Peter May

Modern improvements in potentiometric technology (apparatus and digitisation) have overcome many persistent electrochemical issues yet important fundamental problems remain unsolved. Junction potentials in cells with transference have, in particular, long been a scientific enigma with serious practical consequences. This review highlights the fundamental role played by junction potentials, which have wide-ranging implications for the measurement of pH in electrolyte solutions of practical interest. Ending the sleepwalking around junction potentials is an essential step in a prospective redefinition of pH. We emphasise here that, relative to a reference electrode shielded by an electrolytic bridge, a junction potential profile often can be determined experimentally; this can be done without thermodynamic ambiguity as a function of electrolyte concentration. Coupled with a practical convention for single ion activity coefficients, measurements using an appropriate electrode sensor in the sample can accurately quantify changes in the junction potential from high to low ionic strengths. These data can then be supplemented by further measurements with well characterised pH buffers, so as to extrapolate the junction potential profile to infinite dilution. In this way pH can be better defined and measured, enabling its present range to be greatly extended. For many scientific applications that require more than just precise comparisons of acidity, improved pH accuracy can be expected. In particular, routine analyses of multicomponent aqueous solutions using glass electrode potentiometry should be greatly enhanced.

Legend: Current pH ailments can be cured by the ubiquitous combination glass electrode if junction potential uncertainties can be resolved.
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Despite wide use, thermodynamic conventions are almost never presented explicitly as a topic in textbooks, lectures or publications covering metrology and uncertainty. Their meaning, and the particular need for them, is typically just taken for granted. Consequently, an uncritical acceptance and lack of understanding of their implications can often start in the classroom and persist long after, with unfortunate consequences that undermine some important metrological definitions and practices. Thermodynamic conventions generally create reference scales that allow properties which can only be measured as differences (such as enthalpy and electrode potentials) to be expressed in absolute terms for practical convenience. However, the arbitrary nature of these choices made when selecting and implementing any such convention is not only puzzling to students but can also confuse thermodynamic or metrological specialists. If, as a consequence, thermodynamic conventions are not properly understood, theoretical and experimental progress can become stuck within prevailing paradigms. In this work, we identify two such cases relating to pH and Gibbs energies of reaction, and show how more nuanced understandings of thermodynamic convention as a concept enable better choices and lead to improved scientific outcomes.

Many important thermodynamic calculations for aqueous systems are profoundly limited because ion specific interactions have not been understood. Here an alternative modeling paradigm with compelling advantages is presented based on fundamental insights regarding ion–ion interactions at higher electrolyte concentrations. We also show how an intense ongoing controversy regarding single ion activity coefficients (SIACs) can be resolved and how SIACs can be quantified in full thermodynamic compliance using an overlooked convention. SIAC values can in fact be determined unequivocally and compatibly from two independent types of measurement at trace concentrations. These developments promise important advances, especially in defining pH and modeling multicomponent aqueous systems.

Understanding the stability of gas hydrates in aqueous electrolyte solutions is pivotal for industrial applications, particularly in oil and gas extraction and desalination processes. Many existing thermodynamic models, while predicting the influence of salts on hydrate dissociation temperatures, are often limited because they require numerous fitting parameters and, hence, are only tailored to specific hydrate systems. Here, we present a model for hydrate stability calculations with 40% fewer parameters but equivalent accuracy to existing approaches. This model has now been extended to include a physically based description of the impact of electrolyte solutions on hydrate equilibria. Utilizing an extended Debye–Hückel equation, we first accurately describe osmotic coefficients for 191 unique single strong electrolyte solutions. Zdanovskii’s rule is then employed to predict osmotic coefficients in mixed electrolyte scenarios without any additional parameters. Finally, the predicted osmotic coefficients are used to correct water fugacities calculated via the cubic-plus-association equation of state. The resulting cage-specific hydrate equilibrium electrolyte (CaSH-e) model reliably describes multicomponent mixtures of hydrocarbon and aqueous compounds that include industrially important thermodynamic inhibitors such as methanol and monoethylene glycol. The CaSH-e model’s performance is shown to either match or exceed that of current existing models, like CPA-hydrates in MultiFlash 7.0 (CPAHYD-MF) and the Ballard and Sloan model in CSMGem, particularly for chloride and bromide salt mixtures.

Available equilibrium constant data for reactions of germanium with inorganic ligands in aqueous solution have been critically evaluated. Even though the relevant literature is sparse and mostly rather old, we have established a working thermodynamic description of germanium in aqueous, multicomponent media for its most important interactions with inorganic ligands. These thermodynamic parameters will be useful in environmental and (eco)toxicology studies. However, within the limitations of the presently available literature, significant uncertainties are inescapable. The implications for thermodynamic modelling in general are far-reaching.

