Aquo Ions Research Articles

From Fe(III) Ions to Ferrihydrite and then to Hematite

The pathways from soluble Fe(III)-aquo ions to various solid polymeric Fe(III) oxides of increasing thermodynamic stability involve intermediate products which are still only partly known. This paper combines results from the literature with own new results from X-ray diffraction, Mössbauer spectroscopy, and electron microscopy. It is maintained that no intermediate phases were positively identified between mono-, di-, and trimers and a range of solid polynuclear phases. This indicates fast polymerisation as the OH/Fe ratio of the system increases. The immediate solid polynuclear phases are poorly crystalline Fe(III)-oxyhydroxy salts and a range of oxyhydroxides, called ferrihydrites in mineralogy, with varying crystallinity and magnetic ordering behavior. The rate of hydrolysis as affected by pH, rate of OH addition and temperature is of paramount importance for the nature of these polymeric phases. In the presence of free or surface-bound water, the transformation of ferrihydrite into the stable end product hematite (α-Fe2O3) proceeds by crystallization within the ferrihydrite aggregate and is enhanced as the pH approaches the zero point of charge as well as by Al in the system. This mechanism is different from that in nonaqueous systems.

Factors influencing the inorganic speciation of trace metal cations in fresh waters

The effects of pH and major ion composition on the chemical speciation of the divalent cations of Co, Ni, Cu, Zn, Pb and Cd have been examined after consideration of the available thermodynamic database for solution complexes of these ions. Calculations were made for two model river waters representing the 1% and 99% extremes in composition of global river waters. The results show that inorganic speciation behaviour is of two characteristic types: (a) Cu, Zn and Co are dominated by bis-hydroxy- complexes at high pH and show the greatest reduction in the fraction of free aquo ion with increasing pH; (b) Pb, Ni and Cd are dominated by carbonato- complexes at high pH and show a more gradual decrease in the fraction of free aquo ion with increasing pH. For Cu, Pb and Ni significant fractions of the labile forms of these metal ions are present as inorganic complexes in the pH range of most natural waters, whereas for Zn, Co and Cd this is true only at moderately high pH (pH >7.5). Complexing with the major ions SO42– and Cl– is important only at low pH in river waters of high ionic strength.

Magnetic field dependence of solvent proton relaxation by solute dysprosium (III) complexes.

Many magnetic resonance imaging (MRI) agents are Gd(III)-based; its half-filled f-shell has an S-ground state and hence a long electronic relaxation time, leading to comparably large effects on 1/T1 and 1/T2 of water protons with no shift in the water-proton resonance frequency. 1/T1 and 1/T2 nuclear magnetic relaxation dispersion (NMRD) profiles of the Dy(III) aquo ion and its chelates have been reported recently. Dy(III) ions differ magnetically from Gd(III); the large spin-orbit interaction of its non-S-ground state reduces the electronic relaxation time 100-fold, and can have a large effect on proton 1/T2 and resonance frequency. Relaxation theory is well-developed and applicable to both ions but, for Dy(III), the phenomena are more wide-ranging. Recent interpretations have suggested that the data are anomolous, requiring a new mechanism for their explanation. The authors explain published Dy(III) data in terms of known theory, guided by experience with Gd(III) agents. For fields below 1 T, the authors incorporate the shortened electronic relaxation time into the usual low-field theory for magnetic dipolar interactions between water protons and Dy(III) magnetic moments. Both inner- and outer-sphere relaxations are included. At higher fields (and unusual for small single-ion agents) one must include dipolar interactions of protons with the magnetization of the Dy(III) moments. This "Curie magnetization" causes a quadratic dependence of 1/T1 on field, and--through dipolar-induced shifts--an even greater quadratic dependence of 1/T2. All published data can be explained by magnetic dipolar interactions. For Dy(III), the Curie term has a longer correlation time than the low-field term, namely, the rotation of solute for 1/T1 and the even longer water exchange lifetime tau M for 1/T2. This exchange modulates the shift, producing phenomena not seen with Gd(III). Relaxation by Dy(III) chelates can be explained by the same well-established theory of dipolar interactions used for their Gd(III) analogs. Interestingly, for MRI applications, tau M should be long for Dy(III)-based agents and short for Gd(III)-based agents.

