Dissolved Oxygen Probes: Membrane and Luminescence
Since the publication of a simple, accurate and direct dissolved oxygen analytical procedure by Winkler, the analysis of dissolved oxygen levels for water have been key to the determination of surface water purity and ecological wellness. The Winkler method is still one of only two analytical techniques used to calibrate oxygen electrode meters, the other procedure based on oxygen solubility at saturation as per Henry's law. Though many researchers have refined the Winkler analysis to dissolved oxygen levels in the low PPB range the method does not lend itself to automation.
The development of an analytical instrument that utilizes the reduction-oxidation (redox) chemistry of oxygen in the presence of dissimilar metal electrodes was introduced during the 1950s. This redox electrode utilized an oxygen permeable membrane to allow the diffusion of the gas into an electrochemical cell and its concentration determined by polarographic or galvanic electrodes. This analytical method is sensitive and accurate to down to levels of ± 0.1 mg/l dissolved oxygen. Calibration of the redox electrode of this membrane electrode still requires the use of the Henry’s law table or the Winkler test for dissolved oxygen.
During the last two decades, a new form of electrode was developed based on the luminescence emission of a photo active chemical compound and the quenching of that emission by oxygen. This quenching photophysics mechanism is described by the Stern-Volmer equation for dissolved oxygen in a solution:
The determination of oxygen concentration by luminescence quenching has a linear response over a broad range of oxygen concentrations and has excellent accuracy and reproducibility. There are several recognized EPA methods for the measurement of Dissolved Oxygen for BOD, including the following methods:
- Standard Methods for the Examination of Water and Wastewater, Method 4500 O
- In-Situ Inc. Method 1003-8-2009 Biochemical Oxygen Demand (BOD) Measurement by Optical Probe.
Read more about this topic: Biochemical Oxygen Demand
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