Hg Electrode Research Articles

Electrochemical Liquid‐Liquid‐Solid Growth of Ag‐In Crystals with Liquid Indium Alloy Electrodes

AbstractSynthesis of intermetallic crystals by electrodeposition of Ag from alkaline aqueous electrolytes containing AgCN onto liquid metal electrodes via an electrochemical liquid‐liquid‐solid (ec‐LLS) process has been performed. X‐ray diffraction, scanning electron microscopy, and transmission electron microscopy were performed to identify crystalline products. Ec‐LLS experiments performed with pure liquid Hg and Ga electrodes resulted in the formation of polycrystalline Ag2Ga and Ag2Hg3. Experiments performed with In‐containing liquid metals preferentially yielded Ag9In4 and AgIn2 products with liquid Hg0.35In0.65 and Ga0.86In0.14, respectively. The product distribution with liquid Hg0.35In0.65 depended on the level of Ag supersaturation during the electrodeposition. A mechanism that accounts for the aforementioned observations is presented and discussed. This work described the formation of Ag−In intermetallic phases by the isothermal electroreduction of Ag into different liquid metal solvents via ec‐LLS. Electrodeposition of Ag into a pure Ga or pure Hg liquid metal pool yielded precisely the compounds predicted from isothermal cross‐sections of the respective binary phase diagrams. These compounds were not found when using liquid Hg or Ga containing appreciable In. The smaller enthalpy of formation for AgIn2 was consistent with its synthesis in Ga0.86In0.14. However, the observed product of Ag9In4 in Hg‐containing liquid metals could not be rationalized solely from thermodynamic factors. Instead, this observation was consistent with a kinetic pathway based on the lability of Hg‐metal bonds and nearly identical crystal structures of Ag9In4 and Ag2Hg3. Site exchange of Hg for In is consistent with our prior observations[23] of In exchange into Hg−Pd structures during Pd electrodeposition. This mechanism is not based on any direct role of electrochemistry other than aspects that dictate the operative supersaturation of the metal solute.

Characterization of polysulfides (Sx2−) in seawater euxinic conditions by electroanalytical methods

Polysulfides (Sx2−) are important reduced sulfur species (RSS) that play a role in numerous environmental processes. A sound analytical method for the measurement of Sx2− in euxinic waters is lacking. In this work, differential pulse voltammetry (DPV) at the Hg electrode was used to measure the presence of Sx2− in a model seawater solution and an euxinic marine lake (Rogoznica Lake - RL Croatia), in which the concentration of RSS, mainly HS−, varies between 100 and 4000 μM. In the DPV, an adsorption phenomenon associated with Sx2− reduction on Hg produces a characteristic current minimum at −1.0 V (vs. Ag/AgCl), the magnitude of which is proportional to the concentration of polysulfidic sulfur.The DPV current minima were recorded in the model solution K2Sx NaCl/NaHCO3 (pH ∼ 8.2) in a concentration range from 10 to 100 μM of polysulfidic sulfur. Total RSS was measured by cyclic voltammetry, and sampled DC voltammetry showed the ratio between HS− and S0 within the Sx2−. Using the same methodology, the presence of Sx2− below the chemocline enriched by photoptrophic sulfur bacteria was measured in the euxinic layer of RL at concentrations of up to 70 μM of polysulfidic sulfur. The results suggest that euxinic RL samples can be considered as polydisperse solutions of S0 or of S-rich compounds that can change their physicochemical properties and speciation during sample manipulation. These changes can be detected by electrochemistry. Atomic force microscopy proved the release of bacterial cellular S0 during the acidification and purging step in the electrochemical measurements, which contributed to the voltammetric RSS signal.

(Digital Presentation) Experimental Results on the Determination of the Billiter Potential

