Symmetric Potentiometric Cells for the Measurement of Unified pH Values

Recently the UnipHied consortium published a new article – Symmetric Potentiometric Cells for the Measurement of Unified pH Values. Symmetry 2020, 12, 1150, where we compared the performance of different cells and instruments used to measure acidities on the unified pH scale (pHabs). The advantage of the unified pH scale is that it enables expressing the acidities of any systems – non-aqueous, solutions, colloidal systems, etc – via pHabsH2O values, which are directly comparable to the conventional pH values of the aqueous pH scale.

This is the first time unified acidities have been measured outside of our group. Nine partners from Europe – national metrology institutes and universities – have set up systems to measure pHabs and validated them with standard aqueous buffers.

The results show that the method is robust and does not depend on the cell nor the instrument (pH meter, potentiostat, or electrometer). The inherent symmetry of the cell design helps to reduce the experimental workload and improve the accuracy of the obtained results.

The next step is to measure unified acidities of non-aqueous systems, which could be used by routine laboratories to calibrate their pH electrodes.

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.

Our collection of acidity and basicity data in non-aqueous media has been updated!

The collections of experimental acidity and basicity data in non-aqueous media measured by our group are now up to date on our Chair of Analytical Chemistry webpage. 

Both tables include the compounds name, CAS number and SMILES code. The acidity collection contains 190 compounds, where the compounds available experimentally measured pKa in acetonitrile (MeCN) and pKip in 1,2-dichloroethane (DCE) are given. The pKa in DCE is calculated/estimated based on pKip value. 

The basicity data collection includes 353 compounds and their pKip in tetrahydrofuran (THF) and in DCE. The pKa values in MeCN were re-evaluated taking into account all (close to 700) measurements of 279 bases. Therefore, these pKa values can be considered the most reliable pKa values measured in MeCN available!

Our acid-base research at 19th European Symposium on Fluorine Chemistry 2019

On 26.08.19 at 19th European Symposium on Fluorine Chemistry 2019 Ivo Leito gave a talk Acidities of molecules and media: the united pKa-pHabs scale in 1,2-dichloroethane.

For the first time in a low-polarity solvent, the scale has a dual nature, being both a pKa scale of the involved acids (i.e. describing acidity of molecules) and at the same time a pH scale, describing the acidity of the solutions (media).

The scale spans 28 orders of magnitude (pH or pKa units) and is composed of 87 acids of diverse nature. The scale is linked to the unified acidity (pHabs) scale and expressed in values, which makes the acidities of the solutions directly comparable to the conventional aqueous pH scale. This in turn makes the scale a useful tool in future studies of acid-driven processes (reaction mechanisms, catalysis, strong oxidants, etc) in low-polarity media.

This work has been published in: Chem. Sci., 2017, 8, 6964-6973. A unified view to Brønsted acidity scales: do we need solvated protons?.

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

48th International Symposium on High-Performance Liquid Phase Separations and Related Techniques

Between the 16th to 20th of June our group presented itself in HPLC 2019 in Milan. It was a 5-daylong and intense learning opportunity with more than 300 speakers and around 500 posters.

Topics ranged from fundamentals of HPLC, miniaturization to different omics, pharmaceutical analysis and innovative technologies (can HPLC have a FID as a detector?). For the first time, a whole section was dedicated to 3D printing technologies – a technique that is used to build 3 dimensional separation modules: for example, fascinating talks on using 3D printing to do liquid chromatography in 3 dimensions.

The conference gala dinner was held at the beautiful central courtyard of University of Milan under relieving cool evening sky opposing the hot temperatures of the day. HPLC 2019 also had two new additions that hopefully will become annual traditions: Separation Science Slam and HPLC Tube, offering an opportunity for scientists to express their love for their work in modern ways. The competitions were extremely creative and both the audience and participants were thoroughly enjoying the events. Participants from our analytical chemistry chair gave multiple contributions to the conference.

Ecstasy content in tablets is uneven

Max giving his talk

Max Hecht, MSc, presented an oral presentation on the evaluation of MDMA (also known as ecstasy and  ‘Molly’) content in 412 tablets and dissolution properties in 247 tablets, collected in the UK in the time period of 2001-2018. It was found that there are no physical tablet characteristics which correlate to dissolution rate classification, hence no way of users knowing a priori whether tablets were more likely to be fast or slow-releasing. Further, large within-batch variation in the dose and also dissolution rate was observed, giving the combined result of increasing significantly the danger of over-dosing.

