 5.8
3 13.2 12 10.6 21 8.8 30 7.5 39 6.6 48 5.7
4 12.9 13 10.4 22 8.7 31 7.4 40 6.5 49 5.6
5 12.5 14 10.1 23 8.4 32 7.3 41 6.4 50 5.5
6 12.2 15 10 24 8.3 33 7.2 42 6.3
7 11.9 16 9.8 25 8.2 34 7.1 43 6.2
8 11.6 17 9.6 26 8.1 35 7 44 6.1
The presence of oxygen in water is necessary to keep it fresh and sparkling. The quantity of
6ppm of dissolved oxygen is essential in water to be used for drinking purpose and 4ppm for
fish and aquatic life. Excess of dissolved oxygen create corrosion of pipe material due to the
oxidation.
PROCEDURE
1. Preparation of electrolyte:
One drop of readymade Electrolyte OXEL-03 was prepared in ring with DO membrane
and was fitted it into electrode.
2. Making instrument ready:
The electrode was connected at the socket of the DO meter. The electrode was cleaned
with distilled water and was wiped it with clean cloth.
3. Calibration of instrument:
The instrument was powered by pushing power On/Off" button under wide and
ventilating environment. It displayed ‘O2%’ and ‘Temp.’ value. Further operations was
carried when Stable value of ‘O2%’ was displayed.
The ‘Hold’ button was pressed and was assured to display ‘Hold’ indicator. Press
‘Record’ button was pressed to display ‘CAL’ indicator and countdowns was allowed
from 30 to 0.
When ‘CAL’ indicator was turned to ‘End’ and upper display showed value 20.9 or
20.8 for ‘O2%’,the calibration process is finished and instrument is ready for use.
'FUNC' button was pressed continuously for 2 seconds then it displayed ‘O2%’ in mg/l.
4. Testing the Sample:
Electrode of the DO meter was inserted in that stirred distilled water sample to a depth
of at least 10 cm.
The value of DO was taken when the reading on the display was stable.
OBSERVATION TABLE
Sample Measured DO value (ppm)
S. N Remarks
No.
Observation 1 Observation 2 Observation 3 Average
1 A 4.7 5.9 4.2 4.9
2 B 4.2 5.1 3.9 4.4
RESULT & CONCLUSION
Sample A: Dissolved Oxygen (DO) = 4.9 ppm
Sample B: Dissolved Oxygen (DO) = 4.4 ppm
Both the samples of water doesnot have enough presence of dissolved oxygen for it to be
drinkable, however, the water may be good for aquatic life.
