Potentiometric Titration Experiments Research Articles

Proton and Cd adsorption onto natural bacterial consortia: Testing universal adsorption behavior

Bacterial surface adsorption can control metal distributions in some natural systems, yet it is unclear whether natural bacterial consortia differ in their adsorption behaviors. In this study, we conduct potentiometric titration and metal adsorption experiments to measure proton and Cd adsorption onto a range of bacterial consortia. We model the experimental data using a surface complexation approach to determine thermodynamic stability constants. Our results indicate that these consortia adsorb similar extents of protons and Cd and that the adsorption onto all of the consortia can be modeled using a single set of stability constants. Consortia of bacteria cultured from natural environments also adsorb metals to lesser extents than individual strains of laboratory-cultivated species. This study suggests that a wide range of bacterial species exhibit similar adsorption behaviors, potentially simplifying the task of modeling the distribution and speciation of metals in bacteria-bearing natural systems. Current models for bacteria-metal adsorption that rely on pure strains of laboratory-cultivated species likely overpredict the amount of bacteria-metal adsorption in natural systems.

Characterization of metal-cyanobacteria sorption reactions: a combined macroscopic and infrared spectroscopic investigation.

In this study, we conducted synchrotron radiation Fourier transform infrared (IR) spectroscopy, potentiometric titration, and metal sorption experiments to characterize metal-cyanobacteria sorption reactions. Infrared spectra were collected with samples in solution for intact cyanobacterial filaments and separated exopolymeric sheath material to examine the deprotonation reactions of cell surface functional groups. The infrared spectra of intact cells sequentially titrated from pH 3.2 to 6.5 display an increase in peak intensity and area at 1400 cm(-1) corresponding to vibrational COO- frequencies from the formation of deprotonated carboxyl surface sites. Similarly, bulk acid-base titration of cyanobacterial filaments and sheath material indicates that the concentration of proton-active surface sites is higher on the cell wall compared to the overlying sheath. A three-site model provides an excellent fit to the titration curves of both intact cells and sheath material with corresponding pKa values of 4.7 +/- 0.4, 6.6 +/- 0.2, 9.2 +/- 0.3 and 4.8 +/- 0.3, 6.5 +/- 0.1, 8.7 +/- 0.2, respectively. Finally, Cu2+, Cd2+, and Pb2+ sorption experiments were conducted as a function of pH, and a site-specific surface complexation model was used to describe the metal sorption data. The modeling indicates that metal ions are partitioned between the exopolymer sheath and cell wall and that the carboxyl groups on the cyanobacterial cell wall are the dominant sink for metals at near neutral pH. These results demonstrate that the cyanobacterial surfaces are complex structures which contain distinct surface layers, each with unique molecular functional groups and metal binding properties.

Points of Zero Charge and Potentiometric Titrations

Based on the various categories of surface charge and the definitions of points of zero charge proposed by Sposito (1984, 1998), the determination of the adsorbed proton charge (σH) and the point of zero net charge (PZNPC) of insoluble oxides and layer silicates in aqueous solution was examined in detail using potentiometric titration experiments. The confusion, various interpretations and difficulties associated with points of zero charge, including the point of zero net proton charge (PZNPC), the point of zero net charge (PZNC) and the point of zero charge (PZC), were discussed and clarified. The importance of comparing potentiometric titration curves for pure insoluble oxides in the presence and absence of specific adsorption and of layer silicates with a permanent negative charge was emphasized. The dependency of expressions of points of zero charge on the model assumed and the weakness of the method of potentiometric titrations widely used for the determination of points of zero charge were discussed.

Surface complexation modeling of uranium(VI) sorbed onto lanthanum monophosphate

Sorption/desorption are basic processes in the field of contaminant transport. In order to develop mechanistically accurate thermodynamic sorption models, the simulation of retention data has to take into account molecular scale informations provided by structural investigations. In this way, the uranyl sorption constants onto lanthanum monophosphate (LaPO 4) were determined on the basis of a previously published structural investigation. The surface complexation modeling of U(VI) retention onto LaPO 4 has been performed using the constant capacitance model included in the FITEQLv3.2 program. The electrical behavior of the solid surface was investigated using electrophoretic measurements and potentiometric titration experiments. The point of zero charge was found to be 3.5 and surface complexation modeling has made it possible to calculate the surface acidity constants. The fitting procedure was done with respect to the spectroscopic results, which have shown that LaPO 4 presents two kinds of reactive surface sites (lanthanum atoms and phosphate groups). The uranyl sorption edges were determined for two surface coverages: 40 and 20% of the surface sites that are occupied, assuming complete sorption. The modeling of these experimental data was realized by considering two uranyl species (“free” uranyl and uranyl nitrate complex) sorbed only onto phosphate surface groups according to the previously published structural investigation. The obtained sorption constants present similar values for both surface complexes and make it possible to fit both sorption edges: log K U=9.4 for P(OH) 2+UO 2 2+↔P(OH) 2UO 2 2+ and log K UN=9.7 for P(OH) 2+UO 2NO 3 +↔P(OH) 2UO 2NO 3 +.

PH-dependent absorption and emission properties of a ReI complex working as a carboxylate ligand for Cu2+

The fac-Re(CO)3(bipy-COOH)Cl complex (bipy=2,2′-bipyridine) bears a carboxylic function in the 5-position of the bipy ligand: due to the kinetic inertness of the ReI center, the complex behaves as a R-COOH carboxylic acid. In aqueous solutions, it coordinates Cu2+, according to pH-dependent equilibria, to give the dimeric species Cu[fac-ReI(CO)3(bipy-COO)Cl]2 and {Cu[fac-Re(CO)3(bipy-COO)Cl]2(OH)2}2−, whose formation constants have been determined by means of potentiometric titration experiments. The fac-Re(CO)3(bipy-R)Cl core is luminescent, and interestingly, its emitting properties are pH-dependent. In particular, its luminescence intensity increases with the formation of the {Cu[fac-Re(CO)3(bipy-COO)Cl]2(OH)2}2− species, visually signaling the formation of this hydroxo-containing multicenter complex.

Cationic sigma-phenylplatinum(II) complexes with carboxylic acid functionality: pK(a) determinations and x-ray structures.

The preparation and characterization of a novel series of cationic sigma-phenylplatinum(II) complexes of the type trans-[Pt(sigma-C(6)H(5))(L)(2)A]OTf (A = picolinic acid, L = PPh(3) (4) and PMePh(2) (7); A = nicotinic acid, L = PPh(3) (5) and PMePh(2) (8); A = isonicotinic acid, L = PPh(3) (6), PMePh(2) (9), and PEt(3) (10)) are described. The pK(a) value for the carboxylic acid functionality in selected complexes was found to follow the order 7 (pK(a) = 5.23 +/- 0.09) > 8 (4.85 +/- 0.10) > 9 (3.51 +/- 0.08) > 6 (3.26 +/- 0.07) approximately 10 (3.21 +/- 0.08) by means of potentiometric titration experiments in 50% (v/v) EtOH/H(2)O solution at 295 K. The X-ray crystal structures of 9 and 10 were also determined. The asymmetric unit of each of 9 and 10 comprises a univalent complex cation, a triflate anion, and a solvent CH(2)Cl(2) molecule of crystallization. Centrosymmetrically related pairs of complex cations in 9 associate via the familiar carboxylic acid dimer motif, whereas with 10, the carboxylic acid dimer motif is absent. Instead, the carboxylic acid residue forms both donor and acceptor interactions to the triflate anion and CH(2)Cl(2) solvent of crystallization, respectively, to afford a 10-membered ring structure. Possible reasons for the observed differences in the solid-state structures of 9 and 10 are presented.

Ligand-Modified Colloid Enhanced Ultrafiltration. Use of Nitrilotriacetic Acid Derivatives for the Selective Removal of Lead from Aqueous Solution

In ligand-modified, colloid-enhanced ultrafiltration (LM-CEUF), a ligand that selectively complexes target ions (e.g., lead) also associates with a water-soluble colloid, such as a surfactant micelle or polyelectrolyte. The colloid, associated ligand, and target ion are then concentrated using ultrafiltration, producing a filtrate with a low concentration of the target ion. Dialysis, ultrafiltration, and potentiometric titration experiments have been used to investigate the effectiveness of four nitrilotriacetic acid (NTA) derivatives in the removal of lead from aqueous solution through LM-CEUF. The ligands are 2-phenyl nitrilotriacetic acid (PNTA), 2-[N, N-di-(carboxymethyl)]amino-octanoic acid (HNTA), 2-[N,N-di-(carboxymethyl)]amino-dodecanoic acid (DNTA), and 2-[N,N-di-(carboxymethyl)]amino-3-sulfopropionic acid (SNTA). The colloids used were the cationic polyelectrolyte poly(diallyldimethylammonium chloride) (PDADMAC) and the cationic surfactant cetylpyridinium nitrate (CPNO3). In equilibrium dialysis and ultrafiltration (UF) experiments, SNTA and PDADMAC systems provided effective lead removal. In semiequilibrium dialysis and UF experiments, DNTA and CPNO3 systems also provided effective lead removal. The effects of pH, ionic strength, competing ions, and colloid concentration were investigated for each ligand system. Ligand protonation constants and ligand–metal stability constants were obtained for SNTA in both water and solutions of PDADMAC and for DNTA in solutions of CPNO3. Ligand regeneration through pH adjustment and metal precipitation with various anions was also demonstrated.

Synthetic carbons activated with phosphoric acid: I. Surface chemistry and ion binding properties

Synthetic activated carbons were prepared by phosphoric acid activation of a styrene–divinylbenzene copolymer at various temperatures in the 400–1000 °C range. The resulting carbons were characterized by elemental analysis, cation-exchange capacity measurement, infrared spectroscopy, potentiometric titration with calculation of proton affinity spectra, and copper adsorption from solution. The results indicate that the synthetic carbons obtained possess acidic character and show cation-exchange properties similar to those of oxidized carbons. However, the acidic compounds arising from treatment with phosphoric acid are tightly bound to the carbon lattice and are chemically and thermally more stable than those introduced by oxidative treatments. The largest amount of cation-exchange surface groups is introduced after activation at 800 °C. Infrared investigations showed that phosphorus compounds may be polyphosphates bound to the carbon lattice. Proton affinity distribution curves calculated from potentiometric titration experiments showed four types of surface groups on synthetic phosphoric acid activated carbons. Among them phosphorus-containing groups are the most important for the adsorption of heavy metal ions (copper) from acid solutions. Thus, carbons activated with phosphoric acid may be regarded as prospective cation-exchangers for the removal of heavy metals from water solutions.

Surface acid–base characteristics of natural manganese mineral particles

In this paper, the acid–base characteristics of natural manganese mineral collected from Guangxi Province, China, are investigated by potentiometric titration and solubility experiments. The constant capacitance surface complexation model is used to interpret experimental data. The calculations are performed with the least-squares FITEQL program 2.0. All the model parameters are determined on the base of the experimental data. This study indicates that the proton reactions in the acidic blank can be successfully described by those of Mn 2+ in solution. To explain the surface acid–base behavior of manganese mineral particle, two protonation models are proposed Model I: MnOH 2 +↔ MnOH+ H +, K a1 int =3.52&0xFFFD;3.60 MnOH↔ MnO −+ H +, K a2 int =4.04&0xFFFD;4.88 Model II: MnOH↔ MnO −+ H +, K a int =5.06–5.62 Optimization results indicate that these two models can simulate the experimental results very well. The applicability of the constant capacitance model (CCM) in describing the surface reaction of natural manganese mineral is further verified.

Molecular rearrangements controlled by pH-driven Cu2+ motions

Ligands 1–3 contain two coordinatively distinct compartments: AH2, featuring a bis-amino bis-amido donor set, and B, featuring a bis amino-heterocycle donor set. Their coordinative tendencies towards Cu2+ have been studied in dioxane–water solution (4 ∶ 1 v/v) by means of potentiometric titration experiments. The complex species which form in the pH 2–12 range have been individuated and their formation constants calculated. At neutral or slightly acidic pH values, the Cu2+ cation forms a 1 ∶ 1 complex with the neutral ligands, residing in the B compartment. If the pH is then raised to distinctly basic values, the AH2 compartment loses its two amide protons and Cu2+ moves inside it, forming neutral complexes with a bis-amino bis-imide donor set. The nature of these complexes has been ascertained by means of UV/Vis spectroscopy and by comparison with results for ligand 5, which also binds Cu2+ by losing its two amide protons, but does not possess further binding groups appended to the amine nitrogens. With ligands 1–3, when Cu2+ is coordinated by the bis-amino bis-imide donor set, one of the two appended heterocycles of the B compartment is still bound to it, in one of its apical positions, so that, on changing the pH between neutral and basic values, the Cu2+ movement induces an overall molecular rearrangement, which was found to be reversible.

Elucidation of the ion binding mechanism in heterogeneous carbon-composite adsorbents

A two-stage analysis of the mechanism of metal ion binding by a heterogeneous adsorbent was developed. In the first stage, a continuous proton affinity distribution was calculated from potentiometric titration data using the CONTIN method with a Langmuir kernel. Electrostatic effects were accounted for using a diffuse layer model. In the second stage, the parameters obtained from the continuous distribution function (i.e. the number of different types of surface sites, site densities and their protonation constants) were utilized in a discrete distribution to represent the adsorbent in surface complexation and double layer models using the GRFIT speciation code. This information on the surface groups was applied to metal ion potentiometric titration experiments to calculate the surface complexation equilibrium constants of the metal ions and hence elucidate the mechanism of ion binding to these sites. The proposed method was applied successfully to the adsorption of Sr and Cu ions on carbon-composite adsorbents, KAU-mod and SCN-mod. The continuous distribution method (CONTIN) revealed three types of surface sites within these carbon-composite adsorbents with p K values ranging between 3.5–4.1, 5.3–6.3 and 7.7–8.2. The analysis of the metal ion adsorption data using the GRFIT speciation code showed that only the first two surface sites were capable of forming surface complexes with the Sr ions, and that only the first site governed the adsorption of the Cu ions.

Sorption Behavior of Dye Compounds onto Natural Sediment of Qinghe River

The objective of this study is to assess the adsorption behavior of C.I. Basic Yellow X-5GL, C.I. Basic Red 13, C.I. Direct Blue 86, C.I. Vat Yellow 2, and C.I. Mordant Black 11 on natural sediment and to identify sediment characteristics that play a predominant role in the adsorption of the dyes. The potentiometric titration experiment is used to investigate acid–base properties of the sediment surface with a constant capacitance surface complexation model. The parameters controlling the sorption such as solution pH and ion strength, as well as the influence of organic carbon and Ca2+ ion on the adsorption, are evaluated. It is shown that the titration data can be successfully described by the surface protonation and deprotonation model with the least-squares FITEQL program 2.0. The sorption isotherm data are fitted to the Freundlich equation in a nonlinear form (1/n=0.3–0.9) for all tested dyes. With increasing pH value, the sorption of C.I. Mordant Black 11 and C.I. Direct Blue 86 on the sediment decreases, while for C.I. Basic Yellow X-5GL and C.I. Basic Red 13, the extent of sorption slightly increases. In addition, ion strength also exhibits a considerably different effect on the sorption behavior of these dye compounds. The addition of Ca2+ can greatly reduce the sorption of C.I. Basic Red 13 on the sediment surface, while it enhances the sorption of C.I. Direct Blue 6. The removal of organic carbon decreases the sorption of C.I. Mordant Black 11 and C.I. Direct Blue 86. In contrast, the sorption of C.I. Basic Red 13 and C.I. Basic Yellow X-5GL is obviously enhanced after the removal of organic carbon. The differences in adsorption behavior are mainly attributed to the physicochemical properties of these dye compounds.

Absorption and luminescence as a function of pH for carboxylic acid-functionalized Re I tricarbonyls

Six new ester- and carboxylic acid-derivatives of the fac-Re I(CO) 3(2,2′-bipyridine)Cl and [ fac-Re I(CO) 3(2,2′-bipyridine)(pyridine)]CF 3SO 3 type have been prepared, in which the functions reside on the bipy or py fragments, in their 5- and 3-positions, respectively: the compounds are Re I(CO) 3(bipyCOOCH 3)Cl ( 2), Re I(CO) 3(bipyCOOH)Cl ( 3), [Re I(CO) 3(bipyCOOH)(pyCOOCH 3)]CF 3SO 3 ( 4), [Re I(CO) 3(bipyCOOCH 3)(pyCOOH)]CF 3SO 3 ( 5), [Re I(CO) 3(bipyCOOCH 3)(pyCOOCH 3)]CF 3SO 3 ( 6) and [Re I(CO) 3(bipyCOOH)(pyCOOH)]CF 3SO 3 ( 7). These Re I complexes are air- and moisture-stable, and it has been possible to study their properties in aqueous solution (water–dioxane 1:4), as a function of pH. The p K as of the carboxylic acid(s)-containing species have been determined by means of potentiometric titration experiments and the absorbing and emitting properties of the six complexes related to the deprotonation processes. In particular, it has been possible to ascertain that the MLCT band of the rhenium chromophore is related, as expected, to an electron transfer process taking place always towards the bipy fragments, even in the py containing species 4– 7, independently on the substituent (ester, carboxylic acid or carboxilate) residing on it. Interestingly, the sharp and intense luminescence versus pH variation found for 3 makes it a suitable candidate for a luminescence-based pH sensor.

Inframolecular Studies of the Protonation of Adenophostin A: Comparison with 1-d-myo-Inositol 1,4,5-Trisphosphate

Adenophostin A is a glyconucleotide natural product with the highest known potency for the d-myo-inositol 1,4,5-trisphosphate receptor. Using synthetic adenophostin A we have investigated the macroscopic and microscopic protonation process of this compound by performing 31P NMR, 1H NMR, and potentiometric titration experiments. The logarithms of the first to the fourth stepwise protonation constants are, respectively, log K1 = 8.48, log K2 = 6.20, log K3 = 4.96, and log K4 = 3.80. The latter constant refers to the protonation equilibrium involving the N1 adenine nitrogen. From the microconstants the protonation fractions of each individual phosphate group can be calculated. Remarkably, the ionization state of the phosphates of adenophostin A at near physiological pH is very similar to those of inositol 1,4,5-trisphosphate, indicating that differences in phosphate charge cannot account for the high potency of this molecule. The analysis of the 1H chemical shifts vs pH provided complementary conformational information. In particular, a slight “wrongway shift” of H1" can be related to the protonation of P2, thus indicating a short H1"–P2 distance. Our results are in line with a recently published model in which, however, a certain degree of constraint would keep the ribose 2′-phosphate moiety close to the glucose ring phosphates.

PH-Dependent Transfer Hydrogenation of Water-Soluble Carbonyl Compounds with [Cp*IrIII(H2O)3]2+(Cp* =η5-C5Me5) as a Catalyst Precursor and HCOONa as a Hydrogen Donor in Water

This paper reports a pH-dependent hydrogenation of water-soluble carbonyl compounds by hydrogen transfer from HCOONa as a hydrogen source (transfer hydrogenation) promoted by [Cp*IrIII(H2O)3]2+ (1, Cp* = η5-C5Me5) as a catalyst precursor in water. Complex 1 has been characterized by X-ray structure analysis, 1H NMR, and potentiometric titration experiments. The active catalyst, a dinuclear μ-hydride complex [(Cp*IrIII)2(μ-H)(μ-OH)(μ-HCOO)]+ (2), has been isolated and characterized by 1H NMR, IR, and electrospray ionization mass spectrometry (ESI-MS). The rate of this transfer hydrogenation shows a sharp maximum at pH 3.2 because the active catalyst 2 is generated from the reaction of 1 with HCOONa at pH 3.2 in the highest yield. The series of the carbonyl compounds consists of a straight chain aldehyde (n-butyraldehyde), a cyclic aldehyde (cyclopropanecarboxaldehyde), a ketone (2-butanone), an aldehyde-acid (glyoxylic acid), and a keto-acid (pyruvic acid). Products were determined by 1H NMR and atmospheri...

O-Alkylated Derivatives of 1,3,5-Triamino-1,3,5-trideoxy-cis-inositol: Triamine Ligands with Unexpectedly High Affinity toward Divalent Transition- and d(10)-Metal Ions.

The ligands all-cis-2,4,6-trimethoxycyclohexane-1,3,5-triamine (tmca) and all-cis-2,4,6-tribenzoxycyclohexane-1,3,5-triamine (tbca) were prepared almost quantitatively by using [Ni(taci)(2)](2+) (taci = 1,3,5-triamino-1,3,5-trideoxy-cis-inositol) as precursor, where Ni(2+) acted as a very efficient protecting group for the nitrogen donors. The structure of tmca in solution was investigated by NMR spectroscopy. A strongly solvent-dependent conformational equilibrium was observed. In CD(3)CN, a chair conformation with three axial amino groups formed exclusively, whereas in D(2)O, the conformation with three equatorial amino groups predominated. This effect, as well as conformational changes in the course of stepwise protonation, is discussed in terms of hydrogen bonding effects. The crystal structure of H(3)tmca(3+) exhibits a chair conformation with three equatorial ammonium groups and three axial methoxy groups. The trihydrochloride hydrate crystallizes in the monoclinic space group P2(1)/n, a = 11.057(5) Å, b = 9.960(6) Å, c = 14.671(6) Å, beta = 93.79(3) degrees, Z = 4 for C(9)Cl(3)H(26)N(3)O(4). A variety of bis complexes [M(tmca)(2)](2+) (M = Ni, Cu, Zn, Cd) and [M(tbca)(2)](2+) (M = Ni, Cu) were prepared and they were isolated as solid, crystalline trinitrate or trichloride salts. Crystal data: [Ni(tmca)(2)](NO(3))(2).4H(2)O, triclinic, space group P&onemacr;, a = 8.919(11) Å, b = 9.293(9) Å, c = 9.942(11) Å, alpha = 96.73(9) degrees, beta = 100.66(9) degrees, gamma = 101.95(9) degrees, Z = 1 for C(18)H(50)N(8)NiO(16); [Cu(tmca)(2)](NO(3))(2), tetragonal, space group P&fourmacr;2(1)c, a = 13.017(6) Å, c = 15.985(10) Å, Z = 4 for C(18)CuH(42)N(8)O(12); [Ni(tbca)(2)](NO(3))(2).MeCN.H(2)O, monoclinic, space group P2(1)/c, a = 12.930(8) Å, b = 19.324(10) Å, c = 22.724(14) Å, beta = 97.21(5) degrees, Z = 4 for C(56)H(71)N(9)NiO(13). The formation constants of [M(tmca)](2+) and [M(tmca)(2)](2+) were determined by means of a series of potentiometric titration experiments. A comparison of the taci complexes with the corresponding tmca complexes revealed an unexpected increase of stability for the latter. This increase is more than 2 orders of magnitude for the 1:1 complexes and about 5 orders of magnitudes for the 1:2 complexes. Possible reasons for this unexpected increase in stability are discussed.

Acid-base properties of poly(propylene imine)dendrimers

Potentiometric titration experiments of poly(propylene imine) dendrimers (up to the fifth generation) were carriedout at salt concentrations of 0.1, 0.5 and 1.0 M KCl and NaCl. The experiments were performed at two different locations on different instruments and were converted to titration curves using two different methods, resulting in a consistent experimental data set for the dendrimers measured. The titration curves feature two distinct steps around p H 6 and 10 with an intermediate plateau at 2/3 of the total ionizable groups. This protonation pattern reflects short-ranged repulsive interactions between ionizables sites and can be modeled using an Ising model with nearest-neighbour pair interactions. The intermediate plateau results from the stability of an onion-like structure where all odd shells of the dendrimer are protonated, while the even ones remain deprotonated. The Ising model permits a quantitative analysis of the titration curves. For larger dendrimers, this Ising approach is shown to be superior to the classical analysis in terms of successive protonation equilibria.

Hydrolysis of the Organometallic Aqua Ion fac-Triaquatricarbonylrhenium(I). Mechanism, pKa, and Formation Constants of the Polynuclear Hydrolysis Products

Hydrolytic pathways of the organometallic aqua ion [Re(CO)3(H2O)3]+ (2) in aqueous media were investigated by means of potentiometric titration experiments. The aqua complex 2 was obtained quantitatively by dissolving (NEt4)2[Re(CO)3Br3] (1) in water. Conventional alkalimetric titrations (0.1 M KNO3, 25 °C) allowed the determination of the formation constants of [Re3(CO)9(μ2-OH)3(μ3-OH)]- (3) and [Re2(CO)6(μ2-OH)3]- (4). The neutral dinuclear [Re2(CO)6(μ2-OH)2(H2O)2] (5) was observed as a minor species. A fast titration technique was used to investigate a rapid preequilibrium, consisting of the formation of the mononuclear deprotonation products [Re(CO)3(OH)(H2O)2] (6) and [Re(CO)3(OH)2(H2O)]- (7). The corresponding pKa values are 7.5(2) and 9.3(3). The immediate extraction of an aqueous solution of 2 with diethyl ether after base addition (1 equiv) led to the quantitative isolation of the well-known cubane cluster [Re(CO)3(OH)]4 (8), which was obtained as a DMF (8b) or OPPh3 adduct (8c). Attempts to isolate the aqua complex 2 as a crystalline material by precipitating the bromo ligands of 1 with AgCF3COO resulted in the formation of (NEt4)2[Re(CO)3(CF3COO)3] (9). The structures of 8b, 8c, and 9 were elucidated by single-crystal X-ray analysis.

Electrochemically Switched Anion Translocation in a Multicomponent Coordination Compound

Complex [3a]2+ has been synthesized, which contains one NiII cyclam-like and one bipyridine fragment (cyclam = 1,4,8,11-tetraazacyclotetradecane). Spectrophotometric and potentiometric titration experiments in aqueous solution demonstrated that in the examined pH range (2−10), the NiII cyclam-like subunit remains square planar, i.e., it does not interact with the bipyridine unit, which is instead allowed to bind external metal centers. In particular, in the presence of 0.5 equiv of CuII, the trinuclear multicomponent coordination compound [CuII(3a)2(H2O)]6+ is the only species existing in solution over a wide pH range. The water molecule bound to the CuII center can be substituted by inorganic anions, forming [CuII(3a)2(anion)]5+ species, and the binding constants of a series of mononegative anions have been determined, demonstrating the effectiveness of [CuII(3a)2(H2O)]6+ as an anion receptor. Voltammetric experiments have shown that the two NiII centers in [CuII(3a)2(H2O)]6+ can be reversibly oxidized to NiIII, a d7 cation which needs further coordination in its axial positions. In the case of the [CuII(3a)2(anion)]5+ species, differential-pulse voltammetry experiments demonstrated that on oxidation of the peripheral NiII centers, the anion bound to the central CuII cation jumps onto one of the peripheral NiIII centers. Thus, the translocation of an anion from the center to the periphery (and vice versa) can be electrochemically switched for the multicomponent coordination compound considered here.

Ion exchange and adsorption of some heavy metals in a modified coconut coir cation exchanger

Potentiometric titration experiments were carried out to determine the relative ability of sodium and different heavy metal cations in replacing protons from ionogenic sites of a modified coconut coir cation exchanger. The relative preference for the cations follow the sequence: Na ≪ Ca(II) < Mn(II) < Ni(II) < Cu (II) < Pb(II). This preferential series was confirmed by determining the concentrations of the divalent cations and sodium which were adsorbed by the material during the titration process. Adsorption isotherms at pH 4 and pH 6 were obtained to determine the Langmuir adsorption constants for the heavy metal cations. The adsorbent material adsorbed high amounts of Pb(II), Cu(II) and Ni(II) (in decreasing order) from solutions whose concentrations were comparable to those of wastewaters. Our results show that the modified coconut coir cation exchanger has good potential as an adsorbent of heavy metals in industrial wastewaters.