Automatic potentiometric determination of dissolved oxygen

Abstract

Commercially available automatic titration systems were tested in 1988 for potentiometric titration of liberated iodine by the Winkler method of oxygen determination. The potentiometric equivalence point was also compared to the manual starch end point. Finally the automatic method was used in order to estimate belowhalocline respiration in the Kattegat, Sweden. Standard deviations of 0.007 ml O₂ l⁻¹ or 0.1 to 0.3% coefficients of variation (% standard deviation of the mean) were achieved when titrating 25 ml from replicate 60-ml oxygen bottles using the automatic method, or 50 ml manually. The precision for replicate titrations of 50-ml aliquots of 0.001N KIO₃ was <0.05% (0.002 ml 0.01N Na₂S₂O₃) for the automatic method. Titration time for 25-ml aliquots was 2 to 4 min, somewhat longer than for manual titrations (1 to 1.5 min). However, during titration the operator is free to perform other tasks. It is not possible to use automatic sample changers, due to rapid iodine volatilization. The equipment can be handled by relatively unskilled analysts and is suitable for use on board research vessels or in field stations [weight for a Mettler^TM titrator (Mettler Instrumente AG, Greifensee, Switzerland) <10 kg, volume <0.1 m³]. Below-halocline oxygen consumption in the SE Kattegat ranged from 0 to 6 ml O₂ m⁻³ h⁻¹ (mean values for September and October 1988=1.69 and 0.66 ml O₂ m⁻³ h⁻¹, respectively, with 95% confidence limits of ca. ±0.6 ml O₂ m⁻³ h⁻¹).