In this posting we will discuss measurement of dissolved ozone. Knowing dissolved ozone concentration is important in a variety of situation, most especially in water disinfection. The ozone residual is critical for calculating the CT value as we have discussed in previous postings. CT in turn is the key parameter for determining pathogen inactivation. In applications where organic destruction is being considered, the residual ozone can help establish the mass balance for ozone in the process and therefore the amount of ozone required to treat a given amount of water. It may also be a useful marker for the end point in the end point of the reaction indicating that further ozone addition is not needed.
Measurement of dissolved ozone can be done directly or indirectly using both wet chemical methods and analytical instruments. Direct measurement of ozone in water can be done using UV or electrochemical methods. Measurement can accurately be made in the ppb range. The electrochemical method uses selective polarographic membrane sensors, dissolved ozone can be monitored interference-free.
The UV method employs UV absorption method. The amount of UV radiation absorbed is directly proportional to the amount of ozone in solution. Measurements in the low ppm range are possible with this approach. Both UV and electrochemical monitors provide direct digital readout ozone concentration. As a result they are extensively used in the water treatment industry.
In terms of wet chemical methods, the indigo method relies of the fact that in acidic solution ozone decolorizes indigo. The decrease in absorbance is linear with increasing concentration of ozone. A colorimeter can be used to measure absorbance. Alternatively a color wheel can be used, although the measurement than relies on the operator to judge the match between the color wheel and the change in color in the water. The indigo method is quantitative, selective and simple. It can be used to calibrate the digital instruments based on the electrochemical or UV method.
If the absolute value of ozone concentration is not needed, the oxidation reduction potential (ORP) can provide the operator with a rapid and single-value assessment of the disinfection potential of water and an indirect measure of ozone concentration. ORP monitors are generally less expensive than other ozone monitors, but since they do not give a direct read out of ozone concentration they are also less valuable. In addition, ORP is not specific to ozone and will measure any oxidant present in the water, e.g. chlorine. So if there is a chance for interference, ORP should not be employed.