Making pH measurements in low-polarity solvents possible

From left: Paulo and Jaan

For the first time, potentiometric pH measurement traceable to the conventional aqueous pH scale has been successfully demonstrated in a low-polarity solvent, 1,2-dichloroethane (1,2-DCE)!

Paulo and Jaan from our group achieved this remarkable result, which has now been published: “Experimental Unified pH Scale in 1,2-Dichloroethane” (Phys. Chem. Chem. Phys., 2025, 27, 3810–3816).

The measurement approach is rooted in the unified pH scale (pHabs scale) concept and is based on pairwise differential potentiometric comparisons of solutions yielding their pHabs differences (ΔpHabs values). 85 such pair-wise measurement comparisons were carried out between solutions prepared in 1,2-DCE, some solutions in mixed solvents and aqueous standard pH buffer solutions. The resulting pHabs “ladder” is pictured in Figure 1.

Figure 1. pHabs scale of solutions in 1,2-DCE, linked to the standard aqueous buffer solutions and comparison against reported pHabs values in the literature.

Applying a least squares minimization approach to the ΔpHabs values and taking into account the reference values of the aqueous standard buffers allowed us to assign pHabs values to 19 equimolar buffer solutions in 1,2-DCE, yielding a pHabs range of −2.9 to 11.0. The consistency standard deviation of 0.17 pH units—higher than similar measurements in polar solvents—reflects the experimental challenges of working in low-polarity media.

Multiple salt bridge configurations (shown in Figure 2) were tested to improve measurement stability. Out of the four tested configurations, a separate salt bridge configuration (SB4) with PEEK capillary tubes yielded the most stable and reliable results, extending measurements for up to five hours without significant drift.

Figure 2. Different salt bridge setup configurations used in the work.

Direct pH measurement of 1,2-DCE solutions against standard aqueous pH buffers is challenging because of the very different nature of the solvents and possible water contamination of 1,2-DCE can significantly affect the measurements. Therefore, bridging solutions (a 60:40 acetonitrile/pH 4 formate solution and buffered ethanol) were employed to facilitate measurements against standard aqueous pH buffers. This approach allows the pHabs values in 1,2-DCE to be directly comparable to the aqueous pH values.

The obtained results confirm that potentiometric pHabs measurements are possible in low-polarity solvents, paving the way for experimentally linking many low-polarity solvents into a unified pH scale. Employing pHabs to compare solution acidity across different media will improve our understanding of how pH impacts processes in catalysis, liquid chromatography, sustainable energy, and the interpretation of acid-base processes in various solvents.

Unified pH Scale – from Concept to Applications

On April 16, 2024, prof. Ivo Leito presented the topic of “Unified pH Scale – from Concept to Applications” at the 8th Baltic Electrochemistry Conference: Finding New Inspiration 2 (BEChem 2024), held in Tartu, Estonia. 

At the beginning of the presentation, Ivo gave a brief overview of why pH, a widely used value for expressing acidity, has serious limitations. Thus, it cannot be used to compare the acidities of solutions in different solvents, which can hinder the understanding of chemical, biological, environmental, and industrial processes.

A much more practical approach could be the usage of an absolute/universal pH, which would allow the comparison of acidities in different solvents. Thus, the main part of the presentation was focused on the theoretical and practical aspects of unified pH (pHabs). In recent years, our Acid-base Studies workgroup in the Analytical Chemistry Chair has worked extensively with the development and measurement of unified pH values for various systems in different solvents (liquid chromatography modile phases, electrocatalytic mixtures, strongly acidic solutions, etc). A differential potentiometry method for pHabs has been developed in the workgroup to obtain the unified pH values and has been now distributed to a number of groups in Europe in the framework of the UnipHied project.

For now, the unified pH has matured from a concept into a practical tool and has been accepted both by the European metrology community (Euramet) and by IUPAC. The unified pH could be utilized in various applications, such as liquid chromatography, catalytic systems, and acidity at the interfaces between phases. 

The presented research is largely the result of the EMPIR programme project 17FUN09 “UnipHied”.

Unified pH – what, why and how?

On March 10, 2023, Ivo Leito presented “Unified pH – what, why and how?” at the Chemistry and Chemical Technology 2023 Vilnius (Lithuania).

