YSI 58 - Manual (Page 14)

SECTION 4 OPERATION
4.1 PRINCIPLES OF OPERATION
The probes for use with the YSI 58 are a Clark type voltammetric (polarographic) sensor of dissolved
oxygen. An oxygen permeable membrane covers an electrolytic cell consisting of a cathode and an
anode. This membrane acts as a diffusion barrier and an isolation barrier preventing the fouling of the
cathode surface by impurities in the environment. The cathode is the gold electrode, and the anode is
the silver electrode that completes the electrolytic cell and acts as a reference electrode.
Upon entering the cell through the membrane, oxygen is reduced at an applied potential of -0.8 V
referenced to the silver electrode. The reduction current at the cathode is directly proportional to the
partial pressure of oxygen in liquid (expressed as %-air saturation) that is also proportional to the
concentration of dissolved oxygen (in mg/L) at a particular temperature. Thus the same partial
pressure of oxygen (%-air saturation) in liquid gives different concentrations of dissolved oxygen
(mg/L) at different temperatures because of the different solubility of oxygen at different
temperatures.
Figure 7
Cathode (gold)
Anode (silver)
Temperature sensor
Cathode reaction: O
2
+ 2H
2
O + 4e ==> 4OH
- -
Anode reaction: Ag + Cl ==> AgCl
-
THE 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. 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.
THE 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. This 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 -0.8 V versus the
YSI Incorporated YSI 58 DO Meter 10