The experimental effects at the Billiter potential are measured in narrow intervals of the equilibrium potentials (A, B). The Billiter potential + 0.475 V SHE [1-3] is confirmed by the following experimental results [4,5,7- 9]. The change in the direction of the current between a dropping mercury and a mercury bottom in solutions of redox systems Fe(CN)6 3 - - Fe(CN)6 4 - and quinone-hydroquinone at a given potential E equil . = + 475 V SHE has been established with exactness ± 1-2 mV [4, 5]. This result was confirmed in [6]. At E equil. > +0.475 V SHE, the current flows from a mercury bottom to dropping electrode and at E equil. < +0.475 V SHE - in the opposite direction. Electrocapillary curves have been found by drop-weight method in these solutions in narrow range E equil. ~+0.4 ÷+0.55 V SHE with a maximum at E equil. ~ + 0.475 V SHE [4].A self-recording galvanometer was used to record the reversal of the current between a mercury bottom and a dropping electrode in solutions of redox systems [7-9]. In a solution of quinone-hydroquinone redox system, a recorded positive current decreases exactly to zero at E = + 228 mV SCE (+ 475 mV SHE), and then a negative current increases with continuous dropping of mercury. E equil. changes by itself since an adsorbed quinone on mercury drops are removed from solution onto mercury bottom (A).The current reversal at E ~+ 0.22 ÷ + 0.23 V SCE (~+0.475 V SHE) has been recorded by a recorder between a mercury bottom and a dropping electrode in acidified Hg2(NO3)2 solutions with the addition of complexing agents (triethanolamine, sodium ethylenediaminetetraacetate, thiocarbamide, others) [7-9].The electrocapillary curves (ECC) have been measured by weight-drop method in these solutions (B; with sodium ethylenediaminetetraacetate). After passing the electrocapillary maximum for reversible Hg2 2+/ Hg electrode at E ~ + 0.22 ÷ + 0.23 V SCE, we have obtained an ascending branch of ECC of an ideally polarizable electrode at E equil. < ~+0.190 ÷ +0.210 V SCE. A registered second current reversal at these E equil. corresponds to the transition from a small descending branch of ECC of Hg2 2+/Hg electrode to an ascending branch of ECC of an ideally polarizable electrode. The electrocapillary maximum was also obtained using diluted acidified solutions of Hg2(NO3)2.An increase in a polarization resistance and a change in polarization capacitance have been detected when impedance measuring with reversible electrodes at Ee ~ +0.475 V SHE. These effects were also measured on mercury drops in the diluted 10-10 N solution of Hg2(NO3)2 + 0.1÷ 0.001 N KNO3 The increase in polarization resistance was explained by the achievement of chemical equilibrium of half-reactions at Ee quil. ~ +0.475 V SHE [6,7,16].Measurements by the immersion method with redox systems and solid electrodes of the second kind confirm the reversal of the current at E ~+ 475 V SHE. Submersible small solid electrodes must be absolutely clean and dry [5]. This method is under development.According to [8 - 10], Billiter potential corresponds to some electrode with zero chemical activity (transitional from metals to non-metals with the intermediate electron work function 5.19 eV). The same zero activity is obtained at chemical equilibrium of half-reactions when the sum of changes of chemical potentials of electrons and ions Δμion + Δμelectron = 0, and potential drops in double - electric layers are compensated. The components Δμion= 0 and Δμelectron≠ 0 correspond to potentials of zero charge Ep. z.c. For mercury, the electron work function is 4.52 eV and Δμelectron = 4.52 eV - 5.19 eV = - 0.67 eV. In order Δμion + Δμelectron = 0, Δμion should be equal + 0.67 eV. Indeed, if E p.z.c. = - 0.193 V SHE for mercury is changed by + 0.67 V we obtain the chemical equilibrium of half-reactions at ~ +0.475 V SHE. REFERENCES J.Billitzer, Methods for Determination of the Absolute Potentials, Trans. Amer. Electrochem. Soc., 1930,57, 351.The Encyclopedia of Electrochemistry, C.A. Hampel (ed.), (Reinhold,New York, NY) (1964).H.J.Oel, H Strelow,Z. Electrochem, 1954,58,665.A.I.Chernomorskii, Soviet Electrochemistry, 1981, 17, 874.A.I.Chernomorskii, Russian Journal of Physical Chemistry,1981, 55, p.206.Ya.I.Tur’yan, Redox Reactions and Potentials in Analytical Chemistry, Khimiya, Moscow, 1989.A.I. Chernomorskii,N.S.Polizan, Reports of Academy of Sciences of the Uzbekistan,1990, 6, 35.A.I.Chernomorskii, Thernodynamics of Electrodes, FAN, Tashkent, 1993.A.I.Chernomorskii, The Intermediate Electron Bond and Half-reactions, Scientific Resources, New York, 1999.A.I. Chernomorskii, Journal of The Electrochemical Society, 2021, 168,116514. Figure 1

A new Hg (II) electrode for the analysis of mercury in sea food

An Hg(II) solid state electrode (ISE) was fabricated using HgS and the sparingly soluble sulfur salts of Cu and Ag, and the highest sensitivity of the ISE was obtained with 10% HgS (w/w) composition. The ISE shows a slope of about 29 mV towards the Hg (II) ion (pHg 1,0-7,0 mol L-1). Hg-SE has a response time of 20 s and a linear slope between pH 1.0 and 5.0. Its lifetimes are 1 year. It was determined that ISE is not sensitive to other ions. The LOD value of the electrode was measureted as 5x 10-8 M. In addition, after the validation of the electrode, Hg measurements in seafood were carried out.

