Hydrodynamic voltammetry at the liquid|liquid interface: facilitated ion transfer and the rotating diffusion cell

A new approach to the voltammetric investigation of facilitated ion transfer processes is reported. The technique uses a rotating diffusion cell approach to induce laminar flow in the organic phase of a liquid|liquid electrochemical cell. The interface between two immiscible electrolyte solutions (ITIES) was stabilised against rotation with either γ-alumina or a track-etched polyester membrane. The resultant voltammetry is shown to be consistent with the Koutecký–Levich equation enabling kinetic parameters associated with facilitated transfer of sodium by dibenzo-18-crown-6 across the water|1,2-dichloroethane interface to be evaluated. In particular, the use of the more hydrophilic alumina membrane permits the uncertainties regarding the use of the membrane-stabilised ITIES, namely the interfacial position, to be eliminated.     

Determination of Minocycline by Semi-Differential Cyclic Voltammetry at a Liquid/Liquid Interface

Abstract

The voltammetric behaviour of minocycline at a water/nitrobenzene interface has been studied by normal, semi-differential and semi-integral cyclic voltammetry. The semi-differential cyclic voltammetry can be used as a rapid, simple method for determination of the antibiotic minocycline in the range of 5–200 ug/ml and the relative standard deviation is within 3%. Drug samples were analyzed and satisfactory results were found as compared with US Pharmacopoeia method.     

Study of the effect of the membrane composition on ion transfer across a supported liquid membrane

The rate of ion transfer across the supported liquid membrane (SLM) is studied in the rotating diffusion cell (RDC), varying the chemical composition of the SLM from net-cloth supported gel membranes to radiation-grafted polymer membranes. Steady-state current–voltage curves are measured as a function of the rotation rate, and values for the standard rate constant, k⁰, are determined for a series of tetraalkylammonium cations from the analysis of the initial slopes and the diffusion limiting currents. The analysis gives values for k⁰ of the order of 10⁻²–10⁻⁴ cm s⁻¹, which is in rather good agreement with the values found in the literature for this type of the system. As controlled delivery of ionic drugs can be achieved by control of the electric current, whereby the SLM acts as a drug reservoir, the study is extended to the release of the anti-Alzheimer drug Tacrine, where ion-exchange fibers are embedded in the membrane as the drug carrier. Our previous transient experiments are also discussed, and it is suggested that their interpretation is seriously hampered by the non-uniform potential distribution, which brings about high capacitive currents.     

Ion chromatography with the indirect ultraviolet detection of alkali metal ions and ammonium using imidazolium ionic liquid as ultraviolet absorption reagent and eluent

Indirect ultraviolet detection was conducted in ultraviolet‐absorption‐agent‐added mobile phase to complete the detection of the absence of ultraviolet absorption functional group in analytes. Compared with precolumn derivatization or postcolumn derivatization, this method can be widely used, has the advantages of simple operation and good linear relationship. Chromatographic separation of Li⁺, Na⁺, K⁺, and NH₄⁺ was performed on a carboxylic acid base cation exchange column using imidazolium ionic liquid/acid/organic solvent as the mobile phase, in which imidazolium ionic liquids acted as ultraviolet absorption reagent and eluting agent. The retention behaviors of four kinds of cations are discussed, and the mechanism of separation and detection are described. The main factors influencing the separation and detection were the background ultraviolet absorption reagent and the concentration of hydrogen ion in the ion chromatography‐indirect ultraviolet detection. The successful separation and detection of Li⁺, Na⁺, K⁺, and NH₄⁺ within 13 min was achieved using the selected chromatographic conditions, and the detection limits (S/N = 3) were 0.02, 0.11, 0.30, and 0.06 mg/L, respectively. A new separation and analysis method of alkali metal ions and ammonium by ion chromatography with indirect ultraviolet detection method was developed, and the application range of ionic liquid was expanded.     

