YSI 95 - Manual (Page 17)

Principals of Operation Section 5
YSI Incorporated Model 95 13
5.2 5.2 5.2 5.2 DO READINGS FROM THE CATHODE REDUCTION DO READINGS FROM THE CATHODE REDUCTION DO READINGS FROM THE CATHODE REDUCTION DO READINGS FROM THE CATHODE REDUCTION
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. The same %-air
reading (same partial pressure) would give a higher mg/L reading at a lower temperature than at a
higher temperature. The higher the salinity, the lower the solubility (mg/L) is for the same %-air
reading at the same temperature.
5.3 5.3 5.3 5.3 FORMATION OF AgCl AT THE ANODE FORMATION OF AgCl AT THE ANODE FORMATION OF AgCl AT THE ANODE FORMATION OF AgCl AT THE ANODE
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. Furthermore, the oxidation of silver at the chloride medium provides a stable potential that
the cathode potential is referenced to (for instance, the polarization potential of the cathode is -1.0
V versus the potential of the Ag/AgCl redox couple at the silver anode). Since the current of the
MEA sensor is so small, there should not be any significant accumulation of AgCl at the anode for
3 to 4 years.
5.4 5.4 5.4 5.4 FUNCTION OF THE EL FUNCTION OF THE EL FUNCTION OF THE EL FUNCTION OF THE ELECTROLYTE ECTROLYTE ECTROLYTE ECTROLYTE
There are two main functions for the electrolyte:
1. Supply the chloride (Cl - ) to the anode/reference electrode for the counter reaction of the
oxygen reduction at the cathode.
2. Provide the ionic conduction of electricity inside the cell, especially in the thin layer
between the gold cathode and the membrane.
Under normal operating conditions, such as measuring oxygen around 100%-air saturation (8.27
mg/L) at 25°C, the electrolyte should last up to 500 hours. This translates into about 62.5 working
days at 8 hours per day operation. The actual electrolyte life, however, may be shorter since, in
most environmental applications, membrane fouling determines the life of the
electrolyte/membrane.