EUTECH INSTRUMENTS CyberScan DO 5000, Instruction Manual, Page: 98 / 105

Instruction Manual CyberScan DO5000
95
10 DO FUNDAMENTALS
Meter Operation
The CyberScan DO 5000 meter uses the polarographic method of
determining dissolved oxygen concentration. A voltage is maintained
across two electrodes (a gold cathode and a silver anode) and an
electrolyte separated from the sample solution by an oxygen
permeable membrane. Oxygen diffuses form the sample across the
membrane where it is reduced at the cathode.
1) 0
2
+ 2H
2
0 + 4e
-
4OH
-
The current generated, I, by such an electrode is proportional to the
oxygen concentration of the sample. The rate of oxygen reduction at
the cathode is significantly faster than the rate of diffusion to the
electrode. Therefore, the rate of oxygen reduction is diffusion limited.
2)
I = k D [O2]
I is the measured current
K is a proportionally constant
D is the diffusion coefficient of oxygen
Equation 2 is a simplified description of current generated by the
oxygen electrode. Many other factors such as electrode surface area,
membrane thickness, membrane permeability coefficient,
incorporated into the constants k and D. as the partial pressure of
oxygen in the water is equal to that in the atmosphere, the electrode
output is an indicator of oxygen partial pressure. The oxygen partial
pressure differential between the sample and the electrolyte inside
the electrode is the driving force bringing oxygen to the electrode.
Oxygen Solubility
Oxygen Solubility in water is a critical parameter in many arenas
including biochemistry, industrial processes, and particularly
environmental science and engineering. Sufficient dissolved oxygen
is an absolute requirement for good water quality. It is essential for
aerobic life and the natural purification processes to which they
contribute. Dissolved oxygen concentrations below 5 ppm will result
in substantial damage to the aquatic ecosystem, and concentration
below 2 ppm can result in fish kills and growth of harmful bacteria.