A lattice-gas model for specific ion adsorption at liquid | liquid interfaces

A lattice-gas model is used to investigate the specific adsorption of ions at the interface between two immiscible electrolyte solutions. From Monte Carlo simulations, the profiles of particle densities and of the electrostatic potential are obtained. Specific adsorption is shown to affect the potential distribution markedly. In some cases an overshoot of the potential can be observed, an effect that is well known from specific adsorption at metal electrodes. This redistribution of charge and potential can increase the interfacial capacity, shift the potential of zero charge, and influence the rate of electron-transfer reactions.     

Cyclic Voltammetry of Ion Transfer for Phenylpropanolamine Hydrochloride at Water|Nitrobenzene Interface

The transfer on phenylpropanolamine ion, PPAH⁺, has been studied at the Interface between Two Immiscible Solutions (ITIES). The polarizable potential range was determined by cyclic voltammetry at the interface between an aqueous solution of lithium chloride (LiCl) and a nitrobenzene (NB) solution of electrolyte tetrabutylammonium tetraphenylborate (TBATPB). The half‐wave potential of ion transfer for phenylpropanolamine accross the water|NB interface was found 465.3 mV. The peak separation, the diffusion coefficient, and the standard ion transfer potential of PPAH⁺ were observed to be 59.1 mV, 1.7 × 10^?6 cm²/s, and 104.6 mV, respectively. The temperature of experiment was kept constantly at 25 ± 1 °C using water flow thermostate.     

Simultaneous extraction and determination of trace amounts of diclofenac from whole blood using supported liquid membrane microextraction and fast Fourier transform voltammetry

A novel, simple, and inexpensive analytical technique based on flat sheet supported liquid membrane microextraction coupled with fast Fourier transform stripping cyclic voltammetry on a reduced graphene oxide carbon paste electrode was used for the extraction and online determination of diclofenac in whole blood. First, diclofenac was extracted from blood samples using a polytetrafluoroethylene membrane impregnated with 1‐octanol and then into an acceptor solution, subsequently it was oxidized on a carbon paste electrode modified with reduced graphene oxide nanosheets. The optimal values of the key parameters influencing the method were as follows: scan rate, 6 V/s; stripping potential, 200 mV; stripping time, 5 s; pH of the sample solution, 5; pH of the acceptor solution,7; and extraction time, 240 min. The calibration curves were plotted for the whole blood samples and the method was found to have a good linearity within the range of 1–25 μg/mL with a determination coefficient of 0.99. The limits of detection and quantification were 0.1 and 1.0 μg/mL, respectively. Using this coupled method, the extraction and determination were merged into one step. Accordingly, the speed of detection for sensitive determination of diclofenac in complex samples, such as blood, increased considerably.     

Measurement of nitrophenols in air samples by impinger sampling and supported liquid membrane micro-extraction

A sensitive analytical technique for the detection of trace nitrophenols in air has been developed. The steps in this process are impinger sampling to capture the nitrophenols in an aqueous phase, which is then followed by supported liquid membrane micro-extraction (SLMME) and analytical detection. The nitrophenols were analyzed by reverse-phase high performance liquid chromatography (HPLC) and did not require any derivatization. Method detection limits (MDL) of 0.5-1.0 ng L⁻¹ from aqueous solutions and 3.1-46.7 ppbV from air extractions were observed. The high enrichment of nitrophenol in SLMME allowed low detection limits even with HPLC-UV detection. SLMME is an inexpensive, easy to use procedure that employs disposable membrane fibers.     