Standard substance free quantification in LC/ESI/MS

Anneli giving her talk

Dr. Anneli Kruve presented the recent work of her group on standard substance free quantification of metabolites in green tea samples. In the metabolomics studies, the standard substances for all detected and identified metabolites are hardly ever available. The peak areas obtained from LC/HRMS analysis are also generally usable as different compounds ionize with vastly different ionization efficiencies; the differences may reach 100 million times. With the aid of ionization efficiency predictions, this shortcoming can be overcome and the absolute concentrations estimated. The current prediction accuracy for the green tea metabolites is 1.7 times, which allows comparison of different tea samples and also the identification of the samples with different origin. Importantly, the standard substance free quantitation allows transferring quantitative data from one lab to another. Anneli has also summarized the current status of standard substance free quantitation for the last issue of LCGC. You can find out more about it from kruvelab.com and quantem.co.

Novel eluent additives diversify analyst´s toolkit

Ruta giving her talk

Ruta Veigure, MSc, showed that fluoroalcohols, such as perfluoropinacol (PP) and 1,1,1,3,3,3-hexafluoro-2-methyl- 2-propanol (HFTB), are very useful alternatives to common eluent additives in RP HPLC-MS analysis, acting, among other effects, as weak ion-paring reagents. Novel eluent additives influenced elution of protonated bases by significantly improving analyte’s retention on C18 stationary phase as well as reduce the retention of acidic analytes, which are deprotonated. A comparison was performed to commonly used ammonium acetate and ammonium bicarbonate mobile phase additives. Her research will be rather influential for the analysis of pharmaceuticals, from whom the majority are basic.

Revolutionalizing pH measurements

Prof. Ivo Leito presented a poster introducing a conceptually new approach of measuring pH of mixed-solvent liquid chromatography (LC) mobile phases: the pHabs approach. The new approach is based on the recently introduced unified pH scale (pHabs scale), which enables direct comparison of acidities of solutions made in different solvents or solvents mixtures based on chemical potential of the proton in the solutions. The viewers praised the fact that real numerical values are now available showing how different the conventional pH values are from pHabs, as well as the educative aspect of the whole endeavour. Some visitors were eager to start immediately applying pHabs in their own work.

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.

Automate what can be automated in method validation

Dr. Asko Laaniste introduced the progress on ValChrom, a software for the automation of chromatographic method validation. The development of ValChrom is ongoing and feedback is being gathered from potential users in different fields of chromatography in order to adapt to real needs (contact at valchrom@ut.ee). Feedback from viewers cemented the understanding of the problem that often validation is done in spreadsheets and textual software, that are prone to error. Viewers of the poster approved the endeavour for aiming to help small and medium-sized laboratories that do not have an affordable alternative. They were equally excited to promote the software further to their colleagues in the validation department.

How to make maximum use of the available pKa data in non-aqueous solvents?

On 11.04.2019 Ivo Leito gives a presentation titled “How to make maximum use of the available pKa data in non-aqueous solvents?” at 26th Croatian Meeting of Chemists and Chemical Engineers (26HSKIKI), in Šibenik (Croatia). The presentation discusses the principles of acid-base behaviour in solvents and on how to estimate the pKa values of compounds in solvents where direct experimental data are not available.

 

Guests from Mülheim learning our pKa measurement method

Starting from last week the Analytical chemistry group of University of Tartu has the pleasure to host Dr Monika Lindner and Hendrik van Thienen from the group of professor Benjamin List – a worldwide known guru in the field of strongly acidic catalysts working at the Max-Planck-Institut für Kohlenforschung (Mülheim, Germany).

The purpose of their stay is to learn our pKa measurement method and set it up in Mülheim. This is the logical continuation of our group’s collaboration with the List group – the pKa values of a number of their catalysts have been measured here at Tartu (see Nature Chemistry 2018, 10, 888-894 and Angew. Chem. Int. Ed. 2017, 56, 1411-1415) – and we are glad that our pKa measurement method thereby gains even wider acceptance than it has now. We wish Monika and Hendrik all the success in learning the measurements!

