Horiba Scientific LAQUAact D-75G, Instruction Manual

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Technical note
89
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Salinity concentration correction
When a solution is in contact with air and is in a state of perfect equilibrium (a state of
saturation), the relationship between the DO contained within the solution C (mg/L) and
the partial pressure of the oxygen in the air Ps (MPa) is shown by the following equation.
The H in this equation is referred to as the “Henry constant” and has a different value
depending on the composition of the solution. Generally, the higher the salinity
concentration within a solution, the larger H becomes, and, consequently, the smaller C
becomes.
DO tips actually detect the Ps that occurs in the above equation. This means that even if
a DO tip is immersed in pure water that is saturated with air or in an aqueous solution
containing salt, the output current will not change, which gives rise to a problem.
For this reason, it is necessary to correct the salinity concentration, to enable the correct
DO to maintain a current, even in an aqueous solution containing salt, and resolve the
problem.
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Air pressure correction
Amount of oxygen that can be dissolved in water is affected by air pressure in the
location where measurement is taking place. Threfore, it is necessary to pay attention to
the change of air pressure in using the instrument. This instrument has a built-in air
pressure corrction function. Set the air pressure in the instrument before calibrating to
correct the pressure influence.