Processes regulating cellular metal accumulation and physiological effects: Phytoplankton as model systems

Trace metals exist in a variety of redox states and coordination species which markedly influences their geochemical behavior and biological availability. Most exist in natural waters as metal cations that are complexed to varying degrees by inorganic and organic ligands. Metal ions are generally taken up into cells by membrane transport proteins designed for acquisition of nutrient metals (Mg, Fe, Mn, Zn, Co, Cu, Mo). Organic ligands compete with these transport proteins for binding metal ions, and consequently, organic complexation substantially decreases metal uptake rates. Typically, the chelated metal is not directly available for cellular uptake, and metal uptake is controlled by either the concentration of free aquo ions or that of kinetically labile inorganic species. The coordination sites of transport proteins, however, are not entirely specific for intended nutrient metals, and consequently will bind with and transport non-nutritive or toxic metals. Competition for membrane transport sites and for intracellular metabolic binding sites can substantially influence the uptake of both nutrient and toxic metals and resultant effects on growth rate. Such growth rate effects provide feedback on cellular metal concentrations since the amount of metal accumulated within cells represents a balance between the rate of metal uptake and the cellular growth rate, the effective biodilution rate. Phytoplankton have provided useful model systems for investigating the processes and associated chemical and biological factors regulating cellular metal accumulation and resultant physiological effects.

Modelling the chemical speciation of trace metals in the surface waters of the Humber system

Calculations have been performed to estimate the chemical speciation at equilibrium of six divalent trace metals (Co, Ni, Cu, Zn, Cd, Pb) in riverine, estuarine and marine surface waters of the Humber system. The Windermere Humic Aqueous Model (WHAM) was used to compute distributions of dissolved metals. In the rivers, the free aquo ion (M 2+) is a major part of dissolved Co, Ni, Zn and Cd, but accounts for less than 1% of Cu and Pb. The main complexes are formed with carbonate ligands and dissolved natural organic matter, represented by fulvic acid. In the low-salinity region of the estuary and in seawater, complexation with fulvic acid is less significant, although most of the Cu is still in this form, while the speciation of Cd is dominated by chloride complexes. Adsorption of metals by suspended particulate matter was calculated with a simple model involving the concentrations of the free aquo ions (M z +) and H +, together with a constant for each metal estimated from laboratory adsorption data. Calculated adsorbed concentrations were used to predict the partition coefficient ( K D) for each metal under different circumstances. The values can vary by an order of magnitude or more, depending upon solution conditions. Typical values for rivers, low-salinity water and seawater are within one order of magnitude of observations. However, there is a general tendency to underestimate K D, possible reasons being (1) neglect of electrostatic enhancement of adsorption at low ionic strengths; and (2) analytical overestimation of particulate metal in equilibrium with the solution phase. There is a strong case for the development of a more sophisticated adsorption model.

Inorganic Geobiochemical Implications of the Oxic-Anoxic Transition in Ancient Earth Oceans

