Enhanced electrocatalytic degradation of 2,4-Dinitrophenol (2,4-DNP) in three-dimensional sono-electrochemical (3D/SEC) process equipped with Fe/SBA-15 nanocomposite particle electrodes: Degradation pathway and application for real wastewater

2,4-dinitrophenol (2,4-DNP), which is a nitrophenol compound, is a carcinogenic and non-biodegradable pollutant, which is found at high concentrations in industrial wastewater. Degradation of 2,4-DNP using a three-dimensional sono-electrochemical (3D/SEC) process equipped with G/β-PbO₂ anode and Fe/SBA-15 nanocomposite particle electrodes was evaluated in the present study. Investigating the effect of parameters including pH, electrolysis time, current density, and 2,4-DNP concentration on the performance of the 3D/SEC-Fe-SBA-15 process in 2,4-DNP degradation was considered, and optimization of these parameters was done using the Taguchi design technique. Field emission scanning electron microscopy (FESEM), X-ray diffraction analysis (XRD), energy-dispersive X-ray spectroscopy mapping (EDX-mapping), transmission electron microscopy (TEM), and Fourier Transform Infrared Spectroscopy (FTIR)) were the analyses techniques used to support the successful synthesis of Fe-SBA-15 and G/β-PbO₂ anode. The optimum values obtained for pH, electrolysis time, current density, and 2,4-DNP concentration were 5.0, 60.0 min, 5.0 mA/cm², and 50.0 mg/L, respectively. The experimental removal efficiencies of 2,4-DNP, COD, and TOC using 3D/SEC-Fe-SBA-15 process, under the mentioned conditions, were obtained to be 96.3%, 88.28%, and 83.82%, respectively. In addition, the AOS value was developed from ?0.29 to + 0.88; this indicates the high mineralization of 2,4-DNP and improvement of the solution biodegradability. Detecting the intermediates produced during the degradation process was done by LC-MS analysis, and pathways for its degradation was proposed. Results were indicative of the high potential of the 3D/SEC-Fe-SBA-15 process for treating wastewater containing phenolic compounds, e.g., 2,4-DNP, and can provide acceptable efficiency.     

Improvement of overcharge performance using Li4Ti5O12 as negative electrode for LiFePO4 power battery

Liquid state soft packed LiFePO₄ cathode lithium ion cells with capacity of 2 Ah were fabricated using graphite or Li₄Ti₅O₁₂ as negative electrodes to investigate the 3 C/10 V overcharge characteristics at room temperature. The LiFePO₄/Li₄Ti₅O₁₂ cell remained safe after the 3 C/10 V overcharge test while the LiFePO₄/graphite cell went to thermal runaway. Temperature and voltage variations during overcharge were recorded and analyzed. The cells after overcharge were disassembled to check the changes of the separated cell components. The results showed that the Li₄Ti₅O₁₂ as anode active material for LiFePO₄ cell showed obvious safety advantage compared with the graphite anode. The lithium ionic diffusion models of Li₄Ti₅O₁₂ anode and graphite anode were built respectively with the help of morphology characterizations performed by scanning electron microscopy. It was found that the different particle shapes and lithium ionic diffusion modes caused different lithium ionic conductivities during overcharge process.     

Development of a sensitive surface plasmon resonance immunosensor for detection of 2,4-dinitrotoluene with a novel oligo (ethylene glycol)-based sensor surface

A surface plasmon resonance (SPR) immunosensor for detection of 2,4-dinitrotoluene (2,4-DNT), which is a signature compound of 2,4,6-trinitrotoluene-related explosives, was developed by using a novel oligo (ethylene glycol) (OEG)-based sensor surface. A rabbit polyclonal antibody against 2,4-DNT (anti-DNPh-KLH-400 antibody) was prepared, and the avidity for 2,4-DNT and recognition capability were investigated by indirect competitive ELISA. The sensor surface was fabricated by immobilizing a 2,4-DNT analog onto an OEG-based self-assembled monolayer formed on a gold surface via an OEG linker. The fabricated surface was characterized by Fourier-transform infrared-refractive absorption spectrometry (FTIR-RAS). The immunosensing of 2,4-DNT is based on the indirect competitive principle, in which the immunoreaction between the anti-DNPh-KLH-400 antibody and 2,4-DNT on the sensor surface was inhibited in the presence of free 2,4-DNT in solution. The limit of detection for the immunosensor, calculated as three times the standard deviation of a blank value, was 20 pg mL⁻¹, and the linear dynamic range was found to be between 1 and 100 ng mL⁻¹. Additionally, the fabricated OEG-based surface effectively prevented non-specific adsorption of proteins, and the specific response to anti-DNPh-KLH-400 antibody was maintained for more than 30 measurement cycles.     

