This describes how to determine the oxygen content in water according to the TÖDT and TESKE method.
Water intended for use as an ingredient in the production of beer (brewing liquor) or other foods
Oxygen is present in water in varying concentrations. The oxygen content of cold tap water has a significant impact on iron pipes. If the water exhibits an oxygen deficiency (less than 2–3 mg/l O2), iron pitting can occur, since a protective layer between the water and pipe cannot form.
However, for boiler feed water, the absence of oxygen is normally required (depending upon the type of boiler, no more than 0.02–0.5 mg/l should be present) in order to prevent corrosion. The oxygen determination is vitally important (BOD5 value) for testing and evaluating wastewater.
The measurement using the Digox Analyzer occurs without a membrane using the potentiostatic 3-electrode measuring system according to TÖDT und TESKE.
The cathode performing the measurement consists of solid silver, while the anode is made of stainless steel and serves as an auxiliary electrode. The reference electrode consists of silver/silver chloride.
An electrochemical reaction occurs after applying a defined polarization voltage at the cathode. The oxygen molecules are reduced in this reaction.
Cathode (silver):
O2 + 2 H2O + 4 e- → 4 OH- (cathodic process)
Auxiliary electrode (VA):
4 OH- → O2 + 2 H2O + 4 e- (anodic process)
After a specific polarization voltage is applied, an electrochemical reaction takes place at the cathode. The current flowing while the reaction is taking place is directly proportional to the quantity of dissolved oxygen, i.e., if the polarization voltage remains as precise as possible at the level of the diffusion limited current.
If this is the case, the relationship is represented as follows:
I = K × CO2, where K = n × F × A × 1/d
I = current measured
CO2 = oxygen concentration
n = number of electrons exchanged per molecule
F = Faraday constant
A = cathode surface
d = thickness of the natural convection boundary layer
The thickness of the natural convection boundary layer is defined by the hydrodynamic conditions at the cathode, while the movement of oxygen molecules through the barrier layer is determined by temperature-dependent diffusion processes. Those two clearly defined factors are measured precisely and are taken into consideration.
In order to adjust the polarization voltage between both electrodes in a defined manner, a third electrode, the auxiliary electrode, is employed for DIGOX analyzers. This auxiliary electrode remains in electrolytic contact with the surface of the cathode by means of a diaphragm but without the possibility of a mass transfer.