On The Existence of Nonunique Equilibrium States.

Chemists "know" that the equilibrium position in a homogeneous solution is unique and that the one equilibrium is reached, irrespective of how the solution was prepared. This concept has been verified based on the law of mass action applied to concentrations rather than activities. In this contribution, we explore the situation for a very basic equilibrium system, using established approximations for the activity coefficients. We will demonstrate that under the correct application of the law of mass action, bistable equilibria positions are possible, concomitant, of course, with bifurcation in the kinetics of reaching either of the stable equilibria.

Generalized indicator-based determination of solution pH.

pH indicators can be used both fast responsive as well as long-term stable sensors. They have been extensively used for monitoring pH changes in fast kinetic reactions as well as slowly changing pH in oceanic waters. If the pH range that needs to be covered is narrow it is possible to use only one indicator of appropriate protonation constant; otherwise, mixtures of two or more indicators are used for monitoring pH values covering a broad range of pH. In this paper we presented a new methodology for determining pH of solutions using mixtures of pH indicators. The pH calculation is based on the strict application of the basic laws of mass action and mass conservation. The proposed method was evaluated by the successful determination of the pH values of solutions containing three indicators (neutral red, phenol red (two different protonation constants), and methyl orange) covering a wide range of pH values from 0.5 to 9. The method was also applied for rapid monitoring of pH changes in stopped-flow measurements, investigating the reactions of CO2 in aqueous amine solutions.

Patterns of cyanobacterial abundance in a major drinking water reservoir: what 3 years of comprehensive monitoring data reveals?

The present study provides a detailed analysis of the factors influencing variation in cyanobacterial communities of a large shallow off-river drinking water reservoir on the east coast of Australia. Receiving multiple inflows from two unprotected mixed land-use catchments, the Grahamstown Reservoir is a model example of a reservoir which is highly vulnerable to adverse water quality issues, including phytoplankton blooms and the resulting filtration, toxin and taste and odour problems produced. The spatial and temporal distributions of cyanobacteria were assessed for a period of 3 years (January 2012-December 2014) based on samples collected from three monitoring stations within the reservoir. Relationships between cyanobacterial abundance and a range of environmental factors were evaluated by application of multivariate curve resolution-alternating least squares (MCR-ALS) analysis.Results of the analysis indicated that among the 22 physico-chemical variables and 14 cyanobacterial taxa measured, the vertical temperature gradient within the water column and nutrient availability were the most powerful explanatory factors for the observed temporal and spatial distribution patterns in the densities of cyanobacterial taxa. The abundance patterns of the dominant cyanobacterial taxa-Aphanocapsa, Aphanothece, Microcystis and Pseudanabaena-were strongly linked with rainfall and run-off patterns into the reservoir, while Coelosphaerium and Microcystis were the taxa most influenced by the apparent occurrence of thermal stratification. The findings demonstrate the capacity of rigorous multivariate data analysis to identify more subtle relationships between water quality variables, catchment factors and cyanobacterial growth in drinking water reservoirs.

Activity-based analysis of potentiometric pH titrations.

The discrepancy between concentrations and activities is a predicament well known to the analytical chemist. Because of the difficulty of determining activity coefficients, the standard technique for quantitative equilibrium studies is to work under a particular 'constant ionic strength' by adding an excess of an inert salt. Under such conditions, activity coefficients are approximately constant and can be taken into the equilibrium constants which are defined for the chosen ionic strength (I). Here we propose a fundamentally different approach. Throughout the numerical analysis of the titration data, activity coefficients for all individual species are approximated by well-known equations based on the work of Debye-Hückel. The computational analysis of the measurements strictly obeys the law of mass conservation and obeys the law of mass action only approximately. The main novelty is that now the addition of inert salts is no longer required and measurements are done at minimal I. Consequently, the thermodynamic equilibrium constants are now determined much more robustly based on experiments taken at low I. The approach has been tested and validated with the two very well investigated 3-protic phosphoric and citric acids. In summary: the technique of artificially keeping ionic strength constant has been replaced by improved computational analysis.

A chemical equilibrium modelling strategy for tuning the apparent equilibrium constants of the chemical systems.

