Hach-Lange 9187 sc - Manual (Page 41)

39
Appendix B 9185sc Theory of Operation
B.1 Theory of Operation
The 9185sc Ozone Analyzer is an on-line, single-channel industrial analyzer that
measures ozone in drinking water treatment plants, distribution networks, and other
applications that require monitoring ozone at the ppb and ppm levels.
This instrument uses an amperometric method to measure O
3
concentration. A membrane
allows the selective diffusion of O
3
molecules to the amperometric sensor ( Figure 1 on
page 6 ). The measurement is compensated for pH and temperature.
B.1.1 Principle of Operation
Ozone is a gas that is highly soluble in water (13 times more than oxygen). It is unstable
when dissolved in water.
Effects on solubility:
• Some parameters e.g. temperature and pH can influence the stability of the
measurement. The solubility of ozone in water decreases rapidly with temperature.
Effects of pH: Ozone reacting with OH – hydroxide ions: the greater the number of these
ions (high pH), the greater the degree of breakdown. Conversely, at a low pH, breakdown
will be slower (
Figure 14 ).
Figure 14 Time of Breakdown of Dissolved Ozone
Finally, it is worth noting that, as the OH – ion is a by-product of ozone breakdown in water,
the reaction between OH – and O
3
can be sustained until complete disappearance of the
ozone. This is more pronounced if air is mixed with the water sample.
The exposure to free air of ozone water results in significant degassing: as the ozone
content of ambient air is very low compared to that of the sample, exchange therefore
occurs, with rapid loss of ozone in the sample.
This problem is even more pronounced if air/water mixing occurs. All of these phenomena,
therefore, require that certain precautions be taken concerning the sampling line
(
section 3.2 on page 10 and section 3.3 on page 10 ).