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Current IssueJuly 31, 2015
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Ask The Expert: Ozone concentration in distilled water

We are putting ozone gas into distilled water and are wondering, if we are working at room temperature and normal ambient pressure, what concentration would be expected when the water is fully at saturation? How much more concentration could be expected if the water is chilled almost to freezing as the ozone is bubbled in? Thanks for your help.

William C. Domb, DMD, Upland Calif.



The solubility of ozone in water is dependent on several factors, including: concentration of ozone in the gas; water temperature; absolute pressure of the water; ozone demand in the water; contactor efficiency. Given these variables, the only way to really know the concentration of ozone in your specific system is to measure it empirically using a dissolved ozone sensor. That said, at gas phase ozone concentrations of one to three percent, you should expect 3.5 to 10.6 mg/L at 25C. The concentration of dissolved ozone will be higher at a lower water temperature. You can expect 7.4 to 22.2 mg/L at 5C at gas phase ozone concentrations of one to three percent. (See Ozone, a Reference Manual, pg. 19-20; by the WQA Ozone Task Force, Joseph F. Harrison, Technical Editor; published by the Water Quality Association, 2004).

Bob Smith-McCollum, Pacific Ozone Technology

This is covered by Henry's Law for Ideal Gases and is a function of water temperature and partial pressures (ozone gas phase concentration). As ozone is not an ideal gas, reactions take place that can lower the dissolved ozone level; however,if the water has little or no demand as would be expected with distilled water, you can use it.

Gases dissolve in liquids to form solutions. This dissolution is an equilibrium process for which an equilibrium constant can be written. For example, the equilibrium between ozone gas and dissolved ozone in water is O3(aq) <-> O3(g). The equilibrium constant for this equilibrium is K = p(O3)/c(O3). The form of the equilibrium constant shows that the concentration of a solute gas in a solution is directly proportional to the partial pressure of that gas above the solution. Stating the pressure-concentration ratio as an equation and use of the usual modern symbol for the Henry's Law constant on a concentration basis (Kc) gives the following form of Henry's Law: p = Kcc

In this form, p is the partial pressure of the gas, c is its molar concentration and Kc is the Henry's Law constant on the molar concentration scale. Henry's Law is found to be an accurate description of the behavior of gases dissolving in liquids when concentrations and partial pressures are reasonably low. As concentrations and partial pressures increase, deviations from Henry's Law become noticeable. This behavior is very similar to the behavior of gases, which are found to deviate from the Ideal Gas Law as pressures increase and temperatures decrease. For this reason, solutions which are found to obey Henry's Law are sometimes called ideal dilute solutions.

Paul Overbeck, International Ozone Association

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