Lauri Jalukse

Measurement Uncertainty Estimation in Amperometric Sensors: A Tutorial Review

This tutorial focuses on measurement uncertainty estimation in amperometric sensors (both for liquid and gas-phase measurements). The main uncertainty sources are reviewed and their contributions are discussed with relation to the principles of operation of the sensors, measurement conditions and properties of the measured samples. The discussion is illustrated by case studies based on the two major approaches for uncertainty evaluation–the ISO GUM modeling approach and the Nordtest approach. This tutorial is expected to be of interest to workers in different fields of science who use measurements with amperometric sensors and need to evaluate the uncertainty of the obtained results but are new to the concept of measurement uncertainty. The tutorial is also expected to be educative in order to make measurement results more accurate.

Micro-Winkler titration method for dissolved oxygen concentration measurement.

In this report a gravimetric micro-Winkler titration method for determination of dissolved oxygen concentration in water is presented. Mathematical model of the method taking into account all influence factors is derived and an uncertainty analysis is carried out to determine the uncertainty contributions of all influence factors. The method is highly accurate: the relative expanded uncertainties (k=2) are around 1% in the case of small (9-10 g) water samples. The uncertainty analysis carried out in characterizing the uncertainty of the method is the most comprehensive published for a micro-Winkler method, resulting in experimentally obtained estimates for all uncertainty sources of practical significance (around 20 uncertainty sources altogether).

Model-based measurement uncertainty estimation in amperometric dissolved oxygen concentration measurement

In this paper, uncertainty sources in amperometric dissolved oxygen (DO) concentration measurement are explored and the ISO GUM uncertainty estimation procedure based on a detailed measurement model is presented. The procedure is applied to two different commercial amperometric DO measurement instruments of galvanic type differing in cathode and membrane area and membrane thickness. The complete uncertainty budgets of several typical measurement processes of the two instruments are discussed. From this comparison evidence is provided that the deciding influential factors may be different for the investigated instruments under otherwise comparable measurement conditions, even though the instruments follow the same working principles. Furthermore, the uncertainty as well as the uncertainty budget of the same instrument under different measurement conditions may differ significantly. In this study, variations in the relative expanded uncertainty between U = 0.8% and U = 9% (k = 2) were observed for the same instrument under different conditions. At DO concentrations lower than below 4 mg l?1 (depending on other conditions), the background current of the sensor becomes the dominating uncertainty source. At DO concentrations above that range, a variety of factors become relevant depending on the specific conditions, for instance stirring speed and membrane properties. The high importance of the cathode and membrane area, membrane material and membrane thickness on the uncertainty is demonstrated. Based on these results, a set of recommendations for DO sensor design is formulated.

Unified pH Values of Liquid Chromatography Mobile Phases

This work introduces a conceptually new approach of measuring pH of mixed-solvent liquid chromatography (LC) mobile phases. Mobile phase pH is very important in LC, but its correct measurement is not straightforward, and all commonly used approaches have deficiencies. The new approach is based on the recently introduced unified pH (pH(abs)) scale, which enables direct comparison of acidities of solutions made in different solvents based on chemical potential of the proton in the solutions. This work represents the first experimental realization of the pH(abs) concept using differential potentiometric measurement for comparison of the chemical potentials of the proton in different solutions (connected by a salt bridge), together with earlier published reference points for obtaining the pH(abs) values (referenced to the gas phase) or pH(abs)(H₂O) values (referenced to the aqueous solution). The liquid junction potentials were estimated in the framework of Izutsus three-component method. pH(abs) values for a number of common LC and LC-MS mobile phases have been determined. The pH(abs) scale enables for the first time direct comparison of acidities of any LC mobile phases, with different organic additives, different buffer components, etc. A possible experimental protocol of putting this new approach into chromatographic practice has been envisaged and its applicability tested. It has been demonstrated that the ionization behavior of bases (cationic acids) in the mobile phases can be better predicted by using the pH(abs)(H₂O) values and aqueous pKa values than by using the alternative means of expressing mobile phase acidity. Description of the ionization behavior of acids on the basis of pH(abs)(H₂O) values is possible if the change of their pKa values with solvent composition change is taken into account.

A highly accurate method for determination of dissolved oxygen: Gravimetric Winkler method

A high-accuracy Winkler titration method has been developed for determination of dissolved oxygen concentration. Careful analysis of uncertainty sources relevant to the Winkler method was carried out and the 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.

Using MOOCs for teaching analytical chemistry: experience at University of Tartu

The term MOOC (massive open online course) was coined in 2008 by Dave Cormier and George Siemens [1]. Since then, MOOCs have attracted much attention and public opinion on MOOCs has changed remarkably during the last few years. At the beginning of this decadeMOOCs were hailed as the future of higher education and the year 2012 was declared by the New York Times as “the year of the MOOC” [2]. It was even envisaged that higher education as we know it may collapse. Since then MOOCs have received quite some criticism— based on e.g. insufficient interaction between teachers and students, low course completion rates, etc. [3]—and it is now widely acknowledged that MOOCs were originally overhyped [4]. Nevertheless, MOOCs are offered by numerous universities, as well as by several major MOOC providers, such as Coursera [5] or edX [6]. The intense development of MOOCs goes on and rightly so, because MOOCs obviously enrich the higher education possibilities in the world. MOOCs are intrinsically less suited for experimental sciences, compared to e.g. web design, history or business, because it is impossible to offer the experimental/laboratory training via the Web. Nevertheless, there are MOOCs available also in chemistry and, more specifically, in analytical chemistry. The course “Analytical chemistry/instrumental analysis” (Prof. Vicki Colvin, Rice University) is offered at Coursera, and Udemy offers the course “Analytical chemistry” (Oxford Royale Academy). In this paper we present the experience of running a MOOC “Estimation of measurement uncertainty in chemical analysis” [7] at the University of Tartu (UT). We compare teaching in the “MOOC mode” to conventional university teaching as well as to short training courses for professionals.

Comparative validation of amperometric and optical analyzers of dissolved oxygen: a case study

A comprehensive comparative validation for two different types of dissolved oxygen (DO) analyzers, amperometric and optical, is presented on two representative commercial DO analyzers. 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 (at saturation concentration of DO). 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. Our 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. Both analyzers seem to have issues with the accuracy of temperature compensation. The approach described in this work will be useful to practitioners carrying out DO measurements for ensuring reliability of their measurements.