NAVI D-25, Instruction Manual

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7 Reference
7.5 Dissolved oxygen measurement
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7.5
Dissolved oxygen measurement
●
Measuring dissolved oxygen
“Dissolved oxygen” (DO) is the concentration of oxygen
that is dissolved in water. DO is essential in the self-
cleaning mechanism of rivers and seas and for fish and
other aquatic animals. The measurement of DO is also
essential for waste-water treatment and water-quality
management.
The principles of measurement using a DO tip are
explained below.
Fig. 4 DO tip measurement
A precious metal (silver) is used as the cathode, which is
tightly affixed to an oxygen-permeable membrane, and a
base metal (lead) is used as the anode. Both the
cathode and anode are immersed in an alkaline
electrolytic solution. The external circuit between the
anode and cathode is closed. Oxygen that diffuses
through the oxygen-permeable membrane causes the
following chain reaction to occur in the cathode and
allows current to flow in the external circuit,
O
2
+2H
2
O+4e
-
→
4OH
-
whereas, the following oxidation reaction occurs at the
anode:
2Pb
→
2Pb 2+ +4e -
This current is proportional to the amount of oxygen that
is diffused through the oxygen-permeable membrane, so
measuring the current of the sample enables the DO
contained within the sample to be determined.
Electrode
Anode (lead)
Alkaline electrolyte
Gas-permeable membrane
Cathode (silver)