The development of free dioxygen in the Earth's atmosphere around 2.2Ga led to considerable biological changes. In particular, the early Earth biota was dominated by anaerobic prokaryotes including archaebacteria, possibly the most primitive life forms. Subsequent to the development of an oxic environment, aerobic bacteria and eventually prokaryotes developed. The inorganic biochemistry of the two groups are characterized in part by the relative dominance of nickel and cobalt proteins in the primitive anoxic group and the appearance of copper proteins in aerobic bacteria and multicellular organisms (Frausto di Silva and Williams 1996). Nickel and cobalt proteins are less important in aerobic organisms and copper is insignificant in the primitive anaerobes. Iron and zinc appear equally important throughout, although their dominant biochemical functions change. It is well established that base metal concentrations constitute limiting growth factors for present-day aerobic and anaerobic communities. For example, increasing oceanic iron concentrations enhances the growth of planktonic algae (Martin et al 1991). Iron has long been known as a limiting factor for the development of anaerobic sulphate reducing bacteria because of its importance to cytochrome c 3. (Postgate 1965). The present paper presents the results of a study of thermodynamic of oxic and anoxic ocean models based on the recent definition and discovery of a large number of base metal sulphide and polysulphide complexes (Luther et al. 1996; Chadwell et al. in prep). I assume that biological availability potentially embraces any soluble metal species and not only free metal aquo ions. The chemica l models were made using Geochemists Workbench | version 3.4 (GWB). The GWB seawater model has been discussed by Bethke (1996). The input thermodynamic database is a modified selection of the Lawrence Livermore National Labora tory thermodynamic dataset THERMO.COM.V8.R6.FULL. based on a compilation by Thomas Wolery and co-workers which is in turn largely based on Helgeson et al.(1978). The composition of early Earth oceans is not rigorously constrained and both acid and alkaline oceans have been proposed. I take a conservative view in this study, assuming that ocean composition is currently buffered by reaction with oceanic lithosphere and that this effect is likely to have been enhanced in the higher heat flow regimes of the ancient Earth. As a starting point, I therefore take the composition of the modem oxic oceans, which is fairly well defined. I then reduce it through the GWB reaction progress algorithm, basically by titrating electrons. During this process, the total major element concentration (excluding dioxygen) of the oceans remains constant. The pH remains around 8.0. In oxic oceans, inorganic base metal chemistry is dominated by free metal aquo ions, hydroxide, oxyhydroxide and chloride complexes. The database includes a number of simple base metal organic complexes, but these do not appear to affect the model and their stability constants may not be wellconstrained. In a n o x i c oceans , base metal speciation is simplified and their chemistry is largely controlled by sulphide and bisulphide complexes. The introduction of base metals (bi)sulphide complexes into the anoxic ocean model increases the relative dissolved concentrations of all base metals except Mn. The enhancement is approximately 2 magnitudes or more. This relative enhancement is independent of the sulphide mineral substrate. The total dissolved metal concentrations decrease markedly with peak concentrations around -200mV. The concentrations increase from around -300mV through -400mV. A feature of the anoxic ocean is the diminution of the free metal aquo ion concentrations. I took a computed dissolved metal concentration of 10 -15 M as being a limiting value for the presence of the metal in solution. The absolute concentrations of base metals in anoxic systems is not well constrained because of the lack of information about the nature of the metastable mineral sulphides. The increase in solubility of metastable compared with stable iron sulphides is better known. The first precipitated FeS

Humic Acid Binding of Trivalent Tl and Cr Studied by Synchronous and Time-Resolved Fluorescence

The complexation of Tl(III) and Cr(III) with groundwater humic acids (HA) was investigated at pH 4.5 and pH 5.5 using different fluorescence methods. These comprised both quenching measurements of the continuous and synchronous fluorescence of the HA ligand and an ion competition method based on the time-resolved laser-induced fluorescence of HA-bound Eu(III). Modeling of the experimental data using an electrostatic discrete site approach (Model V), yielded comparable values of intrinsic binding constants for each of the studied metal, independently of sample conditions (pH, metal:ligand ratio) and fluorescence method used. The results confirmed the validity of fluorescence techniques in the analysis of low concentration metal ion−HA interac tions and indicated that, beside the aquo ion and the first hydroxo complex, other species may also bind to humic acids.

