Goodbye to S2- in aqueous solution.

New Raman spectra of Na2S dissolved in hyper-concentrated NaOH(aq) and CsOH(aq) cast serious doubt on the widely-assumed existence of S2-(aq). To avoid conceptual and practical problems with sulfide equilibria in numerous applications, S2-(aq) should be expunged from the chemical literature. Thermodynamic databases involving sulfide minerals also need careful revision.

Obtaining the 'best values' of stability constants: the protonation constants of five thioether carboxylates as a case study.

Computer speciation models often require large numbers of equilibrium constant values. For this purpose, state-of-the-art methods for experimental constant determination, best value constant selection from published values, and constant prediction based on quantitative structure-activity relationships, are evaluated through their application to the study of the protonation constants of five thioether carboxylates in aqueous solution at 37 degrees C and I = 150 mmol dm(-3) (NaCl).

Complexation of iron(III) and iron(II) by citrate. Implications for iron speciation in blood plasma.

Estimates of the concentrations and identity of the predominant complexes of iron with the low-molecular-mass ligands in vivo are important to improve current understanding of the metabolism of this trace element. These estimates require a knowledge of the stability of the iron-citrate complexes. Previous studies on the equilibrium properties of the Fe(III)-citrate and Fe(II)-citrate are in disagreement. Accordingly, in this work, glass electrode potentiometric titrations have been used to re-determine the formation constants of both the Fe(III)- and Fe(II)-citrate systems at 25 degrees C in 1.00 M (Na)Cl and the reliability of these constants has been evaluated by comparing the measured and predicted redox potentials of the ternary Fe(III)-Fe(II)-citrate system. The formation constants obtained in this way were used in computer simulation models of the low-molecular-mass iron fraction in blood plasma. Redox equilibria of iron are thus included in large models of blood plasma for the first time. The results of these calculations show the predominance of Fe(II)-carbonate complexes and a significant amount of aquated Fe(II) in human blood plasma.

Association constants for the NaSO(4)(-) ion pair in concentrated cesium chloride solutions.

Values of the association constant, beta(NaSO(4)(-)), for the weak ion-pair formed by sodium and sulfate ions in aqueous solution have been determined at 25 degrees C by high precision sodium ion-selective electrode potentiometry in solutions of ionic strength ranging from 0.50 to 7.00 M in CsCl media and in 1.00 M Me(4)NCl. The data in CsCl media were fitted to an extended form of the Debye-Hückel equation which yielded log beta(NaSO(4)(-))(0)=0.834+/-0.005 at infinite dilution. Evidence is also presented for the formation of very weak ion-pairs between Cs(+) and SO(4)(2-).

The ionic product of water in highly concentrated sodium perchlorate solutions.

The ionic product of water, pK(w)=-log[H(+)][OH(-)], has been determined in aqueous solutions of sodium perchlorate over the concentration range of 1.0-8.0 M at 25 degrees C from high-precision potentiometric titrations carried out in cells with liquid junction using both glass and hydrogen electrodes. The glass electrode results are systematically lower probably as a result of interference by Na(+) ions.

Complexation of copper(I) by thioamino acids. Implications for copper speciation in blood plasma.

There is mounting evidence that Cu(I) is the most important oxidation state of copper in many physiological systems. Research into Cu(I)-thioamino acid complex formation serves not only to improve the chelation therapy for treating copper intoxication but may also provide a better understanding of many facets of normal copper metabolism. Formation constants for the ternary mixed ligand complexes of Cu(I) with cysteine (Cys), glutathione (GSH) and penicillamine (Pen) are reported here for the first time. Potentiometric titrations, using techniques specially developed for the stabilization of aqueous Cu(I), were performed at 25 degrees C in 1.00 M (Na)Cl. It was found that precipitation severely limits the experimentally accessible pH range and, consequently, the computer analysis of the binary metal-ligand systems; however, it is also found that this is less of a problem when two different ligands are present. This latter fact permitted better models of the binary systems to be developed. The formation constants of Cu(I)-thioamino acids determined in this work were used in an improved computer simulation of copper speciation in blood plasma which, for the first time, incorporates redox equilibria.

The ionic product of water in concentrated tetramethylammonium chloride solutions.

The ionic product of water, pK(w) = - log[H(+)][OH(-)] has been determined in aqueous solutions of tetramethylammonium chloride over the concentration range of 0.1-5.5 M at 25 degrees C using high-precision glass electrode potentiometric titrations. pK(w) data relating to aqueous potassium and sodium chlorides at ionic strengths up to 5 M are markedly lower than the tetramethylammonium chloride results. These differences are almost certainly due to weak associations between potassium and (especially) sodium and hydroxide ions.

Cyanide thermodynamics 2. Stability constants of copper(I) cyanide complexes in aqueous acetonitrile mixtures.

