Yorck-Michael Neuhold

Photoreactions of Mercury in Surface Ocean Water: Gross Reaction Kinetics and Possible Pathways

We present pseudofirst order rate constants for gross photoreduction and gross photooxidation of mercury in surface water from the open Atlantic Ocean, determined under controlled laboratory conditions. Experiments using both unfiltered and filtered ocean water were carried out to characterize the importance of microbes and colloids on reaction kinetics. Results indicate that reduction and oxidation of mercury in ocean water does not follow a simple two-species reversible reaction pathway. We suggest two possible redox pathways that reproduce the pattern of dissolved gaseous mercury (DGM) concentrations observed in our laboratory experiments, and evaluate them using a controlled outdoor experiment. In both proposed pathways Hg(0), the major constituent of DGM, is converted to an unidentified oxidized species that is different from the reducible form present initially. This reaction step plays a major role in the net formation of DGM in our experiments. Our results represent new quantitative information about the gross reaction kinetics for both reduction and oxidation of mercury in open ocean surface water. Pseudofirst order rate constants for reduction reactions that form DGM were determined to be in the range of 0.15-0.93 h(-1) and pseudofirst order rate constants for oxidation of Hg(0) to be in the range of 0.4-1.9 h(-1). Microbes and colloids did not appreciably influence the reduction and oxidation kinetics.

Analysis of reactions in aqueous solution at non-constant pH: no more buffers?

The pH plays an important role in many reactions in aqueous solution. In order to be able to quantitatively analyse reactions that involve shifting protonation equilibria, reaction solutions traditionally needed to be buffered to maintain constant pH. Unfortunately, buffer components can interfere with the reaction species in several ways, all of them are difficult to quantify. We propose a generally applicable computational method, which allows the quantitative analysis of reactions at non-constant pH. This circumvents the necessity to add buffer systems. The feasibility of the method is demonstrated by re-analysing the complex formation reaction between cyclam (1,4,8,11-tetraazacyclotetradecane) and Cu2+. In four spectrophotometric measurements, the reaction has been analysed at non-constant pH in the range between 6 and 1.2. Results obtained with the new method compare very well with published values.

Analyses of three-way data from equilibrium and kinetic investigations

Abstract In kinetic or equilibrium investigations it is common to measure two-way multiwavelength data, e.g. absorption spectra as a function of time or reagent addition. Often it is advantageous to acquire experimental data at various initial conditions or even on different instruments. A collection of these measurements can be arranged in three-dimensional arrays, which can be analysed as a whole under the assumption of a superimposed function, e.g. a kinetic model, and/or common properties of the subsets, such as molar absorptivity. As we show on selected formation equilibria (Zn 2+ /phen) and kinetic studies (Cu 2+ /cyclam) from our own research, an appropriate combination of multivariate data can lead to an improved analysis of the investigated systems.

Rank annihilation correction for the amendment of instrumental inconsistencies

Abstract The globalisation of the analysis of a series of individual measurements often results in more robust and reliable outcomes. However, instrumental drifts that can occur between individual measurements destroy the ideal data structure and thus the advantages. A method based on rank annihilation factor analysis (RAFA) is introduced for the correction of several types of instrumental inconsistencies. It can be applied to many series of bilinear datasets. Experimental examples discussed in this paper comprise the successful correction of non-uniform retention time drifts in chromatography due to temperature or pressure changes, wavelength shifts in IR spectroscopy in an industrial control situation, and background absorption shifts in UV–VIS spectroscopy applied to equilibrium investigations.

Simulation of Complex Chemical Kinetics

Abstract Kinetic what-if simulations can be important for both the optimisation of an industrial process as well as the design of future experiments in the evolving process of a detailed kinetic analysis on laboratory scale. From our viewpoint as coordination chemists we present SIMKIN, a tool for an easy intuitive and fast simulation of chemical kinetics. As one of the key features, an intelligent model parser translates conventional chemical equations consisting of virtually any reaction steps including feedbacks into the rate law. The corresponding differential equations are then solved by standard routines for numerical integration, and the concentration profiles of the involved species plotted versus the reaction time. By means of selected kinetic examples of increasing complexity taken from coordination chemistry we demonstrate the capabilities and versatility of the program. A MATLAB® version with a complete graphical user interface can be requested from the authors free of charge.

An electrospray ionization mass spectrometry study of the nitroprusside–cation–thiolate system

The complex anion [Fe(CN)5(NO)]2− (nitroprusside) has been defined intact using electrospray ionization mass spectrometry (ESIMS), with minor daughter ions predictable fragments of the parent [Fe(CN)5NO]2−. Direct evidence for ion pairing of [Fe(CN)5NO]2− with monovalent alkali metal and some alkylammonium cations was found independently using ESIMS and 13C NMR techniques. The results are relevant to the significant influence of the concentration and nature of cations on the solution equilibrium between nitroprusside with thiolates and their nitrosothiol adducts that presumably arises through charge reduction in tight ion pairs facilitating adduct formation. Ion-paired species {M+[Fe(CN)5NO]2−}−, {M3[Fe(CN)5NO]}+ and more complex clusters of the type {Mx[Fe(CN)5NO]y}z−/z+ (up to x = 8 and y = 5) were observed by ESIMS. The trend in the ion pair formation seen by ESIMS follows the series Li+ > Na+ > K+ > Rb+ > Cs+ and Me4N+ > Et4N+ > Pr4N+ > Bu4N+ and is consistent with purely electrostatic expectations for unsolvated ions. Whereas the poorly solvated alkylammonium series bulk solution behaviour resembles that found by the ESI technique, the activity of alkali metal cations proceeds conversely, i.e. Li+ < Na+ < K+ < Rb+ < Cs+, verified by a 13C NMR study, because the hydrated cation radii play a crucial role in the bulk solution in those cases. Differences in changes of the chemical shift upon addition of various cations points to the axial CN− position as especially important for binding to these cations. Further, short-lived adducts with thiolates of the type {M+3[Fe(CN)5NO(C4H4O4S)]4−}− have been defined by their observation in ESIMS directly and through their decomposition products, providing further support for reversible nitrosothiol complex formation. Mercaptosuccinate ion pairing with cations, which would make formation of the proposed nitrosothiol complex easier, is also observed by ESIMS. The dithiolato complex {K+[Fe(CN)5NO(C4H6O4S)2]2−}−, as well as the disulfide anion radical were detected; both species are suggested intermediates in the spontaneous and autocatalyzed redox decomposition of the [Fe(CN)5N(O)SR](n + 2)− complexes. The ESIMS study has thus provided information relevant to both thermodynamic and kinetic processes in a reaction where species of limited stability are involved.

Conjoint Analysis of Kinetic and Equilibrium Data for Mechanistic Elucidation in Polynuclear Complexation Reactions, Exemplified by Metal(II) Helicate Complex Formation

Combined kinetic and equilibrium studies of the complexation of the ligand N,N′-bis[2-(2-pyridyl)methyl]pyridine-2,6-dicarboxamide with copper(ii) and nickel(ii), examined conjointly employing advanced chemometric methods, provides elucidation of speciation and formation pathways in solution for complexes such as M2L2 and M2L3 helicates, respectively.