Electrodeposition of Pd Intermetallics using Hg-In by the Electrochemical Liquid-Liquid-Solid Method

Electrodeposition of Pd from alkaline baths containing Pd(CN)2 and KCN with liquid metal electrodes has been performed. Data are presented that Pd dissolved into and reacted with the liquid metal electrodes via an electrochemical liquid-liquid-solid (ec-LLS) process. HgPd crystals were obtained with liquid Hg electrodes. On solid In electrodes, In7Pd3 was exclusively formed. In contrast, InPd was the primary product with Hg1-xInx alloy electrodes. X-ray diffraction, scanning electron microscopy, and electron backscattering diffraction show that the materials were not a pure phase, as minor components of HgPd and In7Pd3 were observed for various liquid Hg-In compositions. A mechanism is proposed where the InPd intermetallic forms through an intermediate phase of HgPd by the substitution of In atoms for the Hg sites of the unit cell. This study thus motivates further exploration of Hg1-xInx as a versatile medium for intermetallic synthesis by ec-LLS.

Analysis of As(III) By Anodic Stripping Voltammetry Using Disc and Screen Printed Electrodes

Determination of arsenic is possible by various methods such as spectrophotometry, atomic absorption (as arsine), atomic emission, and neutron activation analysis are common. High levels of interference and the high costs of these methods are major drawbacks. Anodic stripping voltammetry (ASV) has proved to be a sensitive, precise, and cost-effective electroanalytical technique for determining trace metals. It is routinely used for detecting Pb (II) and Cd (II). The general procedure of this technique involves depositing metal(s) at a reductive potential into Hg electrodes for a certain time and stripping them back into the solution by scanning the potential in the oxidative direction. In this work, the analysis of As (III) by ASV has been investigated using solid and Screen Printed Gold electrodes. Improved detection limits up to 10 ppb could be achieved with relative ease. A highly optimized procedure has been presented for the BASi Epsilon users that automatically provides electrochemical pretreatment in automated fashion along with the built-in differential pulse stripping voltammetry in the next step.

Scalable Dry-Pressed Electrodes Based on Holey Graphene.

ConspectusHoley graphene (hG) is a structural derivative of graphene with arrays of through-thickness holes of a few to tens of nanometers in diameter, randomly distributed across the nanosheet surfaces. In most bulk preparation methods, the holes on hG sheets are preferentially generated from the pre-existing defects on graphene. Therefore, contrary to intuitive belief, hG is not necessarily more defective than the intact graphene. Instead, it retains essential parent properties, including high electrical conductivity, high surface area, mechanical robustness, and chemical inertness. Furthermore, the added holey structural motif imparts unique properties that are not present in unmodified graphene, making hG advantageous in numerous applications such as sensing, membranes, reinforcements, and electrochemical energy storage. In particular, the presence of holes enhances the mass transport through the nanosheet plane and thus significantly reduces tortuosity. This difference is a key advantage for using hG in energy storage applications where the transport of ions through the thickness becomes more hindered as the electrode thickness increases to meet practical energy density requirements.An unexpected discovery is that the holes of the hG sheets enable the dry hG powder to be directly compressed into robust monoliths. hG not only can be pressed into monoliths by itself but also can host other electrochemically active materials as a compressible matrix. This important yet unique property, which is not available for other carbon materials including intact graphene, significantly broadens the application horizon in energy storage applications. With the dry compressibility, electrodes with ultrahigh mass loading and thus ultrahigh areal capacity may be conveniently fabricated without toxic solvents or parasitic binders, which are required in conventional slurry-based approaches for electrode fabrication. The dry-press electrode preparation process can be completed within minutes regardless of mass loading. In comparison, high-mass-loading electrodes for advanced battery chemistries using conventional fabrication methods often need stringent and time-consuming process control. hG can also be combined with electrochemically active battery materials while maintaining dry compressibility. This has allowed the unprecedented, convenient manipulation of a wide variety of thick electrode compositions and architectures, which provides not only outstanding performance but also new physical insights for various battery chemistries.In this Account, we first present some basic observations on the dry compressibility of hG as well as the mechanistic investigations from atomistic modeling rationalizing this unique property. We then showcase the applications of neat and composite dry-pressed hG electrodes for various energy storage platforms including supercapacitors, lithium (Li) ion batteries, Li-O2 batteries, and Li-S/Se batteries. The preparation and performance of thick electrodes with practical mass loadings and unique electrode architecture manipulation, both enabled by the dry compressibility of hG, are highlighted and discussed.

