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About Dissolved Oxygen

Measurement

Dissolved Oxygen

Eutech DO instruments automatically compensate for

temperature, salinity and barometric pressure. The salinity

value and barometric pressure are either measured by the

instrument or entered by the user.

DO Electrodes

The measurement of DO requires a special DO electrode that is made

up of an anode, a cathode, electrolyte solution and a gas permeable

membrane. The material of the membrane is specially selected to permit

oxygen to pass through. Oxygen is consumed by the cathode which will

create a partial pressure across the membrane. Oxygen then diﬀuses

into the electrolyte solution. In short, a DO meter actually measures the

pressure caused by movements of oxygen molecules in water or any

other medium. Currently, galvanic and polarographic electrodes are the

predominant methods for measuring dissolved oxygen.

The Galvanic Cell consists of two metals, the positive anode and the

negative cathode, connected by a salt bridge between the individual

half-cells. As the metal electrodes leave electrons behind as they

dissolve in the electrolyte. The diﬀerent properties of the two metals

causes them to dissolve at diﬀerent rates, hence a pressure is created

when the number of electrons in either side of the cell diﬀers. The

pressure is translated into an electric current proportion to the oxygen

concentration in the electrolyte if an electrical circuit is created between

the two electrodes. The galvanic electrode does not need polarising

time and is able to assume operation immediately.

During this process, ions of the more active anode are transferred

through the electrolyte to the less active cathode, and deposited there

as a plating. In this way the anode is corroded. When the anode material

eventually corrodes away, the potential drops and the current halts.

Galvanic electrodes are available with most Eutech

Instruments DO meters such as the DO 6+, CyberScan

DO 110 and DO 300.

The Polarographic Cell consists of two electrodes placed in the

electrolyte: One with ﬁxed potential called the reference electrode,

and the other with a variable potential called the polarizable electrode.

As voltage is applied to the polarizable electrode, a redox reaction

occurs, where electrons break away from the electrode to bond

with oxygen in the electrolyte. The rate at which the electrons break

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About Dissolved Oxygen Measurement

What is Dissolved Oxygen?

Dissolved Oxygen (DO) is a measure of the amount of dissolved

gaseous oxygen in a solution. Some gases, such as ammonia, carbon

dioxide and hydrogen chloride, react chemically with water to form

new compounds. However, gases such as nitrogen and oxygen merely

dissolve in water without chemically reacting with it, and exist as

microscopic bubbles between water molecules.

There are two main ways in which dissolved oxygen occurs naturally

in water: From the surrounding atmosphere, where oxygen in the

surrounding air dissolves readily when mixed into water, up to

saturation, during water movements; Via photosynthesis when oxygen is

produced by aquatic plants and algae as a by-product of photosynthesis.

The amount of oxygen dissolved in water is usually measured in percent

saturation, or expressed as a concentration in milligrams per litre water.

Accurate measurement of dissolved oxygen is essential in processes

where oxygen content aﬀects reaction rates, process eﬃciency or

environmental conditions, such as biological wastewater treatment,

wine production, bio-reactions, environmental water testing.

Basic Principle in DO Measurement

In theory, the amount of DO in a solution is dependent on three factors,

namely temperature, salinity and atmospheric pressure.

1. Water Temperature

Solubility of oxygen reduces as temperature increases. Hence, the

colder the water, the more dissolved oxygen it contains. Since

temperature aﬀects both the solubility and diﬀusion rate of oxygen,

temperature compensation is necessary for any standardized

DO measurements.

All Eutech DO meters come with automatic temperature

compensation for accurate readings even in varying

temperature conditions.

2. Salinity

The amount of dissolved oxygen increases as salinity level decreases.

In other words, freshwater holds more oxygen than saltwater. Since

the presence of dissolved salts limits the amount of oxygen that can

dissolve in water, the relationship between the partial pressure and

concentration of oxygen varies with the salinity of the sample.

Eutech meters feature manual salinity correction to compensate

for variations in ionic concentration. Simply enter the salinity

of the sample in parts per thousand (ppt) to ensure the correct

DO measurements.

3. Atmospheric Pressure

There is a direct proportional relationship between the solubility

of dissolved oxygen and the surrounding atmospheric pressure. As

pressure decreases with increase in altitude, the amount of dissolved

oxygen found in water reduces.

To ensure that your dissolved oxygen is not aﬀected by atmospheric

pressure, Eutech meters come with manual barometric pressure

compensation, with an Atmospheric Pressure Correction Chart

included in the manuals for convenient referencing.

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