Determination of dissolved oxygen concentration by electrochemical oxygen sensors with exposed electrodes
The measurement process using a Digox Analyzer works according to the principle of the potentiostatic three-electrode measurement system developed by TÖDT and TESKE and does not require a membrane.
The measuring electrode consists of solid silver, while the counter electrode is made of stainless steel. The reference electrode is composed of silver/silver chloride.
After generating a defined “polarization voltage”, an electrochemical reaction occurs at the measuring electrode, inducing a reduction of the oxygen molecules in the sample.
Measuring electrode (silver):
O2 + 2 H2O + 4 e− → 4 OH− (cathodic process)
Counter electrode (stainless steel):
4 OH− → O2 + 2 H2O + 4 e− (anodic process)
The flow of current as a result of this reaction is directly proportional to the amount of dissolved oxygen in the sample, if the polarization voltage is fixed as close to the level of the diffusion threshold current as possible.
In this case, the relationship can be represented as follows:
I = K × CO2, whereupon K = n × F × A × 1/d
I = sensor current
CO2 = oxygen concentration
F = Faraday constant
n = number of electrons per molecule
A = cathode surface
d = thickness of the “undisturbed layer” along the wall
The thickness of the undisturbed layer along the wall is determined by the hydrodynamic relationships at the measurement electrode and the transportation of oxygen molecules across the boundary layer brought about by temperature-dependent diffusion processes. Both of these clearly defined factors are precisely measured and compensated.
In order to adjust the polarization voltage between the two electrodes, a third electrode, the reference electrode, is employed in Digox measurement devices. This reference electrode remains in contact with the surface of the measuring electrode over a diaphragm in order to prevent mass transfer [1, 2, 3].
Active calibration:
In-line calibration is integrated into the device and is initiated by pressing a button. Taking advantage of Faraday’s Law, an exactly defined amount of oxygen is produced through the electrolysis of water.
I × t = m × F
I = current required for electrolysis
t = time
m = mass, g/mol
F = Faraday constant
The oxygen dissolves in the medium as it flows through and is detected at the measuring cell. The hydrogen liberated during the electrolysis reaction is not relevant for the measurement. The microprocessor monitors the calibration values and carries out any necessary corrective measures. The electrolysis enables the calibration of the device to be carried out under the same conditions and in the same medium as the analysis. Measurement operations are not disrupted during the calibration process [3].
The following applies to Digox 6.1 and all later models: In order to precisely determine the necessary potential for the measurement system, the Digox Analyzer possesses a scanner, which records the product-specific behavior of the medium subject to analysis. This can establish, whether the medium – due to the additives or supplements – must be measured at another potential. In this way, all types of beer-based beverages, non-alcoholic beverages and wine can be analyzed. Moreover, oxygen-reducing substances which can cause the calibration to be inaccurate may also be detected using the calibration scanning process. The device automatically implements the necessary compensative measures with the factors it has determined.
The method describes how to determine acrylamide in drinking water using gas chromatography.
Water intended for use as an ingredient in the production of beer (brewing liquor) or other foods
The method facilitates the determination of trace amounts of acrylamide monomers in aqueous matrices and is based upon the bromination of the acrylamide double bond. The reaction product (2,3-dibromopropionamide) is extracted from the mixture with ethyl acetate after precipitation with sodium sulfate. The extract is purified in a Florisil column and analyzed using gas chromatography (GC/ECD). The detection limit in aqueous matrices is approx. 0.032 µg/l.
Water intended for use as an ingredient in the production of beer (brewing liquor) or other foods
Drinking water intended for use as an ingredient in the production of beer (brewing liquor) or other foods
Due to physico-chemical properties of these substances, a number of effective enrichment processes are available for analysis using gas chromatography and can be summarized as follows:
pentane extraction
adsorption onto solid materials using thermal desorption (purge and trap)
headspace techniques
Pentane extraction
The sample is cooled with ice and extracted using chilled pentane. Subsequently, the pentane phase is separated with a micro separator.
Purge and Trap
The purge gas, as a rule, the carrier gas of the gas chromatograph, passes through the exhaust vessel filled with the water sample. Through stripping, the volatile substances are driven out and then accumulate on the sorbent, e.g., Tenax. After the stripping process is complete, the substances are thermally desorbed by rapidly heating the adsorber column. They are then conveyed to the gas chromatograph through a heated transfer tube.
Headspace techniques
The static headspace method is an ideal technique for the analysis of the volatile substances found in water, due to the simple sample preparation and the substantial sensitivity of the analysis. A further advantage of this procedure is that particulate matter as well as other substances present in the sample with a low volatility and high molecular weight do not interfere with the analysis, since they are not carried by the steam into the headspace and are therefore do not reach the separation system. Moreover, the high degree of automation combined with the aforementioned short time required for sample preparation allows for a rapid, precise and user-friendly analysis for water samples.
Gas chromatography
For the gas chromatographic analysis, an electron capture detector (ECD) is employed due to its high selectivity and high sensitivity. If the ECD is combined with a flame ionization detector (FID), methylene chloride, benzene and its homologues can also be analyzed. With the aid of cryo-focusing, this method can be adapted to detect more volatile substances, such as vinyl chloride or chlorofluorocarbons.
The method describes how to determine the content of tetrachloroethene and trichloroethene in drinking water through extraction and gas chromatography.
Water intended for use as an ingredient in the production of beer (brewing liquor) or other foods
The PAH compounds are concentrated through extraction with cyclohexane and evaporation. The separation is carried out by means of high-performance thin-layer chromatography. The results are evaluated through examination under UV light, followed by comparison of the Rf values with the reference substances.