Role of chemisorbing species in growth at liquid metal-electrolyte interfaces revealed by in situ X-ray scattering

Liquid-liquid interfaces offer intriguing possibilities for nanomaterials growth. Here, fundamental interface-related mechanisms that control the growth behavior in these systems are studied for Pb halide formation at the interface between NaX + PbX2 (X = F, Cl, Br) and liquid Hg electrodes using in situ X-ray scattering and complementary electrochemical and microscopy measurements. These studies reveal a decisive role of the halide species in nucleation and growth of these compounds. In Cl- and Br-containing solution, deposition starts by rapid formation of well-defined ultrathin (∼7 Å) precursor adlayers, which provide a structural template for the subsequent quasi-epitaxial growth of c-axis oriented Pb(OH)X bulk crystals. In contrast, growth in F-containing solution proceeds by slow formation of a more disordered deposit, resulting in random bulk crystal orientations on the Hg surface. These differences can be assigned to the interface chemistry, specifically halide chemisorption, which steers the formation of these highly textured deposits at the liquid-liquid interface.

(Invited) Atomic-Scale Aspects of Nucleation and Growth at Liquid-Liquid Interfaces

Interfaces between liquid metals and liquid electrolytes, in particular the liquid Hg - electrolyte interface, have played a key role in the development of the theory of the electrical double layer and electrochemical adsorption. More recently, liquid-liquid interfaces have regained interest in the field of material synthesis. Unlike solid interfaces, where strain and stress, heterogeneities, and defects strongly influence growth processes, fluid systems provide soft, defect- and stress-free interfaces. In addition, the growth process profits from the high mobility of atoms, molecules, and particles in both liquids, which allows growth from both sides of the phase boundary. A large variety of metallic and non-metallic nanomaterials has been prepared via electrochemical and electroless deposition at such liquid-liquid interfaces. As demonstrated by Maldonado and coworkers, electrodeposition at liquid metal electrodes even allows the growth of nanostructured crystalline semiconductors via a simple one-step, room-temperature electrochemical process [1].Understanding of the fundamental processes in nucleation and growth at liquid-liquid interfaces is hampered by difficulties in studying these interfaces experimentally on the atomic scale. Most surface-sensitive techniques, especially also scanning probe microscopy methods, cannot access these fluidic phase boundaries. For this reason, the majority of studies relies on electrochemical measurements, optical microscopy, and ex situ investigation of the deposit and thus provide little insight on the initial steps of the growth process. We have shown in the past that hard X-ray scattering methods, such as X-ray reflectivity (XRR) and grazing incidence X-ray scattering (GIXS), are unique tools for determining the atomic liquid-liquid interface structure. In this talk, we present case studies of electrochemically induced growth at liquid interfaces from the first monolayer up to several ten nanometer thick films.The first part discusses the growth of ionic compounds, using lead halides on Hg electrodes as an example. In PbBr2 containing NaF we observed previously growth of a PbBrF layer by operando X-ray scattering. This growth exhibited a complex nucleation and growth behavior, involving a crystalline precursor layer prior to 3D crystal growth [2]. The well-defined subnanometer thick precursor layer provided a template for the subsequent quasi-epitaxial growth of oriented 3D crystallites. Detailed studies on the potential-dependent nucleation and growth kinetics revealed with increasing overpotential a crossover from a low surface density film of large crystals to a compact PbBrF deposit with a saturation thickness of 25 nm [3,4]. In addition, growth on the liquid substrate was found to involve micromechanical effects, such as crystal reorientation and film breakup during dissolution. More recently, we extended these studies to growth in solutions containing only one type of halide anion (Br, Cl, or F). Also here, the formation of precursor layers was observed, indicating that this growth behavior is a general phenomenon.In the second part, joint X-ray scattering studies with Maldonado and coworkers on the electrochemical liquid-liquid-solid deposition of semiconductors from aqueous electrolyte are presented, focusing on Ge electrodeposition on Hg and HgxIn1-x alloy electrodes [5]. We provide evidence for the adsorption of GeO3 - anions on the liquid metal surface and the formation of a crystalline GeO2 adlayer at the positive end of the double layer region. Ge electrodeposition results in nanocrystals, which are separated from the Hg electrode by a water cushion. Furthermore, pronounced Hg surface segregation is found in HgxIn1-x, which protects the electrode surface from oxidation in the potential regime of Ge deposition.[1] Carim, A. I., Collins, S. M., Foley, J. M. & Maldonado, J. Am. Chem. Soc. 133, 13292 (2011)[2] A. Elsen, S. Festersen, B. Runge, C.T. Koops, B. M. Ocko, M. Deutsch, O. Seeck, B. M. Murphy, O. M. Magnussen, Proc. Nat. Acad. Sci., 110, 6663 (2013)[3] B.M. Murphy, S. Festersen, O.M. Magnussen, Nanoscale, 8, 13859 (2016)[4] S. Festersen, B. Runge, C. Koops, F. Bertram, B.M. Ocko, M. Deutsch, B.M. Murphy, O.M. Magnussen, Langmuir, 36, 10905 (2020)[5] D. Pattadar, Q. Cheek, A. Satori, Y. Zhao, P.R. Giri, B. Murphy, O.M. Magnussen, S. Maldonado, Cryst. Growth Des., 21, 1645 (2021)