甲基异丁酮碘萃合物-水界面上的循环伏安法间接测定水溶液中氧化性金属离子

利用甲基异丁酮(MIBK)作为萃取剂,等体积法将水中的碘萃取到有机相中,微量进样器将20～40μL有机溶液注入玻璃微管中,使它处于界面上。利用经典的三电极系统研究该有机相在液/液界面的伏安特性。实验结果表明,该过程是一个受扩散控制的不可逆过程。有机相中碘的含量和峰电流大小在一定范围内成正比。利用这一特性可以间接测定水溶液中具有氧化性的金属离子(如Cr2O72-、Cu2 等)的含量,检出限为10-5mol/L。     

Cyclic voltammetry of ion transfer across a room temperature ionic liquid membrane supported by a microporous filter

Cyclic voltammetry is used to study the transfer of a series of cations and anions across a room-temperature ionic liquid (RTIL) membrane composed of tridodecylmethylammonium cation (TDMA⁺) and tetrakis(pentafluorophenyl)borate anion (TPFPB⁻), and supported by a thin (∼112 μm) microporous filter. Essential advantage of the thin membrane system is the substantial reduction of the ohmic potential drop, which is compensated in voltammetric measurements. Reversible partition of TPFPB⁻ allows fixing the potential difference at one membrane interface and polarizing the other membrane interface in a defined way. It is shown that the polarized potential window for the interface between an aqueous electrolyte solution and RTIL attains the value of ca. 0.7 V at the ambient temperature of 25 ± 2 °C. Tetraphenylarsonium tetraphenylborate hypothesis is used for the first time to estimate the standard Gibbs energies of ion transfer from water to RTIL and to establish the scale of the absolute potential differences. A linear Gibbs energy relationship is found for the ion transfer from water to RTIL and o-dichlorobenzene.     

Cyclic voltammetry for a preliminary study of appropriate electro-synthesis reactions of naphthalene derivatives

Cyclic voltammetry on a glassy carbon electrode was used for studies of electro-oxidations of several naphthalene derivatives. The number of electrons participating in the primary reaction at the electrode is derived using the modified Randles-Ševčik equation applied to a series of experimental data of peak current intensity vs. the potential rate. The mechanism of electro-oxidation of two natural products, juglone and lawsone, is discussed with respect to hydrogen bond formation. Correspondence: Roman E. Sioda, Department of Analytical Chemistry and Electrochemistry, Institute of Chemistry, University of Podlasie, ul. 3 Maja 54, 08-110 Siedlce, Poland. An erratum to this article can be found online at http://dx.doi.org/.     

Facilitated ion transfer of protonated primary organic amines studied by square wave voltammetry and chronoamperometry

The transfer of the protonated forms of heptylamine, octylamine, decylamine, procaine and procainamide facilitated by dibenzo-18-crown-6 from water to a solvent polymeric membrane has been investigated by using cyclic square wave voltammetry. The experimental voltammograms obtained are in good agreement with theoretical predictions. The values of the standard ion transfer potential, complexation constant and diffusion coefficient in water have been obtained from these experiments, and have been used to draw some conclusions about the lipophilicity of these species and the relative stability of the organic ammonium complexes with dibenzo-18-crown-6. The results have been compared with those provided by linear sweep voltammetry. Calibration graphs were obtained with both techniques. An interesting chronoamperometric method for the determination of the diffusion coefficient of the target ion in the membrane has been developed and applied to all these protonated amines.     

Flux enhancement of stagnant sandwich compared to supported liquid membrane systems in the removal of Gemfibrozil from waters

Removal of the drug Gemfibrozil (GEM), as a target molecule, from aqueous media by using a carrier mediated transport in supported liquid membrane (SLM) and Stagnant Sandwich LM (SSwLM) systems has been investigated. Optimal chemical conditions to use in the transport tests were determined by means of solubility and liquid–liquid extraction tests. The results showed that the best LM phase to realize stable LM systems was tributylphosphate (TBP) 30% (v/v) in n-decane. Transport tests by using the “traditional” SLM system showed an average flux J_AV(0–CTT) of 0.421 mmol h⁻¹ m⁻² and a system stability of 1410 min. Three different microfiltration membranes, GH-Polypro, FP-Vericel and Supor 200, made of polypropylene, polyvinylidene fluoride and polyethersulphone polymers, respectively, were used to assemble the SSwLM. Contact angle and adsorption measurements evidenced hydrophilic/lypophilic character of the supports. The best results in terms of average flux (0.873 mmol h⁻¹ m⁻²), permeability coefficient (21.88 L h⁻¹ m⁻²) and stability (7170 min ≈120 h) were obtained by using the SSwLM made with the Supor 200 support. The overall results showed that the SSwLM made with this type of support achieves both high flux and high stability compared to the SLM. Thus SSwLMs seems very interesting to employ transport in LM for removing molecular species (e.g. drugs) from aqueous solutions.     