In many equilibrium systems, it is customary to use a simplified, apparent equilibrium reaction instead of the complete reaction model. The resulting apparent equilibrium constants are altered or controlled by changes of the chemical system, usually modifying the total concentration of auxiliary reagents, but the variation of other parameters such the intensity of light in photo-controlled reactions, types and concentration of solvents or composition of mixed solvents, the ionic strength of the chemical media, etc. is also possible. The main goal of this work is to propose a chemical equilibrium modelling based strategy to control the conditions governing equilibrium systems in order to tune the apparent constants. Indeed, by extracting the analytic relationship between the parameters of the complete model and apparent model, the magnitude of the apparent constant(s) can be predicted. At first, a basic strategy for calculating the tunable range of apparent constants of chemical equilibria is proposed. Next, three case studies for tuning the apparent constants are investigated in detail: inclusion complexes with cyclodextrin, aqueous micellar solutions for investigating the solvent effects and the boric acid/Mannitol system that allows the tuning of the apparent protonation constant. In all examples tuning the apparent constant is possible by regulating the chemical composition.

Handling of highly coeluted chromatographic peaks by multivariate curve resolution for a complex bioanalytical problem: Quantitation of selected corticosteroids and mycophenolic acid in human plasma.

The present study describes the analytical performance of a fast-elution protocol and smart methodology based on multivariate curve resolution-alternating least square (MCR-ALS) modeling of high performance liquid chromatography with photodiode-array detection (HPLC-DAD) data for simultaneous determination of prednisolone (Predl), methylprednisolone (Mpredl) and mycophenolic acid (MPA) in plasma samples. The LC method optimized at two isocratic reverse phase over a symmetric C18 column, a 60:40 (v/v) mixture of acetonitrile and water (0.02 M KH2PO4 (pH = 3.7) buffer solution) and a 10:70:20 (v/v/v) mixture of acetonitrile, methanol and water. The most challenges in the present study were the severe coelution of analytes of interest with each other and the matrix interferences, and high spectral similarity of selected corticosteroids, depending on the mobile phase method. To circumvent these drawbacks, the whole chromatographic runs were divided into two sections. Then, the matrix augmentation in spectra and retention time direction were implemented for the first and second regions, respectively. Highly acceptable resolution and quantification results were obtained. The average recoveries were 97.3% and 102.1% for Predl, 96% and 98.6% for Mpredl and 97.7% and 100.8% for MPA, using two mobile phase conditions, respectively. Accurate and precise results, elimination of expensive and time consuming sample pretreatment steps and a very short chromatographic run time, are among the advantages of the presented method.

Handling Different Spatial Resolutions in Image Fusion by Multivariate Curve Resolution-Alternating Least Squares for Incomplete Image Multisets.

Data fusion of different imaging techniques allows a comprehensive description of chemical and biological systems. Yet, joining images acquired with different spectroscopic platforms is complex because of the different sample orientation and image spatial resolution. Whereas matching sample orientation is often solved by performing suitable affine transformations of rotation, translation, and scaling among images, the main difficulty in image fusion is preserving the spatial detail of the highest spatial resolution image during multitechnique image analysis. In this work, a special variant of the unmixing algorithm Multivariate Curve Resolution Alternating Least Squares (MCR-ALS) for incomplete multisets is proposed to provide a solution for this kind of problem. This algorithm allows analyzing simultaneously images collected with different spectroscopic platforms without losing spatial resolution and ensuring spatial coherence among the images treated. The incomplete multiset structure concatenates images of the two platforms at the lowest spatial resolution with the image acquired with the highest spatial resolution. As a result, the constituents of the sample analyzed are defined by a single set of distribution maps, common to all platforms used and with the highest spatial resolution, and their related extended spectral signatures, covering the signals provided by each of the fused techniques. We demonstrate the potential of the new variant of MCR-ALS for multitechnique analysis on three case studies: (i) a model example of MIR and Raman images of pharmaceutical mixture, (ii) FT-IR and Raman images of palatine tonsil tissue, and (iii) mass spectrometry and Raman images of bean tissue.

Insights into the Chemical Mechanism for CO2(aq) and H+ in Aqueous Diamine Solutions - An Experimental Stopped-Flow Kinetic and 1H/13C NMR Study of Aqueous Solutions of N,N-Dimethylethylenediamine for Postcombustion CO2 Capture.

