Environmental Engineering FOURTH EDITION

84 ENVIRONMENTAL ENGINEERING

Microammeter

PI

Electrolyte

solution

0 ring

Membrane

A LL Lead anod>l

Silver cathode

Figure 5-1. Schematic diagram of a galvanic cell oxygen probe.

the electricity recorded is proportional to the concentration of oxygen in the electrolyte

solution.

In the commercial models, the electrodes are insulated from each other with non-

conducting plastic and are covered with a permeable membrane with a few drops of

electrolyte between the membrane and electrodes. The amount of oxygen that travels

through the membrane is proportional to the dissolved oxygen concentration. Dissolved

oxygen probes are convenient for fieldwork, but need careful maintenance and calibra-

tion (usually against Winkler results). Most oxygen probes are sensitive to changes in

temperature and have thermisters attached to the probe so that temperature adjustments

can be made in the field.

BIOCHEMICAL OXYGEN DEMAND

The rate of oxygen use is commonly referred to as biochemical oxygen demand (BOD).

Biochemical oxygen demand is not a specific pollutant, but rather a measure of the