The stability (formation) constants of the binary Cu(I)-CN(-) complexes have been measured in five aqueous mixtures containing from 10% to 70% v/v acetonitrile (MeCN) by glass electrode potentiometry at 25 degrees C and an ionic strength of l M (NaClO(4)). The constants show monotonic increases with MeCN concentration, the changes being greatest for the higher order complexes, consistent with the unfavourable solvation of CN (-) in these mixtures. The sparing solubility of CuCN(s) prevented determination of the stability constant for CuCN (0) (soln.) at low MeCN concentrations.

Biospeciation, by potentiometry and computer simulation, of Sm-EDTMP, a bone tumor palliative agent.

153Sm-EDTMP (ethylenediaminetetra(methylenephosphonic) acid) is of considerable interest as a bone therapeutic radiopharmaceutical but its properties in solution are not yet well characterized. The protonation constants of EDTMP and the formation constants of the complexes of Sm-EDTMP have accordingly been measured potentiometrically by glass electrode titrations at 25 degrees C in 0.15 M NaCl. Six protonation constants (log beta 011 = 9.638, log beta 012 = 17.330, log beta 013 = 23.597, 10g beta 014 = 28.636, log beta 015 = 31.501, log beta 016 = 32.624) and the formation constants of the [Sm(EDTMP)H-1]6-(log beta 11-1 = 4.865), [SmEDTMP]5-(log beta 110 = 12.018), [Sm(EDTMP)H]4- (log beta 111 = 17.892) and [Sm(EDTMP)H2]3- (log beta 112 = 23.437) complexes were determined. Computer simulations indicate that the [SmEDTMP]5- and the hydroxy [Sm(EDTMP)H-1]6- species are the major Sm(III) complexes formed in blood plasma, which explains the high degree of localization in the kidney and urine observed in biodistribution studies. Calcium ions are probably the major competitor for EDTMP in blood plasma. As the presence of secondary skeletal metastases results in a high rate of bone turnover, it is possible that the high concentration of calcium at these sites encourages localization of 153Sm-EDTMP.

JESS, a joint expert speciation system-III. Surrogate functions.

A new, general method of coupling calculations of equilibria with those of other chemical effects and processes is described. The method, based on the use of simple empirical functions, is demonstrated by applying it to changes of ionic strength and temperature in speciation modelling.

JESS, A joint expert speciation system-I. Raison d'être.

JESS is a new computer package for modelling chemical systems in solution and performing numerical analyses on associated experimental data. It was developed to solve problems requiring specialist knowledge of chemical speciation. It currently comprises over 150 programs, 1200 subroutines and 120,000 lines of Fortran code. The main reasons for the development of JESS, and the principles which underpin it, are described. Subsequent papers detail the major facilities which JESS now provides.

Jess, a joint expert speciation system-II. The thermodynamic database.

The thermodynamic database of the JESS (Joint Expert Speciation System) software package is described. It overcomes many existing problems associated with solution-chemistry databases. The system is fully interactive. Reactions can be expressed in any form. Any number of equilibrium constants, enthalpy, entropy and Gibbs-free energy values can be associated with a reaction. Supplementary data such as background electrolyte, temperature, ionic strength, method of determination and original literature reference are also stored. Data can be readily transferred between databases. Currently, the thermodynamic database that is being distributed with JESS contains over 12,000 reactions and over 20,000 equilibrium constants. These data span interactions in aqueous solution of some 100 metal ions with more than 650 ligands.

Iron chelators of the pyridoxal isonicotinoyl hydrazone class. III. Formation constants with calcium(II), magnesium(II) and zinc(II).

Formation constants for the calcium(II), magnesium(II) and zinc(II) complexes of the orally effective iron chelator, pyridoxal isonicotinoyl hydrazone (PIH) and three analogues, pyridoxal benzoyl hydrazone (PBH), pyridoxal p-methoxybenzoyl hydrazone (PpMBH) and pyridoxal m-fluorobenzoyl hydrazone (PmFBH) have been determined by potentiometry at 25 degrees C and I = 0.1 M [KNO3]. The four ligands bind calcium(II) weakly and magnesium(II) only slightly more strongly, as a 1:1 complex which is formed at pH greater than 8. The chelation of zinc(II) for all the ligands studied was greater than that for calcium(II) and magnesium(II), with complexation generally becoming significant at about pH 5. Thus, chelation of zinc(II) but not calcium(II) or magnesium(II) at physiological pH, 7.4 may be expected. Calculated values of the concentration of uncomplexed metal ion indicate that the selectivity of these ligands towards Fe(III) is comparable to that of the clinically used chelator desferrioxamine.

The use of glass electrodes for the determination of formation constants-IV Matters of weight.