In an effort to advance the understanding of multiamine based CO2 capture process absorbents, we report here the determination of the kinetic and equilibrium constants for a simple linear diamine N,N-dimethylethylenediamine (DMEDA) via stopped-flow spectrophotometric kinetic measurements and 1H/13C NMR titrations at 25.0 °C. From the kinetic data, the formation of monocarbamic acid (DMEDACOOH) from the reaction of DMEDA with CO2(aq) is the dominant reaction at high pH > 9.0 (k7 = 6.99 × 103 M-1·s-1). Below this pH, the formation of protonated monocarbamic acid (DMEDACOOH2) via the pathway involving DMEDAH+ and CO2(aq) becomes active and contributes to the kinetics despite the 107-fold decrease in the rate constant between the two pathways. 1H and 13C NMR spectra as a function of decreasing pH (increasing HCl concentration) at 25.0 °C have been evaluated here to confirm the protonation events in DMEDA. Calculations of the respective DMEDA nitrogen partial charges have also been undertaken to support the NMR protonation study. A comparison of the DMEDA kinetic constants with the corresponding data for piperazine (PZ) reveals that despite the larger basicity of DMEDA, the enhanced and superior kinetic performance of PZ with CO2(aq) above its predicted Bronsted reactivity is not observed in DMEDA.

A review of recent methods for the determination of ranges of feasible solutions resulting from soft modelling analyses of multivariate data.

Soft modelling or multivariate curve resolution (MCR) are well-known methodologies for the analysis of multivariate data in many different application fields. Results obtained by soft modelling methods are very likely impaired by rotational and scaling ambiguities, i.e. a full range of feasible solutions can describe the data equally well while fulfilling the constraints of the system. These issues are severely limiting the applicability of these methods and therefore, they can be considered as the most challenging ones. The purpose of the current review is to describe and critically compare the available methods that attempt at determining the range of ambiguity for the case of 3-component systems. Theoretical and practical aspects are discussed, based on a collection of simulated examples containing noise-free and noisy data sets as well as an experimental example.

Enhanced target factor analysis.

Target testing or target factor analysis, TFA, is a well-established soft analysis method. TFA answers the question whether an independent target test vector measured at the same wavelengths as the collection of spectra in a data matrix can be excluded as the spectrum of one of the components in the system under investigation. Essentially, TFA cannot positively prove that a particular test spectrum is the true spectrum of one of the components, it can, only reject a spectrum. However, TFA will not reject, or in other words TFA will accept, many spectra which cannot be component spectra. Enhanced Target Factor Analysis, ETFA addresses the above problem. Compared with traditional TFA, ETFA results in a significantly narrower range of positive results, i.e. the chance of a false positive test result is dramatically reduced. ETFA is based on feasibility testing as described in Refs. [16-19]. The method has been tested and validated with computer generated and real data sets.

Model-based analysis of coupled equilibrium-kinetic processes: indirect kinetic studies of thermodynamic parameters using the dynamic data.

Thermodynamic studies of equilibrium chemical reactions linked with kinetic procedures are mostly impossible by traditional approaches. In this work, the new concept of generalized kinetic study of thermodynamic parameters is introduced for dynamic data. The examples of equilibria intertwined with kinetic chemical mechanisms include molecular charge transfer complex formation reactions, pH-dependent degradation of chemical compounds and tautomerization kinetics in micellar solutions. Model-based global analysis with the possibility of calculating and embedding the equilibrium and kinetic parameters into the fitting algorithm has allowed the complete analysis of the complex reaction mechanisms. After the fitting process, the optimal equilibrium and kinetic parameters together with an estimate of their standard deviations have been obtained. This work opens up a promising new avenue for obtaining equilibrium constants through the kinetic data analysis for the kinetic reactions that involve equilibrium processes.

Catalysis of CO2 absorption in aqueous solution by inorganic oxoanions and their application to post combustion capture.

To reduce CO2 emission into the atmosphere, particularly from coal-fired power stations, post combustion capture (PCC) using amine-based solvents to chemically absorb CO2 has been extensively developed. From an infrastructure viewpoint, the faster the absorption of CO2, the smaller the absorber required. The use of catalysts for this process has been broadly studied. In this manuscript, a study of the catalytic efficiencies of inorganic oxoanions such as arsenite, arsenate, phosphite, phosphate, and borate is described. The kinetics of the accelerated CO2 absorption at 25 °C was investigated using stopped-flow spectrophotometry. The catalytic rate constants of these anions for the reaction of CO2 with H2O were determined to be 137.7(3), 30.3(7), 69(2), 32.7(9), and 13.66(7) M(-1)s(-1), respectively. A new mechanism for the catalytic reaction of oxoanions with CO2 has also been proposed. The applicability of these catalysts to PCC was further studied by simulation of the absorption process under PCC conditions using their experimental catalytic rate constants. Arsenite and phosphite were confirmed to be the best catalysts for CO2 capture. However, considering the toxicological effect of arsenic and the oxidative instability of phosphite, phosphate would be the most promising inorganic catalyst for PCC process from the series of inorganic oxoanions studied.

