How to Predict the pK a of Any Compound in Any Solvent

Acid-base properties of molecules in nonaqueous solvents are of critical importance for almost all areas of chemistry. Despite this very high relevance, our knowledge is still mostly limited to the pK _a of rather few compounds in the most common solvents, and a simple yet truly general computational procedure to predict pK _a's of any compound in any solvent is still missing. In this contribution, we describe such a procedure. Our method requires only the experimental pK _a of a reference compound in water and a few standard quantum-chemical calculations. This method is tested through computing the proton solvation energy in 39 solvents and by comparing the pK _a of 142 simple compounds in 12 solvents. Our computations indicate that the method to compute the proton solvation energy is robust with respect to the detailed computational setup and the construction of the solvation model. The unscaled pK _a's computed using an implicit solvation model on the other hand differ significantly from the experimental data. These differences are partly associated with the poor quality of the experimental data and the well-known shortcomings of implicit solvation models. General linear scaling relationships to correct this error are suggested for protic and aprotic media. Using these relationships, the deviations between experiment and computations drop to a level comparable to that observed in water, which highlights the efficiency of our method.

Mixed Chromate and Molybdate Additives for Cathodic Enhancement in the Chlorate Process

The economic viability of the electrochemical chlorate process depends on toxic chromate to induce cathodic selectivity to hydrogen and mitigate reduction of hypochlorite or chlorate. In this study, it is shown that performance of a pilot plant for chlorate production can be sustained when a 1000-fold reduction in chromate concentration is compensated by addition of molybdate. Laboratory measurements employing a Quartz Crystal Microbalance suggest growth of a nanometre-thick hybrid Mo–Cr-oxide film to induce cathodic selectivity. An optimized energy efficiency for pilot plant operation was obtained using 0.8 mM molybdate and 27 μM chromate, balancing formation of an effective oxide layer and undesired Mo-induced decomposition of hypochlorite to oxygen in solution. Refinement at the pilot scale level is expected to further optimize the energy consumption, thereby increasing safety aspects and the economic viability of chlorate production.

Graphical Abstract

Exploring the mechanism of hypochlorous acid decomposition in aqueous solutions

Hypochlorous acid is an intermediate in important industrial processes such as the production of chlorate but is also used for water treatment and disinfection. In aqueous solutions hypochlorous acid may decompose into oxygen or chlorate. Using density functional theory (DFT) modelling we have for the first time established detailed mechanisms for the respective decomposition pathways. Our calculations indicate, that both oxygen and chlorate formation proceed through an identical set of intermediates. At neutral pH the reaction is initiated by a fast equilibrium between HOCl, OCl^-, Cl₂O and Cl₃O₂^-. The subsequent abstraction of Cl^- to form Cl₂O₂ is rate determining for chlorate formation while it is the decomposition of Cl₂O₂ in the case of oxygen formation. Under alkaline conditions, OCl^- decomposition to chlorate proceeds through chlorite. This reaction path is significantly less active. The highest rate for chlorate or oxygen formation is found at pH 7.1. These results highlight the need to consider a complex mixture of different Cl species when addressing the chemistry of hypochlorous acid containing solutions.