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 Izutsu's 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.

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.

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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.

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