6.1 Dissolved oxygen probes
A solute of great importance to the chemistry and biology of water is oxygen.
Dissolved oxygen (DO) is essential to maintain most forms of aquatic life and to allow the decomposition of organic matter, including organic effluents. This ensures the continued existence of rivers, streams and lakes in their natural state, and as useable resources.
DO is the amount of oxygen dissolved in water, and is expressed either as percentage saturation (% O2) or as a concentration (mg l−1).
Express 1.0 mg l−1 in parts per million.
1.0 mg l−1 = 1.0 ppm
It's important to remember that DO is in a state of dynamic equilibrium, and its solubility in water decreases as temperature is increased. As such, temperature sensors - thermistors (resistors whose resistance changes with temperature) - are often fitted into DO probes to enable automatic compensation for temperature fluctuations. Modern meters are also designed to compensate for the effects of atmospheric pressure and salinity of the solution being measured.
For modern DO probes calibration is carried out against the air, and the zero value may be checked against a sodium sulfite standard.
We'll look at two types of probe here, which as you'll see are capped with a membrane; these must be handled carefully to prevent damage.