Summary

Overview
To date, it is impossible to compare pH values of solutions made in different solvents. This situation is untenable, since it causes confusion and inaccuracies in many fields, extending far beyond the specific field of acid-base chemistry, to almost all spheres of life. The purpose of this project is to overcome this situation by putting the new theoretical concept of a unified pH_abs scale on a metrologically well-founded basis into practice. The project results will ensure that pH measurement systems are available to industry and academia for accurate pH testing in a wide variety of media.

Need
The pH of solutions is probably the most prominent chemical concept widely utilised outside the field of chemistry. It is used in a near uncountable number of areas, e.g., medicine and life sciences, biology, environmental science, agrology, marine sciences, meteorology, material sciences, corrosion science, energy related sciences, etc. It plays an important role in virtually all material related processes, in their production (e.g., processing metals, paper, plastics, glasses, etc.) as well as in their reprocessing (domestic and industrial wastewater, extraction of solid wastes, etc.). Accurate analysis and monitoring of pH values is therefore an extremely important task in a wide variety of technologies. This variety entails a diversity of media in which those processes occur, i.e., different solvents, solvent mixtures, and dispersions. For basic thermodynamic reasons, valid comparability of pH values in different media has been impossible for a long time, even on the theoretical level. As a result, several pH scales exist in parallel, loosely correlated to each other, without the possibility of converting one scale into the other to the necessary level of accuracy.
In 2010, the unified acidity (pH_abs) concept was introduced to overcome this untenable situation, enabling comparability of pH values between all phases, whether they are gaseous, liquid, or solid. Although conceptually excellent, putting this concept into practical use has proved tremendously difficult and has not yet been fully achieved. In order to be practically usable the knowledge of physicochemical quantities is required, access to which is not straightforward. Additionally, suitable measurement and calibration procedures are required, all of which still need to be developed.
This project concerns the practical realisation of the theoretical concept of pH_abs. This includes the determination of the mentioned quantities, as well as the standardisation of the measurement and calibration processes of pH_abs.

Objectives
The major objectives laid out for this project are:

1. To develop and validate a reliable and universally applicable measurement procedure, including uncertainty budgets, that enables the measurement of pH_abs (expressed relative to the aqueous scale, as pH_abs^H₂O-values) in non-aqueous and mixed solvents, colloids, etc., thereby enabling their acidities to be compared to the conventional aqueous pH scale. The target combined standard uncertainties to be 0.15 pH_abs in "good" (mixed aqueous, alcohols, etc.) and 0.25 pH_abs in "difficult" (colloids, aprotic, etc.) systems.

2. To create a reliable method for the experimental or computational evaluation of the liquid junction potential between aqueous and non-aqueous solutions where the bridge electrolyte is an Ionic Liquid, allowing correction of results from, for example, a glass electrode pH measurement setup calibrated with aqueous standards measuring non-aqueous samples.

3. To develop a coherent and validated suite of calibration standards (including procedure and sensor) for standardising routine measurement systems in terms of pH_abs values for a wide variety of media (e.g., industrial mixtures, soils/waters, food products, biomaterials).

4. Based on the outcome of the project, to contribute to the international specifications for bioethanol quality
EN 15490 (e.g., by assigning values to existing certified reference materials) and other relevant standards development organisations, and to disseminate the findings to the European measurement infrastructure.

The Publishable Summary for 17FUN09 UnipHied Realisation of a Unified pH Scale can be downloaded here .