Hydrated ions are crucially important in a wide array of environments, from biology to the atmosphere, and the presence and concentration of ions in a system can drastically alter its behavior. One way in which ions can affect systems is in their interactions with proteins. The Hofmeister series ranks ions by their ability to salt-out proteins, with kosmotropes, such as sulfate, increasing their stability and chaotropes, such as perchlorate, decreasing their stability. We study hydrated perchlorate clusters as they are strongly chaotropic and thus exhibit different properties than sulfate. In this study we simulate small hydrated perchlorate clusters using a basin-hopping geometry optimization search with empirical potentials. We compare topological features of these clusters to data from both computational and experimental studies of hydrated sulfate ions and draw some conclusions about ion effects in the Hofmeister series. We observe a patterning conferred to the water molecules within the cluster by the presence of the perchlorate ion and compare the magnitude of this effect to that observed in previous studies involving sulfate. We also investigate the influence of the overall ionic charge on the low-energy structures adopted by these clusters.

Abstract:The Basin Hopping search method is used to find the global minima (GM) and map the energy landscapes of thiocyanate-water clusters, (SCN − )(H 2 O) n with 3-50 water molecules, with empirical potentials describing the ion-water and water-water interactions. (It should be noted that beyond n = 23, the lowest energy structures were only found in 1 out of 8 searches so they are unlikely to be the true GM but are indicative low energy structures.) As for pure water clusters, the low energy isomers of thiocyanate-water clusters show a preponderance of fused water cubes and pentagonal prisms, with the weakly solvated thiocyanate ion lying on the surface, replacing two water molecules along an edge of a water polyhedron and with the sulfur atom in lower coordinated sites than nitrogen. However, by comparison with Density Functional Theory (DFT) calculations, the empirical potential is found to overestimate the strength of the thiocyanate-water interaction, especially O-H···S, with low energy DFT structures having lower coordinate N and (especially) S atoms than for the empirical potential. In the case of these finite ion-water clusters, the chaotropic ("disorder-making") thiocyanate ion weakens the water cluster structure but the water molecule arrangement is not significantly changed.

We present putative global minima for the micro-hydrated sulfite SO32−(H2O)N and chlorate ClO3−(H2O)N systems in the range 3≤N≤15 found using basin-hopping global structure optimization with an empirical potential. We present a structural analysis of the hydration of a large number of minimized structures for hydrated sulfite and chlorate clusters in the range 3≤N≤50. We show that sulfite is a significantly stronger net acceptor of hydrogen bonding within water clusters than chlorate, completely suppressing the appearance of hydroxyl groups pointing out from the cluster surface (dangling OH bonds), in low-energy clusters. We also present a qualitative analysis of a highly explored energy landscape in the region of the global minimum of the eight water hydrated sulfite and chlorate systems.This article is part of the theme issue ‘Modern theoretical chemistry’.

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Voltammetric methods for determining total arsenic in zinc and cadmium electrolyte process solutions and in treated effluent samples are described. The method for plant electrolyte couples reductillation with cathodic stripping voltammetry at a hanging mercury drop electrode. Reductillation, a process in which AsV is reduced to Asm and Asm is subsequently distilled into a separate compartment, overcomes interference from high levels of copper and cadmium in neutral leach clarifier overflow and in cadmium leach filtrate and reduces the electroinactive As" to the electroactive As"' species. This method is suitable for the determination of arsenic concentrations down to 0.07 mg 1-1 and can be used in either off-or on-line modes. In treated effluent, the arsenic concentration is in the low pg 1-1 range, and the reductillation technique lacks the required sensitivity. However, the concentration of interfering ions is also low and a direct determination of the total arsenic concentration by cathodic stripping voltammetry is possible after reduction of the AsV species. Cysteine was found to be a better reducing agent than sulfite. However, to achieve online arsenic determination, which was an important requirement of the study, sulfite was the preferred reducing agent because of its long-term stability in solution.