Ion insertion into ionic liquid supported toluene generated by electrochemical redox reaction

Ion transfer across the toluene|water, toluene–ionic liquid mixture|water and ionic liquid|water boundary generated by electrochemical redox reaction of tert-butylferrocene (tBuFc) was studied with the glassy carbon (GC) electrode partially covered by the organic liquid deposit and immersed in the aqueous electrolyte solution. The electrooxidation of the redox probe in toluene deposit is followed by ejection of newly formed cation into the aqueous solution. The same reaction in the toluene–ionic liquid deposit promotes anion insertion. This pathway is also found at the electrode modified with ionic liquid.     

Impact of an ionic surfactant on the ion transfer behaviors at meso-liquid/liquid interface arrays

The influence of ionic surfactants,cetyltrimethylammonium bromide(CTAB),self-assembled within silica-nanochannels of a hybrid mesoporous silica membrane(HMSM) on simple ion transfer(IT)behaviors at the meso-water/1,2-dichloroethane(W/DCE) interface arrays supported by such a HMSM was investigated by voltammetry for the first time.Significantly,it is found that the CTAB in HMSM can dramatically enhance the peak-current responses corresponding to ITs of some anions and even lower their Gibbs transfer energies from W to DCE,which could be ascribed to an anion-exchange process between anions and the bromide of CTAB associated with partial ion-dehydration induced by the CTAB.This work will provide a new strategy to study anion transfer processes and improve the electroanalytical performance for anion detection at the liquid/liquid interface.     

Hollow fiber supported ionic liquid membrane microextraction for determination of sulfonamides in environmental water samples by high-performance liquid chromatography

By using ionic liquid as membrane liquid and tri-n-octylphosphine oxide (TOPO) as additive, hollow fiber supported liquid phase microextraction (HF-LPME) was developed for the determination of five sulfonamides in environmental water samples by high-performance liquid chromatography with ultraviolet detection The extraction solvent and the parameters affecting the extraction enrichment factor such as the type and amount of carrier, pH and volume ratio of donor phase and acceptor phase, extraction time, salt-out effect and matrix effect were optimized. Under the optimal extraction conditions (organic liquid membrane phase: [C₈MIM][PF₆] with 14% TOPO (w/v); donor phase: 4 mL, pH 4.5 KH₂PO₄ with 2 M Na₂SO₄; acceptor phase: 25 μL, pH 13 NaOH; extraction time: 8 h), low detection limits (0.1–0.4 μg/L, RSD ≤ 5%) and good linear range (1–2000 ng/mL, R² ≥ 0.999) were obtained for all the analytes. The presence of humic acid (0–25 mg/L dissolved organic carbon) and bovine serum albumin (0–100 μg/mL) had no significant effect on the extraction efficiency. Good spike recoveries over the range of 82.2–103.2% were obtained when applying the proposed method on five real environmental water samples. These results indicated that this present method was very sensitive and reliable with good repeatabilities and excellent clean-up in water samples. The proposed method confirmed hollow fiber supported ionic liquid membrane based LPME to be robust to monitoring trace levels of sulfadiazine, sulfamerazine, sulfamethazine, sulfadimethoxine and sulfamethoxazole in aqueous samples.     