Hydration and Covalence of the Ln2+ and An2+ Ions vs. Alkaline Earth Ions as Reflected in Solubility and Cocrystallization of Their Sulfates

Solubilities of some divalent lanthanide and actinide sulfates were evaluated and compared with the solubilities of alkaline earth sulfates. It has been concluded that the shift of the solubilities of LnSO4 and AnSO4 towards the solubility of BaSO4 is a consequence of differences in the aquo ion structure between [Ln(OH2)9]aq 2+, [An(OH2)9]aq 2+, [Ba(OH2)9]aq 2+ and [Ra(OH2)9]aq 2+ vs. [Sr(OH2)8]aq 2+ and [Ca(OH2)8]aq 2+. This difference in the primary hydration number throws light on the problem of the nomadic behavior of Eu(II) which, depending on the system, migrates from Ca(II) to Ba(II) on plots of the Gibbs solution and complexation energies. The virtual absence of the covalent shortening in the Euaq 2+ aquo ion slightly enlarges the Eu2+ ion, and this makes its primary hydration number higher by 1 than that of the Sr2+ ion. The increase in the coordination number (CN) for the Euaq 2+ species with respect to the Sraq 2+ species is supported by the solution enthalpy and entropy values of the respective sulfates. In particular, the difference in the primary hydration explains why the solubility product of EuSO4 is about two orders of magnitude lower than that of SrSO4 and close to BaSO4, while the stability of the Eu(II) complexes with EDTA, DCTA and 2-picolinic acid is close to that for the respective Ca complexes. The stronger interactions in the octaaquaions(II) than in the nonaaquaions(II) is the reason why, in oposition to Sr2+, the ions of Eu2+, Yb2+ and Es2+ form inner sphere complexes with tetraphenylborate anions in aqueous-ethanolic solutions. The same reason elucidates why, contrary to the Lnaq 2+ and Anaq 2+ species, Sraq 2+ does not form complexes with o-phenantroline and dipirydyl.

Reactions of Small Aggregates of Taurine Conjugates of Dihydroxy Bile Salts with Divalent Transition Metal Ions

Formation of slightly soluble complexes between the small aggregates (dimers to tetramers) of bile salts and divalent cations, Cu2+, Cd2+, and Fe2+(Me2+), have been studied using polarography to determine concentration of free aquo ions. Polarography has been proved to be useful for studying heterogeneous equilibria. Whereas taurocholate does not form such complexes (possibly because it does not form small aggregates), taurodeoxycholate, taurochenodeoxycholate, and tauroursodeoxycholate (all bearing two OH groups) participate in formation of such complexes. Nevertheless, these complexes are much weaker than those formed with parent, unconjugated bile salts. These differences can be due to differences in stacking as well as to exchange of COO−for the less complex-forming SO3−. The stability of complexes were characterized by values of pKi, obtained from shifts of half-wave potentials with concentrations of bile salt in excess. The values of pKj, corresponding to equilibria between Me2+in solution and solids, which can be obtained from decrease of limiting currents with concentration of bile salt, are less suitable for comparison, as it is often not possible to reach sufficiently high concentrations of bile salts. Comparison of pKiwith pKjindicates that for some bile salts–Me2+combinations the same complex predominates in solution and in the solid; for others the composition of these complexes differ. For Cu2+and Cd2+, tauroursodeoxycholate forms the most stable complexes. For all three bile salts studied, Cu2+formed the least stable complexes. Possibility and consequences of formation of complexes of bile salts with Me2+ions in bile should be kept in mind.

Ionic Association and Electron Spin Relaxation Rates in Aquo Gadolinium(III) Complexes

The electron paramagnetic resonance linewidth of aquo gadolinium(III) ion changes with the counter-ion identity and concentration in aqueous solutions. The EPR linewidth of 2 mMgadolinium(III) chloride increases from 49.2 to 89.0 mT when carbonate ion is added and decreases to 17.3 mT when nitrite ion is added. These observations suggest association reactions between aquo gadolinium(III) ion and anions that change the electron spin relaxation rates of the aquo ion. The concentration dependence of the gadolinium(III) EPR linewidth is consistent with binding constants for nitrite and nitrate ion with the aquo gadolinium(III) ion of 37 ± 6 and 2.3 ± 0.3 L mol−1respectively. The decreases in the EPR linewidth factors with these association reactions are difficult to understand unless the anion reactions increase the symmetry of the metal center. Although first-coordination reactions may not be ruled out, the decrease in EPR linewidth is more consistent with an outer-sphere association reaction that also reduces the coordination number of the metal center from 9 to 8.