Acidity is one of the most critical characteristics of solutions. Its measurement is crucial to understanding and controlling essential processes in fundamental chemistry, industry and living organisms, such as catalysis, extraction, chromatography, processes in micelles/bilayers, etc.

Acidity refers to the activity of the solvated proton and is typically expressed as pH. However, the conventional pH scale is well established only in dilute aqueous solutions at medium pH values. It has severe limitations at extreme values, in other solvents or more complex media where most real-life chemistry occurs. Most importantly, comparing the conventional pH values between different media is impossible because every solvent has its pH scale.

Given the above, a decade ago, the concept of a “unified pH scale” was put forward, defining unified pH via the absolute chemical potential of the solvated proton. The merits of this approach are a strict thermodynamic foundation and direct comparability of values between any media.

In recent years, to a large part thanks to the European Union UnipHied (17FUN09) project, the measurement possibilities have been developed and the concept has now been published as an IUPAC technical report.

The first part of the presentation gave theoretical background and explained the need for unified pH. The second part described the experimental method, and the last part gave an overview of the work done and future perspectives.

 

Webinar “Mobile Phase pH in Liquid Chromatography” on April 29, 2021

It is well known that in liquid chromatography, mobile phase pH is an important parameter, significantly affecting the retention of acidic and basic analytes. Yet, mobile phase pH is tricky to measure because mobile phases are usually aqueous-organic mixtures and in the case of gradient elution mobile phase composition gradually changes during elution.

Mobile phase pH is the topic of an upcoming webinar “Mobile Phase pH in Liquid Chromatography”, which we will organise on 29.04.2021 at 13:00 – 16:00 (Central European time: France, Germany, …) via the Zoom platform. Registration to the webinar is now open at this registration link.

The topics that we plan to cover are:

— Different possibilities to express pH in liquid chromatography (LC)
— Unified pH (pHabs): the concept and measurements methods
— The applications and limitations of different pH expressions in LC

The webinar is organised in the framework of the project 17FUN09 “UnipHied”, which is funded from the EU’s EMPIR programme, co-financed by the Participating States and from the European Union’s Horizon 2020 research and innovation programme.

 

 

 

 

 

New publication – retention studies in C18 column using novel fluorinated eluent additives

It is essential to understand the interaction between the analyte, mobile phase and the column chemistry for the best possible separation of molecules. Of all three, the mobile phase in HPLC is the easiest to modify and thus, usually contains different eluent additives or buffers. While using mass spectrometry undoubtedly increases the variety of analytes possible to detect, it also introduces limiting factors such as the requirement of the volatility of the eluent. Most common additives in LC-MS systems are formic and acetic acid, their ammonium salts as well as ammonium bicarbonate. In our recent paper, we have proposed novel – fluorinated, eluent additives (hexafluoroisopropanol, hexafluoro-tert-butyl alcohol, trifluoroethanol as well as nonafluoro-tert-butyl alcohol and perfluoropinacol).  Their influence was shown on rather simple exemplary molecules, which are widely spread over different logP values, containing protonated and deprotonated acids and bases. All novel fluorinated eluent additives demonstrated a strong influence on basic polar analytes in basic medium – they drastically increased retention. A decrease in retention was observed for acidic analytes when these novel eluent additives were used.

Moreover, current research displays a comprehensive overview of retention mechanisms for nonafluoro-tert-butyl alcohol and is the first time ever when perfluoropinacol has been introduced as eluent additive for reversed phase chromatography. Additionally, the influence on MS signal was studied when fluoroalcohols were used as eluent additives. This is also the first time when the absolute pH (pHabs) scale was used for expressing the mobile phase pH.

Current fundamental research forms a basis for a better understanding of the influence of fluoroalcohols as eluent additives and will help in the assay development in a wide range of applications.

This work is part of a larger endeavor – to promote a wider usage the unified pH scale (pHabs) by the research and technology communities, which is currently in progress via the UnipHied.

The UnipHied project is funded from the EMPIR programme (project 17FUN09) co-financed by the Participating States and from the European Union’s Horizon 2020 research and innovation programme.

 

 

Pan-European Network of Fundamental pH Research: UnipHied

Initiated by our group, the pan-European research network of fundamental pH Research UnipHied started in May 2018.

