13.2. Measurement uncertainty estimation in dissolved oxygen determination by electrochemical and optical sensors, as well as by high-accuracy gravimetric Winkler titration method

Dissolved oxygen (DO) is one of the most important dissolved gases in water. Sufficient concentration of DO is critical for the survival of most aquatic plants and animals [2] as well as in waste water treatment. DO concentration is a key parameter characterizing natural and wastewaters and for assessing the state of environment in general. Besides dissolved CO₂, DO concentration is an important parameter shaping our climate. It is increasingly evident that the concentration of DO in oceans is decreasing [3 – 6].

Accurate measurements of DO concentration are very important for studying these processes, understanding their role and predicting climate changes.

The Winkler titration method is considered the most accurate method for DO concentration measurement. Careful analysis of uncertainty sources relevant to the Winkler method was carried out and the results are presented as a „Report on improved high-accuracy Winkler method for determination of dissolved oxygen concentration“. 

In this report it is described how the Winkler method was optimized for minimizing all uncertainty sources as far as practical. The most important improvements were: gravimetric measurement of all solutions, pre-titration to minimize the effect of iodine volatilization, accurate amperometric end point detection and careful accounting for dissolved oxygen in the reagents. As a result, the developed method is possibly the most accurate method of determination of dissolved oxygen available. Depending on measurement conditions and on the dissolved oxygen concentration the combined standard uncertainties of the method are in the range of 0.012 – 0.018 mg dm^-3 corresponding to the k = 2 expanded uncertainty in the range of 0.023 – 0.035 mg dm^-3 (0.27 – 0.38%, relative). This development enables more accurate calibration of electrochemical and optical dissolved oxygen sensors for routine analysis than has been possible before. Most of this report is based on the article I. Helm, L. Jalukse, I. Leito, Anal. Chim. Acta. 741 (2012) 21-31 (ref [1]).

Preparation of this report was supported by the European Metrology Research Programme (EMRP), project ENV05 “Metrology for ocean salinity and acidity”. 

1. I. Helm, L. Jalukse, I. Leito, Anal. Chim. Acta. 741 (2012) 21-31.
2. R.F. Keeling, H.E. Garcia, PNAS, 99(12) (2002) 7848 – 7853.
3. G. Shaffer, S. M. Olsen, J. O. P. Pedersen, Nat. Geosci. 2 (2009) 105-109.
4. R. F. Keeling, A. Körtzinger, N. Gruber, Annu. Rev. Mar. Sci. 2 (2010) 199–229.
5. D. Gilbert, N.N. Rabalais, R.J. Diaz, J. Zhang, Biogeosci. Discuss. 6 (2009) 9127–9160.
6. J.P.A. Hobbs, C. A. McDonald, Journal Fish Biol. 77 (2010) 1219 – 1229.