Effect of Aluminum Hydroxide Precipitation Conditions on the Alumina Surface Acidity

The effect of specific ions present during the precipitation of alumina precursor (aluminum hydroxide) on resulting alumina surface acidity has been investigated. It was found that both the method of pH control and the nature of ions introduced to the slurry apparently affected the phase composition of aluminum hydroxide as well as the strength and distribution of alumina active sites measured by means of the temperature programmed desorption of ammonia (NH{sub 3} TPD). The results were interpreted in terms of processes taking place during both polycondensation of aquo ions and aging of amorphous hydrogel of aluminum hydroxide. The ions present in the medium of precipitation (particular at the beginning of aluminum hydroxide precipitation) apparently influenced the course of the reaction.

Measurement of Tl(III/I) Electron Self-Exchange Rates Using Enriched Stable Isotope Labels and Inductively Coupled Plasma Mass Spectrometry

An approach is described for measuring electron self-exchange rate constants (k 11 ) in solution based upon stable isotope-labeled reactants, chemical separations, and inductively coupled plasma mass spectrometry. The technique is demonstrated for the exchange between Tl III and Tl I aquo ions in aqueous HClO 4 . Tl III is prepared using 203 Tl-enriched Tl 2 O 3 ( 203 Tl abundance, ∼36%), and Tl I is prepared from natural abundance Tl reagents (natural 203 Tl abundance, 29.52%). The exchange is monitored by mixing the labeled and unlabeled reactants and performing timewise separations through selective precipitation of Tl I as TlBr. Isotope abundances are measured in the TlBr precipitate and Tl III solution phases using ICPMS with minimal sample preparation ; an NIST 981 (common lead) spike is added, and the 208 Pb/ 206 Pb is measured as an internal standard to correct for mass discrimination. The self-exchange rate constant is determined from a McKay plot obtained from the 205 Tl abundances of either oxidation state. A k 11 of (1.0 ± 0.1) x 10 -4 M -1 s -1 was obtained in 1.5 M aqueous HClO 4 at 25 °C.

Phenolic Acid Redox Properties: pH Influence on Iron(III) Reduction by Caffeic Acid

AbstractPhenolic acids are of great interest to soil chemists because their redox properties affect the availability of micronutrients to plants. In order to provide information about the mechanisms that regulate the reduction of Fe(III) at the soil‐root interface, the redox activity of caffeic acid (CAF) at different pH values in aqueous solution was investigated. The kinetics of the redox reaction was studied by using systems with Fe(III)/CAF molar ratios ranging from 1.1 to 10.9. At pH < 3, all systems showed the highest redox capacity: a maximum of nine electrons for each molecule of CAF was released at a Fe(III)/CAF molar ratio of 10.9. At pH > 3, the redox capacity dramatically decreased and was very low above pH 4. At pH > 4, the reduction of Fe(III) was considerably inhibited due to the formation of Fe(III)‐CAF complexes as well as of Fe(OH)3 precipitates, which are active in the adsorption of CAF. The amount of Fe(II) determined after 24 h of reaction suggests that not only the aquo ion, but also the hydrolized Fe(III)‐soluble species, are active in the oxidation of the organic molecule. At about pH 4, the ultraviolet/visible spectra revealed the presence of Fe(III)‐CAF complexes, which gave rise to the formation of a black polymeric product. The infrared spectra of this product suggest that the CAF molecule interacts with the positively charged hydroxide through the carboxylate groups. The reaction was not influenced by the presence of metal ions such as Cu(II), Ni(II), Mn(II), Fe(II), and Zn(II). We have proposed a mechanism of the reduction and complexation process consistent with our results.

