Some oxygen is dissolved in every liquid, and physics dictates that every liquid absorbs oxygen until the oxygen partial pressure in the liquid and the air or gas phase in contact with it is in equilibrium. The oxygen concentration depends upon a number of factors, such as temperature and ambient pressure.
At 20 °C and an ambient pressure of 1013 mbar, water contains about 9 mg/l in its saturated state, while in ethanol it can increase to 40 mg/l of oxygen.
Determination of the concentration of dissolved oxygen through electrochemical oxygen sensors with membrane-enveloped electrodes
Suitable for measurement in beer, wort, beer-based beverages, drinking water, carbonated and non-carbonated beverages. This method can be used to measure high concentrations of dissolved oxygen (in wort 10 to 50 mg/l) or to detect low oxygen concentrations (0.01 to 1.0 mg/l in beer).
Compounds, such as sulfur dioxide, hydrogen sulfide, chlorine and formaldehyde can disrupt this analysis. The effects of these compounds vary according to which sensor is employed; however, on the whole, these substances can be considered disruptive to this analysis.
The analytical determination of oxygen using amperometric electrodes is achieved through measurement of the electrical current. The electrodes consist of a cathode and an anode, which are connected conductively through an electrolyte solution (KCl/KOH). Precious metals, such as platinum and gold are chosen for the cathode, and silver, for the anode. The gas-permeable membrane separates the two electrodes from the solution being measured. An appropriate polarization voltage causes diffusion of the oxygen across the membrane into the measurement cell, where it reaches the surface of the cathode and is reduced, producing hydroxide ions.
Reaction at the cathode: O2 + 4e– + 2 H2O → 4 OH−
Reaction at the anode: 4 Ag + 4 Cl− → 4 AgCI + 4e−
This chemical reaction creates an electrical current that is proportional to the partial pressure pO2 of the oxygen. Oxygen must be steadily liberated from the solution being measured for the oxygen electrode to receive a constant supply. The concentration of oxygen in the medium can be determined using HENRY’s law and the solubility coefficient of oxygen [1]. Three different variations on the types of the equipment required for performing this analysis are present below.