Critically assessed data regarding Sn(IV) dioxides and hydroxy complexes have recently been challenged. Differences as large as nine orders of magnitude occur in certain of the published solubility products and other equilibrium constants, despite supposedly being derived from the same 'reliable' measurements. We show how these differing conclusions depend on the assignments of uncertainty in the respective experimental observations and that the divergence is due to error propagation in identifiable thermodynamic analyses. The use of Sn4+ as a 'basis'/'master' species in thermodynamic modelling is deprecated. Automatic methods which enable the necessary calculations to be properly evaluated, as well as easily repeated, help uncover such mistakes. The results from the comprehensive NEA review are substantially confirmed.

The JESS (Joint Expert Speciation System) Thermodynamic Database (v8.9) is now available as a set of freely available PDF files. This reaction database contains about 280,000 thermodynamic parameters (equilibrium constants, enthalpies, etc.) as published in the literature for over 80,000 chemical reactions. It is grounded in the tradition of the Stability Constants Special Publications (by the Chemical Society) but it has become much more extensive and it includes some quantitative indication of parameter reliability based mainly on intra- and extra-reaction consistency.

The JESS Thermodynamic Database of chemical reactions is one of the thermodynamic databases in the ‘Joint Expert Speciation System’ (JESS), a package of software and databases concerned primarily with the modelling of chemical phenomena in water solutions. JESS is now the world’s largest single electronic repository of thermodynamic information relevant to such systems.

The chemical reaction database is available here as a set of PDF files. It has been subdivided by chemical elements. Elements are indicated by the initial letter(s) of the PDF file names. To find a chemical reaction of interest, search first for a chemical species, identifying the JESS symbol for it using the S file corresponding to the most relevant element, and then locate all chemical reactions with that JESS symbol in them, by searching through the element’s corresponding R file. A full description of the data in the PDF files is given in the accompanying Zenodo file named ‘Instructions’. It is advisable to read the preamble documents.

The JESS chemical reaction database continues to be developed so feedback is most welcome. Questions concerning the production of the PDF files should be directed to Josep Bonet (josep.bonet@schema.lu) and concerning the content of the database to either or both Peter May (P.May@murdoch.edu.au) and Montserrat Filella (montserrat.filella@unige.ch).

Background Secondary hyperparathyroidism (SHPT) complicates advanced chronic kidney disease (CKD) and causes skeletal and other morbidity. In animal models of CKD, SHPT was prevented and reversed by reduction of dietary phosphate in proportion to GFR, but the phenomena underlying these observations are not understood. The tradeoff-in-the-nephron hypothesis states that as GFR falls, the phosphate concentration in the distal convoluted tubule ([P]DCT]) rises, reduces the ionized calcium concentration in that segment ([Ca++]DCT), and thereby induces increased secretion of parathyroid hormone (PTH) to maintain normal calcium reabsorption. In patients with CKD, we previously documented correlations between [PTH] and phosphate excreted per volume of filtrate (EP/Ccr), a surrogate for [P]DCT. In the present investigation, we estimated [P]DCT from physiologic considerations and measurements of phosphaturia, and sought evidence for a specific chemical phenomenon by which increased [P]DCT could lower [Ca++]DCT and raise [PTH]. Methods and findings We studied 28 patients (“CKD”) with eGFR of 14–49 mL/min/1.73m2 (mean 29.9 ± 9.5) and 27 controls (“CTRL”) with eGFR > 60 mL/min/1.73m2 (mean 86.2 ± 10.2). In each subject, total [Ca]DCT and [P]DCT were deduced from relevant laboratory data. The Joint Expert Speciation System (JESS) was used to calculate [Ca++]DCT and concentrations of related chemical species under the assumption that a solid phase of amorphous calcium phosphate (Ca3(PO4)2 (am., s.)) could precipitate. Regressions of [PTH] on eGFR, [P]DCT, and [Ca++]DCT were then examined. At filtrate pH of 6.8 and 7.0, [P]DCT was found to be the sole determinant of [Ca++]DCT, and precipitation of Ca3(PO4)2 (am., s.) appeared to mediate this result. At pH 6.6, total [Ca]DCT was the principal determinant of [Ca++]DCT, [P]DCT was a minor determinant, and precipitation of Ca3(PO4)2 (am., s.) was predicted in no CKD and five CTRL. In CKD, at all three pH values, [PTH] varied directly with [P]DCT and inversely with [Ca++]DCT, and a reduced [Ca++]DCT was identified at which [PTH] rose unequivocally. Relationships of [PTH] to [Ca++]DCT and to eGFR resembled each other closely. Conclusions As [P]DCT increases, chemical speciation calculations predict reduction of [Ca++]DCT through precipitation of Ca3(PO4)2 (am., s.). [PTH] appears to rise unequivocally if [Ca++]DCT falls sufficiently. These results support the tradeoff-in-the-nephron hypothesis, and they explain why proportional phosphate restriction prevented and reversed SHPT in experimental CKD. Whether equally stringent treatment can be as efficacious in humans warrants investigation.