Competition of copper and zinc for strong ligands in a eutrophic lake

Free aquo copper ion and zinc ion concentrations were determined in water samples from eutrophic Lake Greifen by means of ligand exchange with catechol and cathodic stripping voltametry (for Cu) and ligand exchange with EDTA and anodic stripping voltametry (for Zn). The ratios of total dissolved Zn (10–40 nM) to dissolved Cu (7–20 nM) were [Zn]: [Cu] = 0.5–3 in samples taken at different seasons of a year; the ratios of the free aquo ion concentrations [Zn2+] : [Cu2 +] were ∼ 106. pCu was in the range of 14.5–15.9 and pZn 8.6–9.5 at different times and depths. The release of Zn from electrochemically inert complexes upon addition of Cu suggested direct competition of Cu and Zn for ligands in all lake‐water samples examined. The selectivity of the natural ligands for Cu over Zn was evaluated as the conditional constant for the reaction Cu2+ + ZnL1 = Zn2 + CuL1; K = (1.4±0.9) × 106 (average in the euphotic zone; pH 8). These highly selective ligands are probably of biological origin.

Kinetic Studies of Metal Speciation Using Inductively-Coupled Plasma Mass Spectrometry

Kinetic studies of uptake of metal ions by the Chelex batch technique were made to determine Cd, Cu and Pb speciation in model solutions, a snow sample and a river surface water sample. Inductively-coupled plasma mass spectrometry (ICP-MS) and graphite furnace atomic absorption spectrometry (GFAAS) were used for direct determination of these metals. ICP-MS with the solution nebulization technique minimized contamination and adsorption problems involved in the discrete sampling technique of GFAAS, and hence, gave more precise and accurate results. Also, ICP-MS allowed collection of many more data points than GFAAS and was able to resolve components with similar rates of dissociation, which could not always be resolved by GFAAS with its discrete sampling technique. ICP-MS was therefore preferable to GFAAS for kinetic studies of metal speciation. The kinetic data were analyzed by the iterative deconvolution method. The applicability of the Chelex batch technique to metal speciation was validated by analysis of model solutions containing these metal ions with or without EDTA, NTA and fulvic acid. Use of the Chelex batch technique for Cd, Cu and Pb speciation in snow and river surface water samples revealed a number of kinetically distinguishable components of these metals (as complexes) ranging from one to three, probably present as aquo ions or inorganic complexes in the snow sample, and bound to macromolecules/and or colloidal materials in the river surface water sample.

Fe(III)-ion-catalysed non-enzymatic transformation of adenosine diphosphate into adenosine triphosphate part II. Evidence of catalytic nature of Fe ions

Non-enzymatic phosphorylation of adenosine diphosphate (ADP) to adenosine triphosphate (ATP) in aqueous solutions using acetyl phosphate has been studied extensively. The reaction proceeds in the presence of Fe(III) ions. Normally, the yield of ATP never exceeds the amount of Fe ions but, when a sufficient amount of Mg(II) ions is added to the reaction system, the yield of ATP increases beyond the amount of Fe ions. This is because the Fe(III) ion is tightly bound to not only ADP but also ATP, the phosphorylation product. When, however, the Fe(III) ion is liberated from ATP by exchanging with an Mg(II) ion, it regains catalytic activity and contributes to the surplus production of ATP. The Fe(II) ion with either a single molecule of 1,10-phenanthroline or 2,2′-bipyridyl exhibits a remarkably high catalytic activity. This suggests that the Fe(II) aquo ion should also be an excellent catalyst.