Why is such network needed? As of now, it is not possible to compare pH values of solutions made in different solvents, as every solvent has its own pH scale. This situation is highly unfortunate, since it causes confusion and inaccuracies into many fields, extending far beyond the specific field of acid-base chemistry. Examples are industrial catalytic processes, food chemistry, liquid chromatograpy, etc. The central goal of UnipHied is to overcome this situation by putting the new theoretical concept of the recently introduced unified pHabs scale on a metrologically well-founded basis into practice.

The most important specific objectives of UnipHied are (1) to develop and validate a reliable and universally applicable measurement procedure that enables the measurement of pHabs; (2) to create a reliable method for the experimental or computational evaluation of the liquid junction potential between aqueous and non-aqueous solutions; (3) to develop a coherent and validated suite of calibration standards for standardizing routine measurement systems in terms of pHabs values for a variety of widespread systems (e.g., industrial mixtures, soils/waters, food products, biomaterials).

The first version of the pHabs measurement procedure has been created by Agnes Heering (Suu) in the framework of her PhD thesis. The main experimental difficulty is evaluation of the liquid junction potential (LJP), which will be thoroughly addressed by UnipHied. The first important steps towards this goal have very recently been made and published as two back-to-back papers: Angew. Chem. Int. Ed. 2018, 57, 2344–2347 and Angew. Chem. Int. Ed. 2018, 57, 2348–2352
The key achievement described in the papers is finding an ionic liquid, namely [N2225][NTf2], that can be used as salt bridge electrolyte and has such properties that two out of three main sources of LJP are eliminated.

The partners of the UnipHied network are LNE (France, coordinator), BFKH (Hungary), CMI (Czech Republic), DFM (Denmark), IPQ (Portugal), PTB (Germany), SYKE (Finland), TÜBITAK-UME (Turkey), Freiburg University (Germany), ANBSensors (United Kingdom), FCiencias.ID (Portugal), UT (Estonia).

UnipHied is funded from the EMPIR programme (project 17FUN09) co-financed by the Participating States and from the European Union’s Horizon 2020 research and innovation programme.

 

Agnes Heering successfully defended her PhD thesis on experimental realization of the unified pH scale

Agnes Heering successfully defended her PhD thesis on experimental realization of the unified pH scale

On December 6, 2017 Agnes Heering successfully defended her PhD thesis titled Experimental realization and applications of the unified acidity scale.

Her work literally redefines the way the pH of non-aqueous and mixed aqueous solution is understood and measured. The main focus of the experiments was on validating the measurement approach and measuring the unified pH values, i.e. pHabs values, of HPLC mobile phases (eluents). Her work introduces a conceptually new approach of measuring pH of mixed-solvent liquid chromatography (LC) mobile phases and has been published in the Analytical Chemistry journal: Unified pH Values of Liquid Chromatography Mobile Phases. Anal. Chem. 2015, 87, 2623–2630.

Mobile phase pH is very important in LC, but its correct measurement is not straightforward and all commonly used approaches have deficiencies. The new and fundamentally correct approach developed by Agnes enables direct comparison of acidities of solutions made in different solvents, based on chemical potential of the proton in the solutions.

The work by Agnes represents the first experimental realization of the pHabs 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 pHabs values (referenced to the gas phase) or pHabsH2O values (referenced to the aqueous solution). The liquid junction potentials were estimated in the framework of Izutsu’s three-component method.
She determined the pHabs values for a number of common LC and LC-MS mobile phases and formed a self-consistent pHabs scale. This scale enables for the first time direct comparison of acidities of any LC mobile phases: with different organic additives, different buffer components etc. Agnes has developed a possible experimental protocol of putting this new approach into chromatographic practice and has tested its applicability. She has demonstrated that the ionization behavior of bases (cationic acids) in the mobile phases can be better predicted by using the pHabsH2O 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 pHabsH2O values is possible if the change of their pKa values with solvent composition change is taken into account.

The defence was successful in every respect. Agnes presented very well, answered questions confidently and convincingly demonstrated to everyone that she is really on top of this whole matter.

(Photo: Agnes Heering and prof. Peeter Burk, the chairman of the defence committee, during defence)