Redox Chemistry of tris(β-diketonate)cobalt(III) Complexes: A Molecular View

Reduction of low spin S = 0 tris(β-diketonate)cobalt(III) complexes leads to an intermediate spin S = 3/2 anion. This electron rearrangement upon reduction could be the reason why a large peak potential difference between the reduction and re-oxidation peaks was observed during electrochemical analysis, using cyclic voltammetry. It is shown that experimental redox potentials obtained from the following methods, namely (i) polarography, (ii) cyclic voltammetry on a hanging Hg electrode, as well as (iii) cyclic voltammetry on a carbon fibre surface, related linearly to various empirical (Gordy scale group electronegativities, Hammett meta-substituent constants, pK a and the Lever electronic parameters) and density functional theory (DFT) calculated (LUMO energy, adiabatic electron affinity, chemical potential, global electrophilicity index and molecular electrostatic potential on Co) reactivity parameters. Deviations from these linear trends were due to a resonance effect of aromatic substituent groups. The linear relationships obtained for complexes containing aliphatic substituent groups can be used to predict redox potential and the electrophilic reactivity of related tris(β-diketonate)cobalt(III) complexes.

Electrochemistry of the inclusion complexes of metallocene derivatives with cucurbit[7]uril on glassy carbon and mercury electrodes

• The electrochemical behavior of ferricenium and cobalticenium cations and supramolecular complexes formed by such cations and ferrocene with a cucurbit[7]uril was studied on GC and Hg electrodes. • Revealed significant differences in the kinetics of heterogeneous electron transfer for these systems due to the principally different adsorption behavior of supramolecular complexes of these metallocenes on GC and Hg. The aim of this work is to compare the electrochemical behavior of ferricenium and cobalticenium cations and supramolecular complexes formed by these cations and ferrocene with a cavitand – cucurbit[7]uril on glassy carbon and mercury electrodes. On the Hg electrode the adsorption properties of these supramolecular complexes were also studied. The substantial differences in the processes of heterogeneous electron transfer for these systems are explained by the principally different adsorption behavior of supramolecular complexes of these metallocene derivatives on GC and Hg. It was found that the mechanism of reactions of these supramolecular complexes on the Hg electrode is determined by the conditions of formation of their adlayers in the potential region preceding the beginning of electrochemical processes. For complexes formed by ferricenium cation and cobaltocene with cucurbit[7]uril we have determined the more precise values for the constants of complex formation as compared to the literature data (7 × 10 9 M −1 and 1 × 10 8 M −1 , respectively).

Electrochemical Synthesis of Glycine from Oxalic Acid and Nitrate.

In manufacturing C-N bond-containing compounds, it is an important challenge to alternate the conventional methodologies that utilize reactive substrates, toxic reagents, and organic solvents. In this study, we developed an electrochemical method to synthesize a C-N bond-containing molecule avoiding the use of cyanides and amines by harnessing nitrate (NO3- ) as a nitrogen source in an aqueous electrolyte. In addition, we utilized oxalic acid as a carbon source, which can be obtained from electrochemical conversion of CO2. Thus, our approach can provide a route for the utilization of anthropogenic CO2 and nitrate wastes, which cause serious environmental problems including global warming and eutrophication. Interestingly, the coreduction of oxalic acid and nitrate generated reactive intermediates, which led to C-N bond formation followed by further reduction to an amino acid, namely, glycine. By carefully controlling this multireduction process with a fabricated Cu-Hg electrode, we demonstrated the efficient production of glycine with a faradaic efficiency (F.E.) of up to 43.1 % at -1.4 V vs. Ag/AgCl (current density≈90 mA cm-2 ).