Kinetic studies of aluminum and zinc speciation in river water and snow

Kinetic studies of chemical speciation by the Chelex-100 batch technique revealed aluminum is present both as an aquo ion and as simple complexes of inorganic ligands in snow, but probably bound to macromolecule complexants and/or colloidal substances in the Rideau River surface water. Two kinetically distinguishable components of aluminum having dissociation rate constants of 1.1 × 10 −2, ≤ 5.4 × 10 −4 s −1 and three kinetically distinguishable components of aluminum having dissociation rate constants of 5.5 × 10 −3, 8.4 × 10 −4, ≤6.3 × 10 −5 s −1 were observed in snow and river surface water, respectively. One kinetically distinguishable component of zinc having a dissociation rate constant of 9.1 × 10 −3 s −1, and three kinetically distinguishable components of zinc having dissociation rate constants of 8.5 × 10 −3, 2.5 × 10 −3, ≤ 6.0 × 10 −5 s −1, were observed in snow and river surface water, respectively. Use of inductively-coupled plasma mass spectrometry (ICP-MS) with the solution nebulizer to follow the kinetics of the uptake of aluminum and zinc by the Chelex resin in the Chelex batch technique allowed collection of many more data points in the same time range than were obtainable by graphite platform furnace atomic absorption spectrometry (GFAAS) with an automatic sampler, with the result that ICP-MS had a greater ability to resolve labile complexes than GFAAS. ICP-MS is therefore preferable to GFAAS for kinetic studies of metal speciation in aqueous samples.

Electronic structure, energy of hydration, and stability of metal aquo ions

The stability of metal aquo ions with respect to redox reactions is determined by the ionization energies of the atoms and the Gibbs energies of hydration for the ions(−ΔhG0). We present critically selected values of −ΔhG0 for 55 metal ions, determined from electrochemical, thermochemical, and spectra data. We consider the factors determining the values of −ΔhG0 (charges, ionic radii, electronic structure, and relativistic effects). For isoelectronic ions, we observe correlations between the ratios of the Gibbs energies of hydration for these ions with different charges and the ratios of their ionic radii. Based on the use of these correlations, we find −ΔhG0 for a number of aquo ions not observed experimentally and we estimate the unknown oxidation-reduction potentials for the pairs of ions M3+/M2+. We formulate the principles for stabilization of unstable oxidation states of the metals by including the corresponding ions in complexes with certain classes of ligands.

Metal-ion-promoted hydrolysis of polyuridylic acid

First-order rate constants for the hydrolysis of polyuridylic acid in the absence and presence of various metal ions and their chelates have been determined under neutral and slightly acidic conditions. The hydrolysis has been shown to proceed by cleavage of non-terminal bonds rather than by stepwise release of monomeric nucleotides. Only the hydrolysis of the 3′,5′-phosphodiester bonds is accelerated by metal ions, not their isomerization to 2′,5′-bonds. The catalytically active species has been shown to be the monohydroxo form of the metal aquo ion. The rate accelerations are parallel, but up to 20 times greater than those obtained with uridylyl (2′,5′)uridine. The mechanism of the metal-ion action is discussed.

Polarographic and coulometric studies on aquo ions of molybdenum in 0.1 M H 2SO 4

Solutions containing 4 × 10 −4 M Mo(VI) in 0.1 M H 2SO 4 exhibited three polarographic waves. The first corresponds to reduction of Mo(VI) to Mo(V) and the second from Mo(V) to Mo(IV). The third one is due to the reduction of a dimeric Mo(V) species (Mo 2O 4 2+ to Mo(III). The dimer is formed because the Mo(V) generated at the electrode is extremely labile, undergoing dimerization. The Mo(IV) species was not isolated experimentally, and its labile formation was based on computational analysis of the second polarographic wave. This molybdenum form reacts in the presence of oxidants, and the existence of a catalytic process in the Mo(VI) + H 2SO 4 system was observed in the presence of nitrate. Under suitable experimental conditions the Mo(IV) form suffers a disproportionation reaction and the second polarographic wave (Mo(V) → Mo(IV)) is kinetically controlled. The kinetic reaction is prevented at low temperatures and short drop times, and under these conditions the reduction of Mo(V) is controlled by diffusion.