What do you need to know?

This is a technical page. What you have to know at least is:
• the electrode consumes oxygen, so you must stir to obtain reliable results! 
• the electrode is calibrated with the potentiometer to 100% by moving it through well aerated water (shaking or bubbling through)
• the electrode is sensitive for temperature differences, so you should measure everything at the same temperature, to obtain reliable results.

Operation of the oxygen electrode

•The O2-electrode consists of a KOH (0,1 M) filled space with two poles, which space is confined by an O2-permeable membrane (see figure below).  One pole consists of lead (Pb), the other of silver (Ag), and brought together in the KOH solution in the space behind the membrane, a galvanic cell is formed with potential differences up to ±800 mV. When no O2 diffuses through the membrane, (submerge electrode in Na-sulfite), no current will pass the system and the potential over the input of the meter will be 0 mV. See animation. When the electrode is held in air or oxygen containing water, O2 diffuses through the membrane, which will be reduced at the silver pole:

                      2 Pb ð  2 Pb2+ + 4 e–
 2 H2O + O2 + 4 e– ð  4 OH–
_______________________________
 2 Pb + 2 H2O + O2 ð  2 Pb(OH)2

and/or:

          2 Pb + 4 OH–  ð   2 PbO + 2 H2O + 4 e–
 2 H2O + O2 + 4 e–   ð   4 OH–  
______________________________________
               2 Pb + O2  ð    2 PbO


The required electrons come from the lead pole, leaving lead-ions, and run through the wires and the potentiometer to the silver pole. The OH-ions formed at the silver pole diffuse via the electrolite (KOH) to the lead pole and will combine with the Pb-ions to lead hydroxide and/or oxide. See animation.  

Twice the amount of oxygen doubles the current. In order to measure the current we measure the voltage over the potentiometer. Ohm's Law tells that R=V/I and because, after calibration, the resistance doesn't change anymore, V/I remains constant. In other words: when current I increases, voltage V increases proportionally.

Nota bene:

a  The electrode uses oxygen and has to be moved (or the water) in order to keep the diffusion through the membrane at a constant rate. 
b  Diffusion is an extremely temperature sensitive process, which makes calibrating and measuring at the same temperature imperative!
c  Reducing oxygen at the Ag-pole causes Pb to be oxidised at the lead-pole. Lead-ions combine with hydroxide-ions to form a gray layer of lead hydroxide, resulting in a decrease of the Pb-area: the electrode declines. The longevity of a freshly prepared electrode can be enlarged by storing the electrode in a jar with 1% Na2SO3-solution, everytime the electrode is not used.
After a few days an electrode will become useless, and the Pb-surface needs to be polished with a soft abrasive and the electrode has to be prepared again. If you want to know everything about the properties of oxygen electrodes, then click on the site next door.