Electrochemical Synthesis of Glycine from Oxalic Acid and Nitrate

AbstractIn manufacturing C−N bond‐containing compounds, it is an important challenge to alternate the conventional methodologies that utilize reactive substrates, toxic reagents, and organic solvents. In this study, we developed an electrochemical method to synthesize a C−N bond‐containing molecule avoiding the use of cyanides and amines by harnessing nitrate (NO3−) as a nitrogen source in an aqueous electrolyte. In addition, we utilized oxalic acid as a carbon source, which can be obtained from electrochemical conversion of CO2. Thus, our approach can provide a route for the utilization of anthropogenic CO2 and nitrate wastes, which cause serious environmental problems including global warming and eutrophication. Interestingly, the coreduction of oxalic acid and nitrate generated reactive intermediates, which led to C−N bond formation followed by further reduction to an amino acid, namely, glycine. By carefully controlling this multireduction process with a fabricated Cu–Hg electrode, we demonstrated the efficient production of glycine with a faradaic efficiency (F.E.) of up to 43.1 % at −1.4 V vs. Ag/AgCl (current density≈90 mA cm−2).

Electrochemical Determination of the "Furanic Index" in Honey.

5-(hydroxymethyl)furan-2-carbaldehyde, better known as hydroxymethylfurfural (HMF), is a well-known freshness parameter of honey: although mostly absent in fresh samples, its concentration tends to increase naturally with aging. However, high quantities of HMF are also found in fresh but adulterated samples or honey subjected to thermal or photochemical stresses. In addition, HMF deserves further consideration due to its potential toxic effects on human health. The processes at the origin of HMF formation in honey and in other foods, containing saccharides and proteins—mainly non-enzymatic browning reactions—can also produce other furanic compounds. Among others, 2-furaldehyde (2F) and 2-furoic acid (2FA) are the most abundant in honey, but also their isomers (i.e., 3-furaldehyde, 3F, and 3-furoic acid, 3FA) have been found in it, although in small quantities. A preliminary characterization of HMF, 2F, 2FA, 3F, and 3FA by cyclic voltammetry (CV) led to hypothesizing the possibility of a comprehensive quantitative determination of all these compounds using a simple and accurate square wave voltammetry (SWV) method. Therefore, a new parameter able to provide indications on quality of honey, named “Furanic Index” (FI), was proposed in this contribution, which is based on the simultaneous reduction of all analytes on an Hg electrode to ca. −1.50 V vs. Saturated Calomel Electrode (SCE). The proposed method, validated, and tested on 10 samples of honeys of different botanical origin and age, is fast and accurate, and, in the case of strawberry tree honey (Arbutus unedo), it highlighted the contribution to the FI of the homogentisic acid (HA), i.e., the chemical marker of the floral origin of this honey, which was quantitatively reduced in the working conditions. Excellent agreement between the SWV and Reverse-Phase High-Performance Liquid Chromatography (RP-HPLC) data was observed in all samples considered.

Determination of ambient dissolved metal ligand complexation parameters via kinetics and pseudo-voltammetry experiments

Dissolved chemical speciation of metals in natural waters encompasses a wide range of inorganic and organic compounds including metal organic ligand complexes, ML. Because of the different filters used, “dissolved” speciation can range from simple metal-ligand complexes with an average size of about 0.66 nm (mass of <3 k-daltons) to nanoparticles of 1 to 100 nm to colloidal forms that are 10 to 200–400 nm in size. Strong metal-ligand, ML, complexes are normally considered to be in <1 nm size fraction. Over the last 3 decades, competitive ligand exchange – cathodic stripping voltammetry (CLE-CSV) titrations have been the method of choice to study complexation. These titrations primarily give information on the excess ligands in the sample rather than the actual ligand in MLunknown complexes because they require adding metal to the sample. Thus, metal-ligand CLE-CSV titrations do not provide much information on the actual ligand present in MLunknown. However, pseudovoltammetry provides the thermodynamic stability constant, Ktherm, for Zn, Cu, Cd and Pb as the MLunknown complex is destroyed by reduction at the Hg electrode to form metal(Hg). Pseudovoltammetry does not require the addition of any reagents to the sample, but cannot be performed for ions such as Fe(III) [and Mn(III)] because reduction of the ion results in the reduction of the metal ion in the complex without destroying MLunknown. For these ions, kinetic experiments to recover the metal in the ML complex can provide information on the MLunknown dissociation rate constant, kd, and the conditional equilibrium constant, KcondML′. In these kinetic experiments, a competitive ligand (Lcomp) is added to the sample, and over time the MLcomp complex is measured by CSV. If all the metal in MLunknown is recovered, kd of MLunknown can be determined. If only a portion of the metal in MLunknown is recovered, equilibrium is achieved and KcondML′ as well as kd can be determined in a single experiment; kf can then be calculated. We describe how these methods can be used to determine information on the actual MLunknown complex. We show that 7 thermodynamic, kinetic and speciation parameters (Ktherm, KcondML′, KcondM′L′, kf, kd, αM′, αL′) for MLunknown complexes can be derived from a combination of two of these experiments. The approaches described here are useful to determine these parameters for known ML complexes once a ligand has been isolated by advanced separation methods (e.g., LC-MS) and reacted with a metal of interest.

