Calibration and field trial of a dissolved-oxygen membrane electrode: Long-term automatic analyser

Calibration and in-field determination of dissolved oxygen (DO) concentrations were carried out with an automatic water-quality monitoring system known as QUALTEL. A calibrated Orion oxygen membrane electrode and Winkler method were employed to measure DO in water with different chlorinities and compared with similar measurements using a Danfoss electrode. From the regression equations, a current of 2.97 mA at 25°C for water-saturated air was determined as being equivalent to a value of 8.68 mg DO L−1 for the Danfoss sensor. From a regression of log (μA mg−1 L−1) versus t (°C), a constant (m) and intercept (b) for water-saturated air was obtained, the values being 0.0263 and −0.759, respectively. A linear relationship with a high correlation coefficient for the salt content of the samples at different temperatures was obtained. The field measurements of DO showed that this system was reliable for the unattended monitoring of DO for 70 days.