The Unified pH Scale: from Concept to Applications
Conventional pH scales: state-of-the-art and drawbacks
Acidity is one of the most important characteristics of liquids/solutions and, in fact, any materials.1,2,I,II It refers to the activity of the solvated proton and is expressed as pH. The exact definition and correct measurement of pH is crucial to understand and control important processes in fundamental chemistry, industry and living organisms, such as catalysis,3,4,III extraction,5–7 chromatography,8,IV processes in micelles/bilayers,9,10 etc. However, the conventional pH scale is well established1 only in dilute aqueous solutions at medium pH values. It has serious limitations in other solvents or more complex media where most of the real-life chemistry takes place. Most importantly, comparison of the conventional pH values between different media (solvents) is impossible.V
The main reason for current limitations is that in the conventional aqueous pH scale the solvated proton activity a(H+, aq), closely related to its equilibrium concentration [H+], is defined on the basis of a dilute aqueous standard state1. pH scales have also been established in other solvents by defining different standard states referring to a(H+, solvent).11,12 However, because of strongly differing solvation energies13 the thermodynamic activities of the same number of protons (or the same proton concentrations) also differ strongly. It is the activities, not just numbers of protons, that influence processes and promote reactions. A simple qualitative example: pH 7 in water is neutral, while pH 7 in acetonitrile is strongly acidic,V although in both cases [H+] is roughly 10-7 mol L-1.
For this reason, a number of different pH scales, often ambiguous, have emerged for the description of practically relevant media, e.g., different pH evaluation approaches in liquid chromatography8 or different pH definitions for seawater.14 This creates confusion and obscures acidity-related trends.14
Although in water strictly established reference standard solutions (as defined in ref 1) are available, limitations of the current pH scale include strongly acidic and basic aqueous solutions (with pH outside of the approximate range 1.5…12.5), since the acidic/basic species in high concentration effectively act as solvent components. Thus, rigorous measurement and intercomparison of pH values between different complex media like widely used aqueous-organic mixtures, non-aqueous solutions,8,IV catalytic3,III and electrocatalytic4,15 systems, or different biological fluids,9,10 where a multitude of important processes is strongly influenced by the proton activity, is currently not possible.
The concept of unified pH scale
In view of the above problems, recently the concept of unified pH scale (Scheme 1) was put forwardV defining unified pH (pHabs) via the absolute chemical potential of the solvated proton and using a universal standard state – proton gas at 1 barV with pHabs = 0 by definition. For easier comparability the pHabs scale has been “aligned” with the aqueous pH scale.IVb I.e. any medium/solution with pHabs 7.00 has the same thermodynamic activity of the solvated proton as aqueous solution with pH 7.00. This conveniently preserves the common way of expressing pH values, while still being related to the universal standard state. This is in contrast to the current situation where standard states are defined individually for every solvent/medium.VI The merits of pHabs: strict thermodynamic foundation and direct comparability of pHabs values between any solvents/media.
Currently the pHabs scale is still mostly a theoretical concept that has yet to experience much practical implementation and has seen limited uptake by the community.
Prospective application areas of the unified pH scale
pHabs is potentially advantageous for any acidity/basicity-related applications, especially in cases where media not covered by conventional pH scales are involved. Here we briefly describe three major application areas and the prospective benefits of using pHabs in each.
(1) Significant extension of the conventional pH scale towards higher acidity by ca 10 orders of magnitude would be possible using the pHabs concept. This enables correct pH measurement across broader pH range, important e.g. in acid anodization for surface coatings,16 conversion of saturated hydrocarbons,17 etc. Such extension has been proposed via acidity functions, e.g. the well-known Hammett H0 acidity function,17 which have serious limitations: (1) are influenced by other factors besides the activity of H+; (2) are based on questionable assumptions18 and (3) there are a number of different acidity functions,19 leading to ambiguity in acidity values. In contrast, pHabs is based on the chemical potential of the solvated proton and would enable rigorous thermodynamic quantification of the pH of highly acidic solutions and comparing acidities in solutions of strong acids. Solutions containing a significant amount of a strong acid (pH below 0) do not cause problems since the pHabs concept does not require constant solvent composition.
(2) Liquid chromatography (LC) and its combination with mass spectrometry (LC-MS), using reversed-phase columns, are the workhorses in proteomics,20 metabolomics,21 environmental analysis,22 etc. The current practice is that the mobile phase pH is characterized via approximations, which have drawbacks8 and can lead to incorrect conclusions about the real acidity in these media and the ionization states of the analytes in them.IV This hinders understanding retention mechanisms and may result in inefficient separations, wasted time and resources.23–25 This can be avoided by evaluation of mobile phase acidity/basicity through pHabs and eventually creating a comprehensive “map” of pHabs of the most commonly used LC mobile phases, expected to be hugely beneficial for LC method development.
(3) Organocatalysis is steadily gaining importance,3,26,III among other reasons for the promise of “green” solutions in chemical industry.26 Brønsted acid catalysis is among the most important types of homogeneous organocatalysis in academia and industry.27 Yet, the current possibilities of predicting/rationalizing catalyst efficiency still have a lot of room for improvement. In general acid catalysis, the catalyst (acidic species) interacts directly with the substrate and the solvated proton is not involved.28,III If stereoselectivity (e.g. enantioselectivity) is desired, it is essential that proton transfer occurs directly from the catalyst to the substrate which are spatially close together:29 the possibility of the proton being carried to the substrate by the solvent molecule(s) would impair the stereoselectivity. For this reason, such reactions are usually carried out in low-polarity media.III,VII The concentration of solvated protons there is negligibly low:II protons are donated to the substrate directly by catalyst molecules. The influence of the catalyst’s acidity on the catalytic process has typically been investigated on the basis of its pKa value.30,31,III,VII However, reliable pKa values in low-polarity solvents are difficult to obtain and the reaction mixture can have different other (basic) constituents that can bind the acid catalyst, thereby decreasing the number of catalyst molecules available for the substrate.
Describing the reaction medium through pHabs is expected to lead to better quantitative evaluation of the relationships between acidity and catalytic reaction rate. The catalytic activities will be comparable across catalysts and across solvents. Of course, the rates of acid-catalytic processes strongly depend on other factors besides medium acidity, such as solvent, steric hindrance, etc. However, by being able to neatly quantify one of the factors (medium acidity) it will be possible to narrow down other influencing factors.
Deeper, more fundamental understanding of the mechanisms will help in scaling up catalytic processes for industrial use. pHabs would be applicable also to heterogeneous (solid acids) processesV as well as the novel hybrid approaches where catalysts are linked to the surface of solid particles to preserve the reaction mechanism while combining the higher selectivity and conversion rate of homogeneous catalysis with the ease of catalyst separation of heterogeneous catalysis.32–34
Practical realization of the pHabs approach
Up to now the main obstacle for wider acceptance of pHabs by the chemical community has been the difficulty in its experimental realization. The initial “proof-of-concept” pHabs measurement method exists,IV but very limited uptake4 and a number of key issues remain:
(1) The prototype method insufficiently accounts for the liquid junction potential (LJP)IV and has been tested for limited systems only. LJP is a very important factor, which in difficult cases can amount to several hundreds of mV, leading to errors of up to several pH units, if not eliminated or accounted for.
(2) No simple methods or reference standards (standard solutions) in non-aqueous media are available that are suitable for practitioners.
(3) There are yet almost no demonstrations of considerable advantages in prominent practical applications of pHabs.
All these problems will be solved by the current project.
References
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