Maintenance And Storage Of Do Electrodes; Salinity Correction - Tintometer Lovibond SD 335 Multi Mode D'emploi

a) b)

6 .4 Maintenance and storage of DO electrodes

The oxygen sensor comprises a platinum cathode, a lead anode, and potassium hydroxide (KOH)
as the internal electrolyte. If oxygen is present, this is reduced at the platinum cathode and the
sensor delivers an electrical signal. The measurement of dissolved oxygen (DO) depletes the
anode, ageing the sensor. The membrane, which is open to diffusion, also causes the sensor
to lose water, especially if it is stored in dry air. If the electrode can no longer be calibrated,
or if it returns unstable measurement values only, it must be serviced or the membrane head
must be replaced. In principle, maintenance is carried out in the same way as an initial filling.
First, the membrane head is unscrewed and the old electrolyte is removed. Any residual
electrolyte solution must be wiped off using a paper towel. If the membrane is undamaged,
the membrane head can be reused, otherwise it must be replaced.
Visible residue inside the membrane cap:
During operation, products from the reaction at the lead anode include lead oxide (brown/
red, from the reaction with oxygen) and lead carbonate (white, from the reaction with carbon
dioxide). Although these substances can collect on the membrane, they do not generally affect
the measuring function and can largely be removed during sensor maintenance. Before the
membrane cap is screwed on, they should be removed in order to avoid trapping particles
between the membrane and the platinum dome. Rapid formation after commissioning,
or excessive lead carbonate build-up, is an indicator of air in the sensor (incomplete filling /
leaks due to the cap / filling screw not being screwed on correctly, or a leak in the membrane).
Storage
Always store oxygen sensors in a humid place! Place the sensor in a storage flask
filled with distilled water or in a vessel containing water.
!
Never use the KOH solution for storage!
After a long period of storage, the membrane must be cleaned carefully to remove
any deposits (e.g. algae or bacteria) using a soft paper towel.

6 .5 Salinity correction

As the salinity (salt content) increases, the solubility of oxygen in the water decreases, i.e.
fewer mg of oxygen are dissolved per litre of water at the same partial pressure of oxygen.
To determine this oxygen concentration, the salinity of the medium must first be entered
into the configuration menu. The salinity correction is adapted to aqueous media that have
a chemical composition equivalent to seawater. The International Oceanographic Tables (IOT)
are used as the basis for the salinity correction.
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Immerse the oxygen sensor in the measuring solution.
Ensure sufficient flow, e.g. by
a) using a magnetic stirrer with agitator (recommended)
b) continuously swirling the sensor in the solution.
Always read off the measurement value while stirring!
SD_335_1 10/2020