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 [3] 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 [4 – 7].

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

Electrochemical and optical sensors are the most widespread means of DO concentration measurement. Both are widely used but the effects of different uncertainty sources on the results are remarkably different and estimation of uncertainty is not straightforward. In order to help practitioners with this, comprehensive comparative validation for these two different types of dissolved oxygen (DO) analyzers, amperometric and optical, was recently carried out on the basis of two representative commercial DO analyzers and published: I. Helm, G. Karina, L. Jalukse, T. Pagano, I. Leito, Environ Monit Assess 2018, 190:313 (ref [1]).

A number of performance characteristics were evaluated including drift, intermediate precision, accuracy of temperature compensation, accuracy of reading (under different measurement conditions), linearity, flow dependence of the reading, repeatability (reading stability), and matrix effects of dissolved salts. The matrix effects on readings in real samples were evaluated by analyzing the dependence of the reading on salt concentration. The analyzers were also assessed in DO measurements of a number of natural waters. The uncertainty contributions of the main influencing parameters were estimated under different experimental conditions. It was found that the uncertainties of results for both analyzers are quite similar but the contributions of the uncertainty sources are different. The results imply that the optical analyzer might not be as robust as is commonly assumed, however, it has better reading stability, lower stirring speed dependence, and typically requires less maintenance. On the other hand, the amperometric analyzer has a faster response and wider linear range. The approach described in this work will be useful to practitioners carrying out DO measurements for ensuring reliability of their measurements.

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 that 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. 2012, 741:21-31 (ref [2]).

The contents of this on-line course can be used as basis for carrying out the measurement uncertainty evaluation described in the above mentioned report. In particular, the Self-test 9.2 A and Self-test 9 B  are directly related to DO concentration measurement.

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

1. I. Helm, G. Karina, L. Jalukse, T. Pagano, I. Leito, Environ Monit Assess 2018, 190:313.
2. I. Helm, L. Jalukse, I. Leito, Anal. Chim. Acta. 2012, 741:21-31.
3. R.F. Keeling, H.E. Garcia, PNAS, 2002, 99(12):7848 – 7853.
4. G. Shaffer, S. M. Olsen, J. O. P. Pedersen, Nat. Geosci. 2009, 2:105-109.
5. R. F. Keeling, A. Körtzinger, N. Gruber, Annu. Rev. Mar. Sci. 2010, 2:199–229.
6. D. Gilbert, N.N. Rabalais, R.J. Diaz, J. Zhang, Biogeosci. Discuss. 2009, 6:9127–9160.
7. J.P.A. Hobbs, C. A. McDonald, Journal Fish Biol. 2010, 77:1219 – 1229.