YSI 5239 Dissolved Oxygen Probe - Manual (Page 12)

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anode/reference electrode which completes the overall
electrolytic reaction in the chloride medium (KCl electrolyte)
behind the membrane. These reactions, at the cathode and the
anode, are as follows:
Cathode reaction: O
2
+ 2H
2
O + 4e
-
==> 4OH
-
Anode reaction: Ag + Cl
-
==> AgCl
T HE DO R EADINGS F ROM THE C ATHODE R EDUCTION
The oxygen reduction current is sampled and processed, by the
meter, and displayed as either %-air saturation or mg/L. While
the parameter of %-air (partial pressure) is independent of
temperature and salinity, mg/L (solubility of oxygen) is a
function of temperature and salinity. For instance, the same %-
air reading (same partial pressure) would give a higher mg/L
reading at a lower temperature than at a higher temperature.
Also, the higher the salinity, the lower the solubility for the
same %-air reading at the same temperature.
T HE F ORMATION OF AgCl AT THE A NODE
While the oxygen reduction current passes through the internal
circuit to be reported as the DO reading, it also passes through
the anode oxidizing the silver and forming a thin layer of
silver chloride. This oxidation of silver at the chloride medium
provides a stable potential which the cathode potential is
referenced to (for instance, the polarization potential of the
cathode is -0.8 V versus the potential of the Ag/AgCl redox
couple at the silver anode). As more and more silver chloride