Implicit and explicit methods of weighting titration data for the purpose of determining formation constants are considered. Many current computer programs make no provision at all for weighting and the exceptions do so in ways that have unknown effects. It is shown that formation constants obtained by optimization can, in practice, vary significantly when different methods of weighting are employed. However, none of these methods consistently yields the best results. Whenever possible, it is advisable to use several approaches to check for agreement between them.

The use of glass electrodes for the determination of formation constants-V Monte Carlo analysis of error propagation.

The precision with which formation constants are calculated can be quantified by a Monte Carlo technique from realistic estimates of the experimental errors in titration parameters. The resulting standard deviations are invariably much poorer than those obtained conventionally because they reflect the true effects of systematic errors on the formation constant calculation. Such effects probably account for many of the discrepancies in the formation constant literature. Better assessment of errors makes it possible to select the set of titration parameters which, when optimized, yields the result with greatest probable accuracy. It is shown that this approach can markedly reduce the differences between formation constants determined by independent observers.

The use of glass electrodes for the determination of formation constants-III Optimization of titration data: The esta library of computer programs.

Difficulties with the analysis of titration data for the determination of formation constants are discussed. Considerable differences between published values can, in part, be attributed to incorrect "model selection", i.e., incorrect selection of chemical species. Experimental errors also contribute to the problem in ways that are neither well understood nor easy to overcome. The library of computer programs described, called ESTA (Equilibrium Simulation for Titration Analysis), has been written to investigate better methods of optimizing formation constants from potentiometric titration data.

The use of glass electrodes for the determination of formation constants--II: Simulation of titration data.

Methods for simulating titration data, including various types of corrections for changes in activity, liquid-junction potential and ion-selectivity of electrodes are described. These form the basis of a new library of computer programs, called ESTA (Equilibrium Simulation for Titration Analysis). They permit the range of titration conditions employed in the determination of formation constants to be usefully extended. Simulations have been performed to illustrate the extent to which the effects mentioned above are manifest in many titrations of practical importance.

Metal binding by pharmaceuticals. Part 5. Interaction of Cd(II), Ni(II) and Pb(II) with the intracellular hydrolysis products of the anti-tumour agent ICRF 159 and its inactive homologue ICRF 192.

Formation constants for the cadmium(II), nickel(II) and lead(II) complexes of DL-NN'-dicarboxamidomethyl-NN'-dicarboxymethyl-1,2-diaminopr opane (ICRF 198) and the 1,2-diaminobutane homologue (ICRF 226) have been measured potentiometrically at 37 degrees C and I = 150 mmol dm-3 [NaCl]. In all titrations a competing ligand, known to complex strongly with the metal ion, and having its formation constants predetermined, was employed. The constants are used in computer simulation models to assess the relative efficacy of the agents in mobilizing these metals from plasma proteins into low-molecular-weight complexes and the results are compared to those for known chelating agents. It is shown that the lead mobilizing potential of the agents is greater than either EDTA or D-penicillamine; they are, however, less adept in the removal of cadmium and nickel than other established agents.

Computer simulation of metal ion equilibria in biofluids. IV. Plutonium speciation in human blood plasma and chelation therapy using polyaminopolycarboxylic acids.

An investigation by computer simulation into the nature of Pu(IV) binding to low-molecular ligands in human blood plasma is described. Particular consideration is given to the interactions of various chelating agents which have been or might be used for treating plutonium intoxication. Formation constants of EDTA and DTPA with Cu(II), Mg(II), Mn(II), Zn(II), and Cd(II) have been measured under biologic conditions of temperature and background electrolyte. The relative ability of these and other chelating agents to cause excretion of plutonium and the concomitant loss of certain essential trace metals has thus been assessed.

Metal binding by pharmaceuticals. Part 4. A comparative investigation of the interaction of metal ions with hydralazine, prizidilol and related compounds.

The metal complexing properties of two antihypertensive drugs, hydralazine (1-hydrazinophthalazine) and prizidilol (a hydrazinopyridazine), and some related ligands, have been studied using potentiometry, elemental analysis, spectrophotometry and computer simulation. The coordination chemistry of 1-hydrazinophthalazine and the hydrazinopyridazines is similar in that Ca(II), Mg(II), and Mn(II) complexes are not formed, whereas Zn(II), Cu(II) and Fe(II)/Fe(III) complexes are produced. Both kinds of ligand react with Fe(II) to form a brightly coloured tetrazene complex which is insoluble for hydralazine but soluble for prizidilol. Computer simulation studies indicate that the most prevalent metal complex of prizidilol in blood plasma is [Fe2+(Priz-)H+]2+ but that this only forms at very high drug concentrations. It is concluded that prizidilol is unlikely to have any direct effects on the metabolism or distribution of the trace elements listed here.