An insight to the filtration mechanism of Pb(II) at the surface of a clay ceramic membrane through its preconcentration at the surface of a graphite/clay composite working electrode

The use of cyclic voltammetry (CV) and linear scan anodic stripping voltammetry (LSASV) to predict the selectivity of microfiltration ceramic membranes made from a lump of local clay towards Pb(II) ions filtration is described. The membranes were characterized by different techniques followed by CV analysis of the Fe(CN)₆^3-/Fe(CN)₆^4- redox couple and Pb(II) on bare graphite, raw clay, and clay-modified carbon paste electrode (clay-modified CPE). The effect of clay loading in the range of 1–10 % (w/w) on the electrodes is studied, where an enhanced peak current is observed for 5 % w/w clay. Moreover, a decrease in the peak current can be seen for bare graphite electrodes, suggesting that the clay mineral had played a substantial role in the sieving of heavy metal ions through the ceramic membrane. The electroactive surface area of 5% w/w raw clay towards Fe(II) ions was found to be in the order of 3.07 × 10^-2 cm² and higher than 5% w/w clay sintered to 1000 °C and bare graphite. CV analysis shows that both, 5 % w/w raw clay and 5 % w/w clay sintered to 1000 °C exhibited high peak currents towards Pb(II) ions. The mobility of the Pb(II) ions is found to increase when 5% w/w clay sintered to 1000 °C is utilized as membrane/electrode, leading to an increase in the amount of reduced Pb(II) ions on the surfaces of the clay membranes/electrodes. The study suggests successful filtration of Pb(II) ions through the proposed membrane/electrode and a much better accumulation than Fe(II) at the surface of the membrane/electrode before being subjected to filtration.     

Theory and application of cyclic voltammetry at a hemispherical microelectrode for a quasi-reversible reaction

The theory of cyclic voltammetry for a quasi-reversible reaction at a hemispherical microelectrode has been deduced. From the results calculated according to the theory, the heterogeneous rate constant and charge transfer coefficient can be measured with the cyclic voltammetric data obtained at an intermediate scan rate on a hemispherical microelectrode. To verify the theory, the kinetic constant for reduction of benzoquinone in acetonitrile has been determined. The result is coincident with that measured by a fast scan rate at a platinum microdisk electrode. The kinetic constant and charge transfer coefficient for reduction of 2,3,5,6-tetrachlorobenzoquinone have also been determined with the method described in this paper.     

Direct electrocatalytic reduction of p-nitrophenol at room temperature ionic liquid modified electrode

Direct electrochemical reduction ofp-nitrophenol （PNP） was investigated on a room temperature ionic liquid N-butylpyridinium hexafluorophosphate （BPPF6） modified carbon paste electrode （CILE）. The cathodic peak potential was positively shifted and the peak currents were increased compared to that obtained on traditional carbon paste electrode （CPE）. The results indicated that the presence of ionic liquid BPPF6 on the electrode surface showed excellent catalytic ability to the electrochemical reduction of PNP. The electrochemical behaviors of PNP on the CILE were investigated by cyclic voltammetry and the conditions such as the scan rate, the buffer pH, the substrate concentration were optimized. The electrochemical parameters were further calculated with the results of the electron transfer number （n）, the charge-transfer coefficient （α） and the surface concentration （Гr） as 1.76, 0.37 and 2.47 × 10^-9 mol/cm^2, respectively, for the selected reductive peak. The results indicated that PNP showed an irreversible adsorption-controlled electrode process on the CILE.     

双通道微米管支撑的微-液/液界面上电荷转移反应的研究

将有机相和水相分别灌入双通道玻璃微米管θ管中的一个管中,利用θ管表面的亲水特征,在灌有有机相的微米管口附近形成微-液/液界面.利用循环伏安法研究了电荷在这种微-液/液界面上的转移反应,包括简单离子(四甲基铵离子TMA⁺)转移、加速离子转移(DB18C6加速K⁺离子)和电子转移(二茂铁/铁氰化钾+亚铁氰化钾体系)反应过程.结果表明,这种双通道微米管所得到的微-液/液界面具有不对称扩散场的特性.此装置是目前最简单的可用于研究液/液界面上的电荷转移反应的装置之一,即所谓的可进行"无溶液"液/液界面电化学及电分析化学研究的装置.