An automated precise Winkler titration for determining dissolved oxygen on board ship

Abstract

An automated titration for the Winkler method is presented for measuring primary productivity in the ocean. The system is based on a microcomputercontrolled titration with potentiometric endpoint detection. By the use of 0.005-N sodium thiosulfate as a titrant and a program designed to shorten the time for measuring, the method achieves a precision of 0.04% coefficient of variation with a range of 0.01 to 0.10% for six replicates of samples at oxygen concentration of 70 to 250 µM. It takes about four to five minutes to measure one sample.

Winter measurements of oceanic biogeochemical parameters in the Rockall Trough (2009–2012)

“…A Metrohm 848 Titrino Plus, with a Metrohm combined Pt electrode was used to determine the endpoint, i.e. potentiometric endpoint determination, measuring the change in redox potential of the sample, which reaches a minimum at the endpoint (Furuya and Harada, 1995). This method of determination was also used effectively by numerous WOCE cruises in the Atlantic and Pacific oceans, and also on some Hawaii Ocean Time Series (HOT) cruises (http://www.soest.hawaii.edu/HOT_WOCE/).…”

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“…A 25 mL aliquot was drawn from the acidified solution with a volumetric pipette and was titrated using sodium thiosulfate (0.0125 N). Titration equivalence points were determined potentiometrically using an auto-titrator (Mettler DL-50) equipped with a platinum combination electrode (Mettler DM 140-SC; Graneli and Graneli 1991;Furuya and Harada 1995;R. Benner pers.…”

“…Therefore, we used the Winkler method with potentiometric auto-titration when measuring the oxygen concentrations in each bottle. This is an efficient, accurate, and precise method to measure oxygen concentration, especially when attempting to detect small amounts of oxygen consumption (Furuya and Harada 1995). Auto-titration was precise within 0.05 ± 0.009 mg O 2 L -1 (mean ± SE, n = 22) for oxygen concentrations ranging from 3.5 to 8.5 mg O 2 L -1 , or 0.5% ± 0.2% (mean coefficient of variation ± SE, n = 22) comparable to precision seen in other studies for Winkler measurements (e.g., Ploug et al 2002).…”

“…For example, an average baseflow suspended POC concentration in the third-order stream from the eastern Piedmont sampled for this study is 0.15 mg POC L -1 (Richardson et al 2009), which would consume < 0.1 mg L -1 oxygen during a 24-h incubation, assuming 20% of POC were mineralized. The precision of the O 2 measurement can be increased with the use of potentiometric auto-titration of the classic Winkler reaction to quantify O 2 concentrations, which has a real-world precision of ± 0.25% to ± 0.5% or approximately ± 0.01 to ± 0.08 mg L -1 (Graneli and Graneli 1991;Benner 1995;Furuya and Harada 1995;Amon and Benner 1996). However, a 10-fold improvement in O 2 measurement precision alone is not sufficient to develop the high precision data for curve fitting various kinetic models.…”

Use of an optical oxygen sensor to measure dissolved oxygen in seawater

“…Although titration has been regarded as the standard method, it is rather tedious and time-consuming, even when used in combination with photometric endpoint detection (Williams and Jenkinson, 1982;Furuya and Harada, 1995). Furthermore, because the sensitivity of the method is low, approximately 100 mL of seawater is required.…”