Instruction Manual
CyberScan DO 1500
43
8. DO THEORY
The polarographic type Oxygen probe used with the CyberScan DO
1500 meter responds to the partial pressure exerted by oxygen in a
given sample. As Oxygen reaches the probe's cathode, it is reduced
according to the following equation:
O
2
+ 2 H
2
O + 4e
-
= 4 OH
-
When oxygen is reduced at the cathode (along with concurrent
oxidation of silver at the anode) current flows through the cell. The
more oxygen that is reduced, the more current is generated. The
relationship is linear, and by measuring the current generated in a
sample, and comparing it to that of a standard, one can compute the
amount of oxygen in the sample.
The primary influence on how much oxygen reaches the anode is the
partial pressure exerted by oxygen on the probes’ membrane. More
partial pressure brings more oxygen, and therefore more current. The
partial pressure that oxygen exerts is fixed for a given temperature
and atmospheric pressure. This fixed level corresponds to the
saturation level for a given temperature and pressure. The
CyberScan DO1500 meter uses the known relationship between
these parameters to permit simple, automatic standardisation.
In sample measurements, however, another factor influences the
relationship among partial pressure, temperature, and dissolved
oxygen. This is the factor of how much salt is contained or dissolved
in the sample. The presence of dissolved salt lowers the sample’s
ability to dissolve oxygen. Therefore a sample with a high level of
dissolved salt will contain less oxygen than a sample with less or no
salt at the same temperature and pressure. Fortunately, the
relationship between dissolved salt or salinity and dissolved oxygen is
well defined. The CyberScan DO 1500 uses this fact to provide
accurate dissolved oxygen measurements in samples whose salinity
range from 0 to 45 ppt.
