Dissolved Oxygen – An Introduction

Oxygen is a vital component for almost all living beings, but also an important element for countless natural and industrial processes.

As is generally known, oxygen is a colorless and odorless gas with a share of about 21 % in our atmosphere, in total (bound) oxygen is the most abundant element in the earth's crust with almost 50 percent by mass and the third most abundant element in the universe. Like almost all gaseous elements (except for noble gases), it occurs on Earth as a gas almost exclusively as a molecule, two oxygen atoms covalently combine to form an oxygen molecule. Its name is derived from ancient Greek and means something like "acidifier". This should not be taken literally, because the "cause" of acid, the hydrogen ion, was only found later.

The reason why we are dealing with oxygen at this point is its solubility in liquids, as a manufacturer of water analysis systems understandably its solubility, especially in water and aqueous solutions. Its solubility is, among other things, crucial for life and most biological and biochemical processes in water.

Oxygen partial pressure

Oxygen, like many gases, dissolves in liquids, but also in plastics, for example, but this is only noticed in passing. This solubility depends on two factors:

1. The partial pressure of oxygen in the atmosphere
2. The temperature of the liquid

Schematic of oxygen partial pressure

The dissolved oxygen measurement is a partial pressure measurement because the pressure caused by the dissolved oxygen in the solution is the same as the pressure of the oxygen in the atmosphere above. In physical chemistry, this is also referred to as the so-called fugacity of a gas.

If you calculate the partial pressure at a normal pressure above sea level of 1013 hPa according to the proportion of oxygen, you get about 212 hPa in dry air. However, since the water vapour pressure above the water surface must also be considered, the value at 20 °C is around 207 hPa.

The oxygen partial pressure is the leading variable in dissolved oxygen measurement, from which two other parameters that are important for practice are derived.

Oxygen saturation

The oxygen saturation in measurements in water and aqueous solutions always means the saturation in air, here the measured partial pressure is set in relation to the theoretically possible pressure (corrected by the water vapor pressure)

• X_O2: Molar fraction
• p_w: water vapor partial pressure
• p_A: actual barometric pressure (unscaled)
• p_O2: Oxygen partial pressure

Since the saturation depends only on the partial pressure, it is also suitable for approximate determination in non-aqueous liquids, provided that the chemical resistance for the probe is given.

The oxygen concentration

Henry's law according to the English chemist William Henry (1774 – 1836) describes the solubility of dissolved oxygen in liquids. This is also based on partial pressure, but also on the ability of a liquid to dissolve substances. Henry's Law looks like this:

• c: Oxygen concentration
• XO₂: Molar fraction
• p_A: actual barometric pressure (unscaled)
• p_w: Water vapor pressure
• MO₂: Molar mass oxygen
• p₀: 1013 hPa (air pressure to normal zero)
• V_M: Molar volume of oxygen
• α: Bunsen absorption coefficient (solvent and temperature dependent)

A guideline value for the solubility of oxygen at 20°C and 1013 hPa is 9.18 mg/l. Due to the Bunsen absorption coefficient, the concentration calculation in the Xylem Analytics systems is limited to the measurement of the concentration in water, the output for other liquids is not possible.

A few conclusions from the above parameters:

1. The dissolved oxygen is always a consequence of the oxygen partial pressure of the protruding air column. It is the same at saturation both inside and outside the solution.
2. The concentration of dissolved oxygen in a solution depends not only on the partial pressure, but also on the temperature and the type of solution itself.

Read more in our other blog articles about dissolved oxygen measurement: