Potentiometric responses of polymeric liquid membranes based on hydrophobic chelating agents to metal ions.

The effect of hydrophobicity of acidic chelating agents as sensing materials on the potentiometric responses of polymeric liquid membranes was investigated. The chelating agents tested were 8-quinolinol (HOx), dithizone (HDz), 1-(2-pyridylazo)-2-naphthol (PAN) and their alkylated analogues, 5-octyloxymethyl-8-quinolinol (HO8Q), di(phexylphenyl)thiocarbazone (C6HDz), 7-pentadecyloxy-1-(2-pyridylazo)-2-naphthol (C15PAN) and a series of N-alkylcarbonyl-N-phenylhydroxylamines (CnPHA, n = 3, 6, 9, 12). The distribution coefficients between membrane solvent and water were determined to evaluate the hydrophobicity of the agents. The potential-pH profiles of the membranes containing hydrophobic chelating agents demonstrated the generation of potentiometric responses, while less hydrophobic agents gave no response. A possible model for the generation of membrane potential is proposed. The charge separation is attained by the permselective uptake of metal cations by the chelating agent anion at membrane/solution interface, where the high hydrophobicity of the agent enables the anionic or deprotonated form of the agents to remain at the membrane/solution interface.     

Ribonucleic acid as a novel ionophore for potentiometric membrane sensors of some transition metal ions

Ribonucleic acid (RNA) is used as a novel ionophore in plasticized poly(vinyl chloride) matrix membrane sensors for some transition metal ions. Membranes incorporating RNA and doped in Cu(2+), Cd(2+) and Fe(2+) display fast near-Nernstian and stable responses for these ions with cationic slopes of 31.1, 31.3 and 35.5 mV per decade, respectively, over the concentration range 10(-6)-10(-2) M and pH range 4-6.5. The cadmium RNA-based sensor shows no interference by Cu(2+), Fe(2+) Hg(2+) and Ag(+), which are known to interfere significantly with the solid-state CdS/Ag(2)S membrane electrode. The copper RNA-based sensor displays general potentiometric characteristics similar to those based on macrocyclic ionophores and organic ion exchangers and has the advantage of a better selectivity for Cu(2+) over some alkaline earth, divalent and transition metal ions. The iron RNA-based membrane sensor exhibits no interference by Hg(2+) and Zn(2+), which are known to interfere with other previously suggested sensors. The nature and composition of the RNA ionophore and its cadmium complex are examined using electrophoresis, Fourier-transform infrared analysis, elemental analysis and X-ray fluorescence techniques.     

Role of axial ligation on potentiometric response of Co(III) tetraphenylporphyrin-doped polymeric membranes to nitrite ions

Two types of Co(III) tetraphenylporphyrins, Co(III)TPPX (I) and Co(III)(N)TPPX (II), where X = C1⁻ or NO⁻₂ and N = C₅H₅N or C₆H₅CH₂C₅H₄N, are used as ionophores to prepare nitrite responsive polymeric membrane electrodes. The influence of the initial axial ligand (X⁻ and N) on the operative ionophore mechanism of these metalloporphyrins within the solvent polymeric membranes is examined. Results from potentiometric and electrodialysis experiments suggest that in the presence of nitrite in the test sample and internal solution, both types of Co (III) porphyrins studied (I and II) act as neutral carriers and that the addition of lipophilic cationic sites (e.g., tridodecylmethylammonium ions (TDMA⁺)) to the organic membrane is essential to improve the selectivity and long term stability of sensors prepared with these species. Membranes formulated with (I) or (II) in the nitrite form along with TDMACl in plasticized PVC films exhibit the following selectivity sequence: SCN⁻ > NO⁻₂ ˜ C1O⁻₄ > Sal⁻ > NO⁻₃ > Br⁻ > C1⁻. Membrane electrodes with added lipophilic cationic sites are shown to exhibit rapid, fully reversible and Nernstian response towards nitrite ions in the concentration range of 10⁻¹–10⁻⁵ M, with good long term stability.     

Multistage separation of metal ions with a series of composite supported liquid membranes

The possibility of performing multistage separations of metal cations, present at low concentrations in aqueous solutions, using a series of composite supported liquid membranes (SLM), interposed between compartments containing identical aqueous electrolyte solutions, is discussed. The multistage separation of Eu³⁺. from Am³⁺ and of Sm³⁺ from Nd³⁺ and Ce³⁺ is theoretically analyzed. A single separation stage, using a thin, three-layer asymmetrical composite SLM, which allows differential permeation of Eu³⁺. and Am³⁺, is described and experimentally studied. The composite SLM consists of a 1 mm thick aqueous slab sandwiched between acidic and neutral supported liquid membranes. The results demonstrate that the permeation of each cation through the thin composite SLM can be described by a single permeation parameter which is equal to the permeability coefficient through the first acidic supported liquid membrane of the composite layer.     

Application of emulsion liquid membranes to recover cobalt ions from a dual-component sulphate solution containing nickel ions

Emulsion liquid membranes (ELMs) containing 2-ethylhexyl phosphonic acid mono-2-ethylhexyl ester (PC-88A) have been applied to recover cobalt II ions from a dilute sulphate solution containing equal amounts of nickel II ions (0.16 g/l). We focused on the study to develop an effective technique to recover cobalt as a target metal. It is found that polyamine (PX 100) membranes allow better permeation rates of cobalt ions than sorbitan monooleate (Span 80) membranes. The separation factor (β_Co/Ni) in polyamine membranes averaged 70 at a carrier concentration of 12 mol/m³ and feed solution pH 5.5. The permeation rate of Co II was found to increase proportionately with feed pH while for Ni II it decreased substantially at pH above 5.5 indicative of slower interfacial reaction rate. We found that short contact time (4–6 min) of feed solution and emulsion improved separation factor (β_Co/Ni) at feed pH above 5.5 and also minimized chances of emulsion break up. We have also observed that Span 80 membranes are hydrolyzed readily in a moderate acidic sulphate solution (pH 4.0–5.5) to form viscous gels. Results have shown that excess carrier [(HR)₂] affects the stability of emulsion and thus the separation factor. The critical ratio of carrier to emulsifier [(HR)₂]/[C_sf] was found to be approximately 0.5. This paper concludes with a discussion on the prospects of ELM system in practical use.     

Used gold nano-particles as an on/off switch for response of a potentiometric sensor to Al(III) or Cu(II) metal ions

The potentiometric response of a carbon paste electrode modified with silica sol-gel and mercaptosuccinic acid (MSA) in the presence and absence of gold nano-particles was studied. The results showed that the electrode with gold nano-particles was responded to Al(3+) ions as a hard metal ion. On the other hand, the electrode without gold nano-particles was responded to copper ions as a soft metal ion. The electrodes without and with gold nano-particles exhibits a Nernstian slope of 29.1 and 19.2 mV decade(-1) for copper and aluminum ions over a wide concentration range of 4.3×10(-7)-1.0×10(-2) and 4.5×10(-7)-1.6×10(-3) mol L(-1), respectively. The detection limits of electrodes were 4.0×10(-7) and 1.6×10(-7) mol L(-1) for copper and aluminum ions, respectively.     

Uptake of transition metal ions using liposomes containing dicetylphosphate as a ligand.

The uptake of Cu2+ was investigated using various types of liposomes composed of phosphatidylcholine (PC), cholesterol (Chol) and dicethylphosphate (DCP). DCP played a role as a ligand for Cu2+. Multilamellar vesicles (MLVs) were more effective for the uptake of Cu2+ compared to unilamellar vesicles prepared by the extrusion technique. The uptake efficiency of MLVs for Cu2+ was dependent on the molar ratio of DCP in MLVs. The uptake percent of Cu2+ was 92% using MLVs having a PC:DCP:Chol molar ratio of 4:3:3; 95% of the total vesicle Cu2+ was bound to DCP of the outer membrane surface of the MLVs, and the remaining 5% of the total Cu2+ was distributed into the interior side of the MLVs. MLVs having a PC:DCP:Chol molar ratio of 4:3:3 were also effective as separation media for Mn2+, Co2+, Ni2+ and Zn2+. The uptake efficiency of the MLVs for the transition-metal ions increased in the order Co2+ < Zn2+ < Ni2+ < Mn2+ < Cu2+.     

Cruciforms as functional fluorophores: response to protons and selected metal ions

The photophysics of dialkylamino- and/or pyridine-containing functional chromophores, 1,4-distyryl-2,5-bis(ethynylaryl)benzenes (cruciforms) was investigated; their fluorescence quantum yields and emissive lifetimes were determined. Depending upon their substituents, the frontier molecular orbitals (FMOs) of these cruciforms are either congruent, i.e., HOMO and LUMO occupy the same real space, or disjoint, i.e., the HOMO is located on one branch of the cruciform while the LUMO is located on the second one. Donor-acceptor substitution leads to a disjoint FMO pattern, while the parent 1,4-distyryl-2,5-bis(phenylethynyl)benzene shows congruent FMOs. The photophysics of the cruciforms was investigated upon addition of either an excess of trifluoroacetic acid or an excess of selected metal (Mg(2+), Ca(2+), Mn(2+), Zn(2+)) trifluoromethanesulfonate salts. Addition of either metal ions or protons led to analogous but not identical changes in the spectroscopic properties of the investigated cruciforms. The collected data suggest that the metals bind preferentially at the aniline nitrogen and not at the electron-rich arene. The spatially separated FMOs permit the independent manipulation of the HOMO and the LUMO of such cruciforms. If the branches contain metal-complexing moieties, metal binding leads to either a hypsochromic or a bathochromic shift in emission via interaction of the metal cations with either the HOMO or the LUMO.     

High preconcentration of ultra-trace metal ions by liquid–liquid extraction using water/oil/water emulsions as liquid surfactant membranes

A high preconcentration method by liquid–liquid extraction using liquid surfactant membranes was developed. The water-in-oil (w/o) emulsion containing dilute hydrochloric acid, 2-ethylhexyl hydrogen 2-ethylhexylphosphonate (PC-88A), liquid paraffin, and kerosene was used for the extraction. In a resulting volume of 1000 cm³ of an aqueous sample solution (pH 5.0) containing less than 1 mg of each metal ion, 2 cm³ of w/o emulsion droplets coated with sorbitan monooleate were dispersed. The analyte metal ions in the outer bulk aqueous phase were extracted into the organic phase to form a complex with PC-88A and successively back-extracted into the inner aqueous phase. The analytes in the resulting inner aqueous phase were determined subsequently by graphite furnace atomic absorption spectrometry applied as a detector. By this procedure, concentration factors of 570, 820, 750, 970, 860, and 880 were achieved for chromium(III), manganese(II), cobalt(II), nickel(II), copper(II), and cadmium(II), respectively, and also the respective detection limits (3σ) of 0.4, 20, 1.2, 18, 18, and 0.7 pg cm⁻³ were obtained.     

Aquacyanocobalt(III)-cobyrinate as a key compound in NO2-sensitive polymeric liquid membranes.

The reactions within a recently introduced NO2-sensitive polymeric membrane based on aquacyanocobalt(III)-cobyrinate are described. The detailed reaction mechanism was investigated in three ways: using UV/VIS-spectroscopy, determining the reaction products and investigating the influence of other gases. It could be shown that the membrane's high sensitivity and selectivity derives from the remarkable reaction of NO2 with this cobyrinate derivative, which yields nitric and nitrous acid. This reaction can be transduced with a consecutive protonation of the included chromoionophore into an optical signal, which allows the detection of NO2 in the ppb-range.     

Complexation reaction of metal ions with peptide systems. Part IV. A potentiometric study of rare earth complexes of glycyl-L-proline

Amino acids and dipeptides are of biological importance and therefore their metal complexes are of special interest. Many workers have studied the complexes of various peptides with different transition metals in solution^{1 – 5}. Although the stability of complexes of lanthanum with amino acids has been studied^{6 – 11}, no work appears to have been done with dipeptide systems. In continuation to our previous work^{12, 13} on the complexation reactions of lanthanides with dipeptide systems, the present communication reports the stability constants and thermodynamic functions of Y(III), La(III), Ce(III), Pr(III) and Nd(III) with glycyl-L-proline in aqueous medium at 25, 35 and 45°C at 0.15 M (KNO₃) ionic strength. The Calvin—Bjerrum titration technique^{14, 15} as modified by Irving and Rossotti¹⁶ was used.     

Indirect ultraviolet detection of alkaline earth metal ions using an imidazolium ionic liquid as an ultraviolet absorption reagent in ion chromatography

A convenient and versatile method was developed for the separation and detection of alkaline earth metal ions by ion chromatography with indirect UV detection. The chromatographic separation of Mg²⁺, Ca²⁺, and Sr²⁺ was performed on a carboxylic acid base cation exchange column using imidazolium ionic liquid/acid as the mobile phase, in which the imidazolium ionic liquid acted as an UV‐absorption reagent. The effects of imidazolium ionic liquids, detection wavelength, acids in the mobile phase, and column temperature on the retention of Mg²⁺, Ca²⁺, and Sr²⁺ were investigated. The main factors influencing the separation and detection were the background UV absorption reagent and the concentration of hydrogen ion in ion chromatography with indirect UV detection. The successful separation and detection of Mg²⁺, Ca²⁺, and Sr²⁺ within 14 min were achieved using the selected chromatographic conditions, and the detection limits (S /N = 3) were 0.06, 0.12, and 0.23 mg/L, respectively. A new separation and detection method of alkaline earth metal ions by ion chromatography with indirect UV detection was developed, and the application range of ionic liquids was expanded.     

Separation and concentration of some platinum metal ions with a new chelating resin containing thiosemicarbazide as functional group

A new chelating ion-exchange resin containing thiosemicarbazide as functional group and based on macroreticular polystyrene-divinylbenzene (8%) has been prepared. Its sorption characteristics for palladium(II), platinum(IV), rhodium(III), ruthenium(III) and iridium(III) have been studied. These platinum metal ions can be quantitatively separated by sorption on this chelating resin and selective elution. The resin is highly stable in acid and alkaline solution.     

Transport of iron ions from chloride solutions using cellulose triacetate matrix inclusion membranes with an ionic liquid carrier

In this study, liquid membranes denoted as polymer inclusion membranes (PIMs) consisting of cellulose triacetate (CTA) as a polymer matrix, o-nitrophenyl octyl ether (NPOE) as a plasticiser and phosphonium ionic liquids, trihexyltetradecylphosphonium chloride (Cyphos® IL 101) and trihexyltetradecylphosphonium bis(2,4,4-trimethylpentyl)phosphinate (Cyphos® IL 104), as carriers of metal ions were developed. The transport of Fe(II) and Fe(III) from chloride aqueous solutions across PIMs was investigated. It is shown that these phosphonium ionic liquids are effective carriers of Fe(III) ions through PIMs. While, for Fe(II), the highest value of extraction efficiency and recovery factor after 72 h does not exceed 40%, by contrast, the values of these parameters for Fe(III) transport ranged from 60% to almost 100%. Additionally, the results indicate the transport rate to be strongly influenced by the amount of carrier in the membrane. The highest initial flux of Fe(III) and permeability coefficient are noted for the membrane containing 40 mass % Cyphos® IL 101. However, it is shown that the transport of Fe(III) increases as the carrier content is increased then decreases at a content of the carrier equal to 40 mass %. It appears that the Fe(III)-carrier complex decomposes with difficulty at the interface of the membrane-receiving phase, hence leading to low values of recovery factor Fe(III).     

Studies on extraction of chromium (VI) from acidic solutions containing various metal ions by emulsion liquid membrane using Alamine 336 as extractant

The extraction of chromium (VI) ions from acidic solutions containing various metal ions by emulsion liquid membrane (ELM) was studied. Liquid membrane consists of a diluent, a surfactant, and an extractant. 0.5 M ammonium carbonate solution was used as stripping solution. Effects of acid concentration in feed solution, type and concentration of stripping solution, mixing speed, surfactant concentration, phase ratio and the influence of membrane characteristics were studied and optimum conditions were determined. Under the optimum conditions, extraction of chromium (VI) was tested and it was possible to selectively extract 99% of chromium from the acidic feed solution. This study also examined the effect of extractant concentration and acid type in the feed solution on the extraction of Cr (VI) ions and almost all of Cr (VI) from the acidic feed solution containing 500 mg/L from each of Co (II), Ni (II), Cd (II), Zn (II), and Cu (II) ions, and 100–500 mg/L Cr (VI) was extracted within 5–10 min.     

Laser induced breakdown spectroscopy (LIBS) as an analytical tool for the detection of metal ions in aqueous solutions

Laser induced breakdown spectroscopy (LIBS) is applied to analyze aqueous solutions of Li⁺, Na⁺, Ca²⁺, Ba²⁺, Pb²⁺, Cd²⁺, Hg²⁺ and Er³⁺ and suspensions of ErBa₂Cu₃O_x particles (d=0.2 m). An excimer (308 nm) pumped dye laser with laser pulse at 500 nm and pulse energy at 22±2 mJ is used to produce plasma in aqueous solution. Plasma emission lines of the elements are detected by a photodiode array detector. Detection limits of the metal ions are 500 mg/l for Cd²⁺, 12.5 mg/l for Pb²⁺, 6.8 mg/l for Ba²⁺, 0.13 mg/l for Ca²⁺, 13 g/l for Li⁺ and 7.5 g/l for Na⁺. No mercury and erbium emission can be detected, even at Hg²⁺ and Er³⁺ concentrations of up to the g/l range. On the other side, for Er in suspensions of ErBa₂Cu₃O_x particles a more than 10³ times higher sensitivity is found than for dissolved Er³⁺. This result gives a possibility to analyze colloid-borne metal ions with an increased sensitivity.     

New class of coordination compounds with noble gas fluorides as ligands to metal ions

A new class of coordination compounds of the type [Mⁿ⁺(L)_p](AF₆⁻)_n and [Mⁿ⁺(L)_r](BF₄⁻)_n, where M is Mg, Ca, Sr, Ba, Cd, Pb, lanthanides, A is P, As, Sb, Bi and L is XeF₂, XeF₄, XeF₆, KrF₂, was studied. A review of all known coordination compounds with L is XeF₂ is given: (a) synthetic routes for the preparation of these compounds; (b) analysis of their crystal structures (molecular, dimer, chain, double chain, layer, strongly interconnected double layers and three-dimensional network); (c) the influence of the ligand XeF₂ (small formula volume, linear, semi-ionic, charge of −0.5e on each F ligand); (d) the influence of the central metal ion; (e) the influence of the anions: AF₆⁻ and BF₄⁻ (the formula volume, Lewis basicity). On the basis of all properties of the metal ions, ligand and anions the obtained variety of the structures is analyzed.     

Functionalized calix[4]arene as an ionophore: Synthesis,crystal structures and complexation study with Na and K ions

Calix[4]arenes with substituents at three of the four OH groups of the lower rim have been synthesized to investigate their properties as ionophores for Na⁺ and K⁺ metal ions. Crystal structures of these trisubstituted compounds revealed that the calixarene moiety has adopted a partial cone conformation, however the precise shape of the molecule, and intra- and intermolecular interactions, are significantly different due to variations of the substituents. Compound L² encapsulated an acetonitrile molecule in the cavity of the calix moiety, held by C–H?π interactions. In the case of L³, the 2-(2-chloroethoxy)-ethanol substituent is involved in strong intramolecular C–H?π interactions with the centroid of the phenyl rings of the calix, bringing the 2-(2-chloroethoxy)-ethanol moiety inward the calix cone, which prevented the entry of any solvent molecule into the cavity. The complexation properties of L²–L⁴ with Na⁺ and K⁺ ions have been investigated in chloroform–methanol mixture by ¹H NMR and an attempt has been made to isolate these complexes in the solid state. Complexation studies reveal that only L³ forms a complex selectively with K⁺, involving 2-(2-chloroethoxy)-ethanol as a coordinating moiety. The association constant (1.4 × 10⁵ M⁻¹) of the complex has also been determined.