Organic Copper Speciation by Anodic Stripping Voltammetry in Estuarine Waters With High Dissolved Organic Matter.

The determination of copper (Cu) speciation and its bioavailability in natural waters is an important issue due to its specific role as an essential micronutrient but also a toxic element at elevated concentrations. Here, we report an improved anodic stripping voltammetry (ASV) method for organic Cu speciation, intended to eliminate the important problem of surface-active substances (SAS) interference on the voltammetric signal, hindering measurements in samples with high organic matter concentration. The method relies on the addition of nonionic surfactant Triton-X-100 (T-X-100) at a concentration of 1 mg L−1. T-X-100 competitively inhibits the adsorption of SAS on the Hg electrode, consequently 1) diminishing SAS influence during the deposition step and 2) strongly improving the shape of the stripping Cu peak by eliminating the high background current due to the adsorbed SAS, making the extraction of Cu peak intensities much more convenient. Performed tests revealed that the addition of T-X-100, in the concentration used here, does not have any influence on the determination of Cu complexation parameters and thus is considered "interference-free." The method was tested using fulvic acid as a model of natural organic matter and applied for the determination of Cu speciation in samples collected in the Arno River estuary (Italy) (in spring and summer), characterized by a high dissolved organic carbon (DOC) concentration (up to 5.2 mgC L−1) and anthropogenic Cu input during the tourist season (up to 48 nM of total dissolved Cu). In all the samples, two classes of ligands (denoted as L1 and L2) were determined in concentrations ranging from 3.5 ± 2.9 to 63 ± 4 nM eq Cu for L1 and 17 ± 4 to 104 ± 7 nM eq Cu for L2, with stability constants logK Cu,1 = 9.6 ± 0.2–10.8 ± 0.6 and logK Cu,2 = 8.2 ± 0.3–9.0 ± 0.3. Different linear relationships between DOC and total ligand concentrations between the two seasons suggest a higher abundance of organic ligands in the DOM pool in spring, which is linked to a higher input of terrestrial humic substances into the estuary. This implies that terrestrial humic substances represent a significant pool of Cu-binding ligands in the Arno River estuary.

Effect of Boron Doping Level on BDD Electrode for Heavy Metal Ion Sensor

Recently, industrial wastewater is being discharged in large quantities due to the development of industry and this wastewater contains a various heavy metal ion. Exposure to high levels of trace metal ions [Cd (II), Pb (II), Cu (II)] causes a variety of health and environmental problems. These toxic species accumulated in the human body through the water source have a low rate of clearance. [1,2]To minimize the exposure of humans to these contaminants through effective water quality monitoring, anode stripping voltammetry (ASV) method was used. ASV method has wide linear dynamic range, low detection limit (ppb), multi-element analysis capability and the simplicity of the instrumentation, which is relatively inexpensive and small in size, requires low levels of electrical power and is sufficiently portable to permit its use in the field. So, ASV has been used successfully for the determination of heavy metals in water. The sensitivity of differential pulse anode stripping voltammetry (DPASV) is higher than other classical voltammetry, because of its enhanced faradaic current and decreased non-faradaic charging current. [2]In generally, Ir, Au, Ag and Hg electrodes have been used for stripping analysis of trace metals, but the Ir, Au, Ag electrodes have low sensitivity, Hg electrode have toxicity and volatility. [2] To solve this drawbacks, we fabricate a Boron-doped diamond (BDD) electrodes. BDD electrodes are candidate for use in the detection of a trace heavy metal ions, because these show good electrical conductivity, low background current, and non-toxicity.In this study, BDD electrodes on Si substrate deposited by hot filament chemical vapour deposition (HFCVD) and applied to DPASV measurement. Through the optimization of the deposition condition of BDD electrodes and measurement condition of DPASV, we increased the detection sensitivity of heavy metal ions. The BDD electrode with the optimum boron doping concentration by controlling the boron doping concentration from 100 to 10000 ppm in the deposition process optimization step has excellent durability and selectivity of heavy metal ions. As a result of electrochemical measurement, BDD electrode with 8000 ppm B was displayed high sensitive response and wide potential window range of -1.7 V to 1.6 V with the detection limit of 0.001 mg/L to 0.01 mg/L ±5 % about heavy metal ions [Cd (II), Pb (II), Cu (II)].[1] M.O. Park et al, Electroanalysis. 2017, 29, pp. 514 – 520[2] J. –H. Yoon et al, Bull. Korean Chem. Soc. 2010, Vol. 31, No. 1This study was supported by the Korea Environmental Industry & Technology Institute[grant number 20190027400012] and R&D Center for Green Patrol Technologies through R&D for Global Top Environmental Technologies [grant number 20180018400013] funded by the Ministry of Environment, Republic of Korea (MOE), Figure 1

