In-Situ® RDO® Methods For Analysis Of BOD, CBOD, And DO

By Edward F. Askew, Ph.D.

Since the publication of the Winkler method for measuring dissolved oxygen (DO) (Winkler 1888), the analysis of DO levels for water has been key to determining 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 being based on oxygen solubility at saturation as per Henry’s Law. Though many researchers have refined the Winkler analysis to DO levels in the low µg/L range (Potter 1957; Potter and White 1957; Potter and White 1957), the method does not lend itself to automation, continuous monitoring, or process control due to the nature of the method.

The development of an analytical instrument that uses the reduction-oxidation (redox) chemistry of oxygen was introduced during the 1950s (Kemula and Siekierski 1950). This redox electrode uses an oxygenpermeable membrane to allow the diffusion of the gas into the electrochemical cell and the redox potential is measured by the sensor. This analytical method is sensitive and accurate down to levels of ±0.1 mg/L DO (APHA 2005). However, the sensing technology is susceptible to drift and requires membrane replacement and regular maintenance and calibration.