horiba U-51, Инструкция по эксплуатации

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6 Reference
105
6.4 DO measurement
6.4.1 Principle of DO measurement
Dissolved oxygen (DO) refers to the amount of oxygen that is contained in water.
The concentration of dissolved oxygen is generally given as mg/L or as a percentage value
(the dissolved oxygen saturation ratio).
Dissolved oxygen is essential for maintaining the self-purifying ability of rivers and seas and
also for fish to live. The concentration of dissolved oxygen acts as an indicator of water quality.
It is often measured when processing waste water and managing water quality. Fig. 1
provides an overview of the principles behind dissolved oxygen sensor measurement.
Fig. 1 Overview of principles behind dissolved oxygen sensor
The polarographic oxygen sensor is an enclosed sensor wherein voltage is applied to a
cathode made of a precious metal (such as gold or platinum) and an anode also made of a
precious metal (such as silver) via an external circuit, and a cap with an oxygen permeable
diaphragm (membrane) is filled with electrolyte solution. As indicated in Fig. 1, the
concentration of dissolved oxygen can be measured by measuring the current proportional to
the amount of reduced oxygen when oxygen that has dispersed through the oxygen
permeable diaphragm produces a reductive reaction on the surface of the active electrode
(gold). The method of measuring dissolved oxygen based on the above principle is called the
Membrane Electrode Method. Compared to the Chemical Analysis Method, which requires
complicated pre-processing to alleviate the effect of reduced materials and oxidizing
materials, this method allows dissolved oxygen to be measured very easily. It is also easy to
remove undesired buildup from the silver electrode by polishing and cleaning if an insulator
forms on it due to oxidation, making the method reusable.
6.4.2 Conditioning of DO sensor
Oxygen goes into membrane cap of DO sensor through membrane without being consumed
by the sensor while U-50's power is off, although polarographic DO sensor consumes oxygen
during the measurement. DO value is higher than usual after turning U-50's power on because
DO sensor is measuring and consuming extra oxygen.
For the easy maintenance, U-50's DO sensor has a big membrane cap and space for oxygen,
so conditioning time is a little long.
Three-Electrode Polarographic Method
Oxygen reduction
Applied voltage
Electrolyte solution
Membrane cap
Reference electrode
Counter electrode (silver)
Active electrode (gold)
Oxygen-permeable
Dissolved oxygen (O
2
)
Cathode: O
2
+ 2H
2
O + 4e
-
→ 4OH
-
Anode: 4Ag + 4Cl
-
→
4AgCl + 4e
-
Current flows between the active electrode
Constant electrical
Counter electrode
Reference electrode
Active electrode
current
potential applied
(silver/silver chloride)
diaphragm
and the counter electrode
Active electrode (gold)
Counter electrode (silver)