Contemporary voltammetric techniques and its application to pesticide analysis: A review

The objective of the paper is to access the usefulness of contemporary voltammetric methodologies using various classical and modified electrode system and surface-active agents for wide range of application. Although in industrial sector the voltammetric techniques have certainly lesser explored than spectroscopic, chromatographic and electrophoretic techniques but offer superior solution for specific tasks due to its qualitative as well as quantitively characterisation ability. Voltammetric methodologies are one of the emerging contemporary assay procedures that allows a easy and fast discrimination and its voltammogramic equivalence with a spectrum in spectroscopy provide ample information on the chemical characteristics of electro active species involved in redox processes. Application of voltammetry is widely explored and exploited in the fields of science and technology inclusive of environmental analysis of pesticides at commercial and residue levels (10−12–10−1 M). Optimisation of voltammetric response is critical to obtain credible and reliable interpretable data in suitable electrolyte-solvent system in applied potential range in electrochemical cell with suitable indicator, reference and counter electrode system. Recent development in chemically modified electrode in term of sensitivity, selectivity and diverse applications leads to revolution in voltammetry techniques. Recent trends revealed that such contemporary voltammetric methods are reasonably explored in pesticide analysis of variety of thiourea, benzimidazoles, organophosphorus, dithiocarbamates, triazines, carbaryl, organochlorine, etc. Diversity and complexity of the pesticide structures create electroanalytical possibility to explored reactions potential range from cathodic to anodic along with a number of explored voltammetric techniques involving Hg, amalgam, Au, glassy carbon, carbon paste and BDD Electrode its further modification for their determination. However, in recent intervals the classical voltammetric techniques have almost been vanished and are being substitute by more advanced voltammetric methodologies for numerous analytical and mechanistic studies.

Voltammetric, FTIR Spectroscopic and Thermal Analysis Studies on Adduct Formations of Rifampicin with Soft Nucleophiles Cysteine and Glutathione

In the present study, the adduct formation equilibria of rifampicin (RIF) with cysteine (CySH) and glutathione (GSH) at different pHs were investigated using cyclic and square-wave voltammetry techniques. With increasing RIF concentration, the reduction currents of mercurous thiolates (Hg2(RS)2) which are main surface products for CySH and GSH on Hg electrode decrease drastically and their reduction potentials shifted towards less negative values due to intermolecular interactions. At the same time, the voltammetric signals (especially the Epa and Epc values of reversible redox process of the α-hydrojuglone moiety) of RIF shifted towards positive values in the presence of CySH or GSH. A plausible mechanism is proposed to explain the formations of some adducts by means of this interaction process. That is that CySH and GSH behave as nucleophile and undergo the addition reaction with RIF. The formation of adducts was also confirmed by FTIR measurements and could be clarified by the disappearance of the stretching band of free thiol group. The stoichiometric ratio of this reaction was found to be 1 : 1 and the binding constants were evaluated using the experimental data of square-wave voltammetry. The pH effect on binding constant was studied by varying pH (4.5, 7.4, 9.0). It was found that the reaction was strongly influenced by the pH. This nucleophilic addition reaction was mostly suitable at pH 9. Also, the constant of the addition reaction of GSH is higher than that of CySH for all studied pHs. The thermal behaviours of these adducts were characterized by thermal analysis techniques (TGA/DrTGA/DTA). Since the thermograms recorded for adducts are not an overlapping of thermal curves of starting materials, this behaviour has been considered a further proof for the formation of adducts.

Effects of ex situ chronopotentiometric analysis on stability of bovine serum albumin on mercury electrodes

Constant current chronopotentiometric stripping (CPS) allows analysis of proteins based on measured peak H resulting from catalytic hydrogen evolution reaction. The technique is label-free and sensitive to the structure and stability of the protein adsorbed at the mercury electrode. Comparison of proteins must be carried out under the same conditions, including ionic strength, temperature, pH, and accumulation potential and time, as all of these factors can affect the protein stability and structure on the electrode surface. Here we show that for bovine serum albumin, uncontrolled disconnection of the cell after accumulation, as is necessary for ex situ CPS measurements, can cause an increased susceptibility to electric field-induced denaturation during the subsequent CPS measurement. This destabilization is attributed to oxidation of the Hg electrode during disconnection. For this reason, care much be taken when ex situ CPS measurement of protein is performed.