(On the photo, from left: Märt Lõkov, Monika Lindner and Hendrik van Thienen)

 

New Publication: Determination of pKa values of fluorocompounds in water using 19F NMR

Our group is well-known for pKa measurements in different solvents. So far, mostly UV-Vis spectrophotometric method has been used. Recently Elisabeth Parman (on the photo) from our group has been developing a method for measuring pKa values of fluorocompounds, using 19F NMR. NMR measurements to determine pKa values have several advantages compared to UV-Vis spectrophotometry. For example, accurate concentrations are not needed, the method is much more tolerant to impurities and several compounds can be measured simultaneously. The newly developed method has now been published in the Journal of Physical Organic Chemistry: Determination of pKa values of fluorocompounds in water using 19F NMR by E. Parman, L. Toom, S. Selberg and I. Leito.
In this work, the developed method was applied to 19 fluorocompounds – fluorinated phenols, carboxylic acids and sulfonamides, hexafluoroisopropanol (HFIP), perfluoropinacol, etc – with the aim of determining their pKa values in water, in the range from about 3.5 to 10. Several advantages of the 19F NMR method were demonstrated in pKa measurement. In particular, using multiple relative measurements against several reference compounds with known acidities it is possible to obtain highly reliable pKa values. Elisabeth convincingly demonstrated that the literature pKa values of two of the investigated compounds were inaccurate and provided more reliable values for them.

New type of pKa – biphasic pKa in octanol water system

It is not frequent that a new quantitative measure of some physicochemical characteristic is introduced. Recently our group managed to do exactly that by introducing a novel approach – termed as biphasic pKa values – of measuring the acidity/basicity of lipophilic compounds embedded in water-immiscible media (membranes or solutions) at equilibrium with aqueous solution.

In this approach, the H+ ion activity is determined in the aqueous phase, while the concentration ratio of protonated and deprotonated forms of acid/base is measured in the lipophilic phase with any suitable technique. The approach has been demonstrated on the example of biphasic pKa values of some lipophilic indicator bases in the octanol: water system, measured using UV-Vis spectrophotometry. The counterion identity and concentration, as well as the overall ionic strength in the aqueous phase, are included in the standard state definition.

This work has been published S. Selberg, S. Tshepelevitsh, I. Leito Croat. Chem. Acta 2018, 91, (4) and will form a part of the PhD thesis of Sigrid Selberg.

Biphasic pKa values are envisaged to be useful for characterizing lipophilic indicators, synthetic receptors, phase transfer catalysts and other implements that are used in hydrophobic media (e.g. in sensor membranes) in contact with water.

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.

(On photo: Sigrid Selberg)

UnipHied EMPIR Network at Baltic Electrochemistry Conference 2018

On Nov 05, 2018 Ivo Leito gave a presentation Unified pH about the pan-European research network of fundamental pH Research UnipHied (www.uniphied.eu) at the 7th Baltic Electrochemistry Conference organized by the University of Tartu.

The presentation started with explaining the need for the experimental realization and measurement capability of unified pH (pHabs). Thereafter the current state of art of measuring pHabs values was described and finally, some first exemplary results were highlighted.

The presentation created a lot of interest from the participants and roughly as many questions were asked as for the other four presentations of the same session put together!

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 chromatography, etc.

The central aim of the UnipHied network is to establish at international level measurement capability of pHabs values that would be applicable also at routine laboratory level. The two key activities for achieving that are creating a reliable method for the experimental or computational evaluation of the liquid junction potential and between aqueous and non-aqueous solutions and developing 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 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, initiator).

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.

 

Milestone in catalysis research: approaching sub-ppm-level asymmetric organocatalysis

Recently our group had the pleasure of participating in a cutting-edge endeavor: characterizing the acidity of some extremely efficient strongly acidic organocatalysts. In the case of the Mukaiyama aldol reaction, the best of them (1) worked at low ppm to sub-ppm level, (2) gave excellent yields and (3) high enantiomeric selectivity as well as (4) turnover numbers (TON numbers) of hundreds of thousands.

The extent to which these four features occurred together in the same catalyst was so remarkable that the results were published in one of the most prestigious journals in chemical sciences: Nature Chemistry 2018, 10, 888-894.

The extremely demanding acidity measurements were performed by Dr Karl Kaupmees. The whole research was led by the group of professor Benjamin List – a worldwide known guru in the field of strongly acidic catalysts working at the Max-Planck-Institut für Kohlenforschung.
These results are expected to open new avenues in the development of powerful new organocatalysts.

(Photo by Andres Tennus: Karl doing acidity measurements in a glovebox under anhydrous conditions)