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About Dissolved Oxygen

Measurement

Dissolved Oxygen

away from the polarizable electrode is linearly proportionate to the

amount of oxygen available in the electrolyte, hence this movement

of electrons is representative of the amount of dissolved oxygen left

in the electrolyte.

The advantage of a polarographic cell is that the cathode remains intact.

The current ﬂow of the polarographic cell is also linearly proportional

to the amount of oxygen present in the electrolyte, enabling the cell to

provide highly accurate measurements at low oxygen levels.

Polarographic self-stirring DO/BOD probes are available for use

with the Eutech DO 700 and DO 2700 bench meters.

BOD & COD

The BOD test measures the molecular oxygen utilized in the

biodegradation of organic material and the oxidation of inorganic

material. By measuring the amount of oxygen dissolved in samples at

the beginning and end of a speciﬁed incubation period, the relative

oxygen requirements of wastewaters, eﬄuents, and polluted waters

can be determined.

BOD

(mg/L) =

- D

BOD

= Oxygen uptake during incubation period t

= DO of diluted sample immediately after preparation (mg/L)

= DO of diluted sample after incubation period t (mg/L)

P = Decimal volumetric fraction of sample used

BOD is similar to the Chemical Oxygen Demand (COD), which

also measures relative oxygen-depletion. However, the possible

presence of non-biologically oxidisable may render the COD test to

be less accurate.

The COD Test is often used to measure the amount of organic

compounds in surface water by measuring the amount of oxygen

required to oxidize and break down an organic compound into carbon

dioxide, ammonia and water. The basis of the COD test is that anything

can be oxidized into carbon dioxide using a strong oxidizing agent in

acidic environments. A blank sample, created by adding all reagents to

distilled water is usually used as a control in COD measurements.

Both the BOD and COD tests are means to measure the relative

oxygen-depletion eﬀect of a waste contaminant, and are widely used

to monitor pollution levels. The BOD test measures the oxygen demand

of biodegradable pollutants whereas the COD test measures the

oxygen demand of biogradable pollutants plus the oxygen demand of

non-biodegradable oxidizable pollutants.

However, because COD measures everything that can be chemically

oxidised and not just the level of biologically active organic matter, the

possibility of non-biological oxidizable may render the COD Test as a

less accurate method compared to the BOD method.

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