Carbonic anhydrase activity of dinuclear Cu(II) complexes with patellamide model ligands.

The dicopper(II) complexes of six pseudo-octapeptides, synthetic analogues of ascidiacyclamide and the patellamides, found in ascidians of the Pacific and Indian Oceans, are shown to be efficient carbonic anhydrase model complexes with k(cat) up to 7.3 × 10(3) s(-1) (uncatalyzed: 3.7 × 10(-2) s(-1); enzyme-catalyzed: 2 × 10(5)-1.4 × 10(6) s(-1)) and a turnover number (TON) of at least 1700, limited only by the experimental conditions used. So far, no copper-based natural carbonic anhydrases are known, no faster model systems have been described and the biological role of the patellamide macrocycles is so far unknown. The observed CO2 hydration rates depend on the configuration of the isopropyl side chains of the pseudo-octapeptide scaffold, and the naturally observed R*,S*,R*,S* geometry is shown to lead to more efficient catalysts than the S*,S*,S*,S* isomers. The catalytic efficiency also depends on the heterocyclic donor groups of the pseudo-octapeptides. Interestingly, the dicopper(II) complex of the ligand with four imidazole groups is a more efficient catalyst than that of the close analogue of ascidiacyclamide with two thiazole and two oxazoline rings. The experimental observations indicate that the nucleophilic attack of a Cu(II)-coordinated hydroxide at the CO2 carbon center is rate determining, i.e. formation of the catalyst-CO2 adduct and release of carbonate/bicarbonate are relatively fast processes.

A speciation study of sulfur(iv) in aqueous solution.

In the current work ultraviolet spectrophotometric titrations at different S(iv) concentrations have been globally analysed using the entire spectral dataset to determine the complete speciation of S(iv) in aqueous solution over a large pH range (from 9.6 to 1). As a result, the dimerisation constant for the formation of disulfite from hydrogen sulfite has been accurately determined. Further, protonated disulfite has been identified and quantified for the first time. In addition, the molar absorptivities of all S(iv) species are also reported over the studied wavelength range.

Determination and visualization of rotational ambiguity in four-component systems.

One of the main problems that limit the use of model-free analysis methods for the resolution of multivariate data is that usually there is rotational ambiguity in the result. While methods for the complete definition of rotational ambiguity for two- and three-component systems have been published recently, the comprehensive and general resolution of rotational ambiguity for four-component systems has eluded chemists for several decades. We have developed an extension of self-modelling curve resolution for a mixture of four-components. The performance of the method was verified by applying it to resolve simulated and real data sets.

Multivariate linear regression with missing values.

This contribution presents and discusses an efficient algorithm for multivariate linear regression analysis of data sets with missing values. The algorithm is based on the insight that multivariate linear regression can be formulated as a set of individual univariate linear regressions. All available information is used and the calculations are explicit. The only restriction is that the independent variable matrix has to be non-singular. There is no need for imputation of interpolated or otherwise guessed values which require subsequent iterative refinement.

Reactions of CO2 with aqueous piperazine solutions: formation and decomposition of mono- and dicarbamic acids/carbamates of piperazine at 25.0 °C.

Piperazine (PZ) is widely recognized as a promising solvent for postcombustion capture (PCC) of carbon dioxide (CO(2)). In view of the highly conflicting data describing the kinetic reactions of CO(2)(aq) in piperazine solutions, the present study focuses on the identification of the chemical mechanism, specifically the kinetic pathways for CO(2)(aq) in piperazine solutions that form the mono- and dicarbamates, using the analysis of stopped-flow spectrophotometric kinetic measurements and (1)H NMR spectroscopic data at 25.0 °C. The complete set of rate and equilibrium constants for the kinetic pathways, including estimations for the protonation constants of the suite of piperazine carbamates/carbamic acids, is reported here using an extended kinetic model which incorporates all possible reactions for CO(2)(aq) in piperazine solutions. From the kinetic data determined in the present study, the reaction of CO(2)(aq) with free PZ was found to be the dominant reactive pathway. The superior reactivity of piperazine is confirmed in the kinetic rate constant determined for the formation of piperazine monocarbamic acid (k(7) = 2.43(3) × 10(4) M(-1) s(-1)), which is within the wide range of published values, making it one of the faster reacting amines. The corresponding equilibrium constant for the formation of the monocarbamic acid, K(7), markedly exceeds that of other monoamines. Kinetic and equilibrium constants for the remaining pathways indicate a minor contribution to the overall kinetics at high pH; however, these pathways may become more significant at higher CO(2) loadings and lower pH values where the concentrations of the reactive species are correspondingly higher.

