Thermo Fisher Scientific CyberScan 6000 Series Meters v.2 - Manual (Page 95)

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Appendix: DO Theory
DO6000 and PCD6500 meters
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Meter Operation
The DO6000 and PCD6500 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 from the sample across the membrane where it is reduced at the
cathode.
1) O
2
+ 2H
2
O + 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 = kD (O
2
)
I is the measured current
k is a proportionality 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,
and oxygen partial pressure are integral to this process and incorporated into the constants k and
D. As the partial pressure of oxygen in the water is equal to that in the atmosphere, 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 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 concentrations below 2 ppm can
result in fish kills and growth of harmful bacteria.
The ultimate amount of oxygen possible in water is a function of temperature, atmospheric
pressure, and the concentration of dissolved salts or salinity. At a given atmospheric pressure,
water will hold a fixed, well-defined amount of oxygen at a given temperature. Water at higher
temperatures will hold less oxygen than water at lower temperatures.
Dissolved salts in water lower its capacity for oxygen. Though the amount of oxygen that water
can hold is reduced by the presence of dissolved salts, the partial pressure of the dissolved
oxygen still equals that of oxygen in the air above it. The DO6000 and PCD6500 meter
compensates for this fact by relating the salinity value of water to its true capacity for oxygen at a
given temperature and pressure.
Biochemical Oxygen Demand (BOD)
The BOD test measures the degree to which water is contaminated with organic material, which
is decomposed by aerobic bacterial processes. This test is a critical importance for municipal and
industrial discharges. Typically, the sample is diluted with oxygen saturated water. The dissolved
oxygen content is measured immediately after dilution, and after a 5-day incubation period. The
BOD is expressed as the mg/L of dissolved oxygen consumed during the incubation.