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Solubility is the property of a solid, liquid, or gaseous chemical substance (called solute) to dissolve in a solid, liquid, or gaseous solvent. Usually we hear of oxygen or dissolved O2. It means that O2 is soluble in water. Ozone gas (O3) is 13 times more soluble in water than O2.

Ozone is soluble in many substances, forming either stable or metastable solutions. Under practicable conditions in water, ozone is more soluble than oxygen but forms a metastable solution.

The ability to dissolve of gases in liquid media is described by Henry’s law (Henry’s law states that the solubility of a gas in a liquid is proportional to the pressure of the gas over the liquid). As Henry’s said this law can only be applied on gasses that do not chemically change in water, during transfer.

Conditions that will affect the level of solubility of a gas in water:

  • Water temperature.

  • Water pressure: the higher the pressure, the greater the the dissolution capacity.

  • Gas pressure: pressurized gas, ie gas that is at an increased pressure, being applied in water, also improves efficiency (Increased pressure, will increase solubility).

  • Concentration: If the gas you are putting into the water is in an increased concentration, this will allow for additional solubility.

  • The efficiency of the diffuser, whose purpose is to transfer the ozone into the water.

It is more complicated to predict ozone solubility than other gases, because the solubility of ozone is influenced by several factors. The degree of solubility of the ozone gas depends on the concentration in the gas and therefore depends on the partial pressure. In addition to the partial pressure, other important factors influencing the ozone solubility are the temperature, the pH and the concentration of ions in the solution.

In summary, the solubility of ozone in water can be increased by:

  • Increase the concentration of ozone in the feed gas;

  • Growing air pressure;

  • Decreasing water temperature;

  • Decreasing the amount of solutes;

  • Decreasing the pH.

Ozone bubbles in water at 30ºC.

Water at 30º C
will have less ozone bubbles than 10º C.

Ozone bubbles rising from the bottom of a container filled with water at a temperature of 10ºC.

Water at 10 ° C
will have more ozone bubbles than 30 ° C.

The table below lists the solubility of 100% ozone in pure water, for the range of 0ºC – 60ºC. This values were divided by 40, 20 and 10, respectively, to obtain the solubility for 2.5%, 5% and 10% by weight (wt.) ozone concentrations.

Temperature (ºC)mg/l (PPM)mg/l (PPM)
(2.5% by wt. O3)
mg/l (PPM)
(5% by wt. O3)
mg/l (PPM)
(10% by wt. O3)
Source: Ulmann’s, 1991

Different temperature and ozone concentration plays on ozone solubility into water. This shows that a small change in water temperature may create a large difference in potential ozone dissolved into water. Also, changes in ozone concentration (dry air to oxygen) will change the ozone solubility dramatically.

The table below shows the difference that water pressure causes on Ozone Solubility for an ozone concentration of 6% by weight and water temperature of 15ºC.

Water Pressure(mg/l)
6% O3 at 15ºC (S=0.31)
0 PSIG (1 atm)26.598
5 PSIG (1.34 atm)35.64132
10 PSIG (1.68 atm)44.68464
15 PSIG (2 atm)53.196
20 PSIG (2.36 atm)62.77128
25 PSIG (2.7 atm)71.8146
30 PSIG (3 atm)79.794
Image showing the effect of water pressure on ozone solubility with a pressure of 5 psi. The image displays two separate containers filled with water, both with ozone diffusing into the water, but one container is under higher pressure causing more ozone to dissolve in the water.
Ozone solubility test at 20psi water pressure

Thus, the higher the water pressure, the greater the ozone solubility and the greater the mass transfer of ozone to the water. However, water pressure can cause differences in gas transfer to pure effect and faster decomposition of the ozone gas.

Above we can see that an increase in water pressure from 5 PSIG to 20 PSIG almost doubles ozone solubility. In some applications, a small increase in water pressure may make a huge difference in ozone mass transfer into water. So, when water pressure decrease ozone solubility also decreases.

Higher water pressures result in higher dissolved ozone levels.