Toward the understanding of chemical absorption processes for post-combustion capture of carbon dioxide: electronic and steric considerations from the kinetics of reactions of CO2(aq) with sterically hindered amines.

The present study reports (a) the determination of both the kinetic rate constants and equilibrium constants for the reaction of CO(2)(aq) with sterically hindered amines and (b) an attempt to elucidate a fundamental chemical understanding of the relationship between the amine structure and chemical properties of the amine that are relevant for postcombustion capture of CO(2) (PCC) applications. The reactions of CO(2)(aq) with a series of linear and methyl substituted primary amines and alkanolamines have been investigated using stopped-flow spectrophotometry and (1)H NMR measurements at 25.0 °C. The specific mechanism of absorption for each of the amines, that is CO(2) hydration and/or carbamate formation, is examined and, based on the mechanism, the kinetic and equilibrium constants for the formation of carbamic acid/carbamates, including protonation constants of the carbamate, are reported for amines that follow this pathway. A Brønsted correlation relating the kinetic rate constants and equilibrium constants for the formation of carbamic acid/carbamates with the protonation constant of the amine is reported. Such a relationship facilitates an understanding of the effects of steric and electronic properties of the amine toward its reactivity with CO(2). Further, such relationships can be used to guide the design of new amines with improved properties relevant to PCC applications.

Kinetics of the reversible reaction of CO2(aq) and HCO3(-) with sarcosine salt in aqueous solution.

Aqueous sarcosine salts are fast carbon dioxide (CO(2)) absorbents suitable for use in postcombustion CO(2) capture in coal-fired power plants. We have developed a detailed reaction scheme including all the reactions in the sarcosine-CO(2)-water system. All unknown rate and equilibrium constants were obtained by global data fitting. We investigated the temperature-dependent rate and equilibrium constants of the reaction between aqueous CO(2) and sarcosine using stopped-flow spectrophotometry, by following the pH changes over the wavelength range 400-700 nm via coupling to pH indicators. The corresponding rate and equilibrium constants ranged from 15.0 to 45.0 °C and were analyzed in terms of Arrhenius, Eyring, and van't Hoff relationships. The rate constant for the reaction between CO(2) and sarcosine to form the carbamate at 25.0 °C is 18.6(6) × 10(3) M(-1) s(-1), which is very high for an acyclic amine; its activation enthalpy is 59(1) kJ mol(-1) and the entropy is 33(4) J mol(-1) K(-1). In addition, we investigated the slow reaction between bicarbonate and sarcosine using (1)H nuclear magnetic resonance spectroscopy and report the corresponding rate and equilibrium constants at 25.0 °C. This rate constant is 5.9 × 10(-3) M(-1) s(-1).

Toward rational design of amine solutions for PCC applications: the kinetics of the reaction of CO2(aq) with cyclic and secondary amines in aqueous solution.

The kinetics of the fast reversible carbamate formation reaction of CO(2)(aq) with a series of substituted cyclic secondary amines as well as the noncyclic secondary amine diethanolamine (DEA) has been investigated using the stopped-flow spectrophotometric technique at 25.0 °C. The kinetics of the slow parallel reversible reaction between HCO(3)(-) and amine has also been determined for a number of the amines by (1)H NMR spectroscopy at 25.0 °C. The rate of the reversible reactions and the equilibrium constants for the formation of carbamic acid/carbamate from the reactions of CO(2) and HCO(3)(-) with the amines are reported. In terms of the forward reaction of CO(2)(aq) with amine, the order with increasing rate constants is as follows: diethanolamine (DEA) < morpholine (MORP) ~ thiomorpholine (TMORP) < N-methylpiperazine (N-MPIPZ) < 4-piperidinemethanol (4-PIPDM) ~ piperidine (PIPD) < pyrrolidine (PYR). Both 2-piperidinemethanol (2-PIPDM) and 2-piperidineethanol (2-PIPDE) do not form carbamates. For the carbamate forming amines a Brønsted correlation relating the protonation constant of the amine to the carbamic acid formation rate and equilibrium constants at 25.0 °C has been established. The overall suitability of an amine for PCC in terms of kinetics and energy is discussed.