Bis(2-mercaptobenzoxazolato)mercury(II) and bis(2-pyridinethiolato)mercury(II) complexes as carriers for thiocyanate selective electrodes

Highly selective poly(vinyl chloride) (PVC) membrane electrodes based on bis(2-mercaptobenzoxazolato)mercury(II) [Hg(MBO)₂] and bis(2-pyridinethiolato)mercury(II) [Hg(PT)₂] complexes as new carriers for thiocyanate-selective electrodes are reported. The electrodes were prepared by coating the membrane solution containing PVC, plasticizer, carriers and additives on the surface of graphite electrodes. Influence of the membrane composition, pH and possible interfering anions were investigated on the response properties of the electrodes. Both sensors exhibited Nernstian responses towards thiocyanate over a wide concentration range of 1×10⁻⁶ to 0.1 M, with slopes of 60.6±0.8 and 57.5±1.2 mV per decade of thiocyanate concentration for Hg(MBO)₂ and Hg(PT)₂ carriers, respectively, over a wide pH range of 3-11. The limit of detection for both electrodes was ∼6×10⁻⁷ M. The sensors have response times of ≤5 s and can be used for at least 2 months without any considerable divergence in their potential response. The proposed electrodes show fairly good discrimination of thiocyanate over several inorganic and organic anions. The electrodes were successfully applied to direct determination of thiocyanate in saliva and as indicator electrodes in precipitation titrations.     

The deposition of nanostructured β-PbO2 coatings from aqueous methanesulfonic acid for the electrochemical oxidation of organic pollutants

Highly crystalline, nanostructured, three-dimensional β-PbO₂ coatings were successfully obtained by galvanostatic deposition from baths containing aqueous lead(II) and methanesulfonic acid (CH₃SO₃H). This constitutes a much more environmentally friendly methodology compared to plating of β-PbO₂ in HNO₃. The deposits exhibited high quality and good adherence. The crystallite size was in the range 20–30 nm and AFM imaging revealed very uniform, rough deposits (i.e., 255–275 nm rms). The oxidative destruction of Methyl Orange azo dye was studied by electrochemical advanced oxidation processes (EAOPs). An electro-Fenton process with a high surface area carbon-felt cathode performed better than the single anodic oxidation. Rapid and complete decolorisation was achieved following pseudo first-order kinetics. The stability of the β-PbO₂ electrodes during the electrolyses was also demonstrated.     

Properties of LiNiO2 cathode and graphite anode in N-methyl-N-propylpyrrolidinium bis(trifluoromethanesulfonyl)imide

Electrochemical properties of LiNiO₂|Li and LiNiO₂|graphite cells were analysed in ionic liquid electrolyte [Li⁺][MePrPyrr⁺][NTf₂^-] (based on N-methyl-N-propylpyrrolidinium bis(trifluoromethanesulphonyl)imide, [MePrPyrr⁺][NTf₂^-]) using impedance spectroscopy and galvanostatic techniques. The ionic liquid is incapable of protective solid electrolyte interface (SEI) formation on metallic lithium or lithiated graphite. However, after addition of VC, the protective coating is formed, facilitating a proper work of the Li-ion cell. Scanning electron microscopy images of pristine electrodes and those taken after electrochemical cycling showed changes which may be interpreted as a result of SEI formation. The charging/discharging capacity of the LiNiO₂ cathode is between 195 and 170 mAh g⁻¹, depending on the rate. The charging/discharging efficiency of the graphite anode drops after 50 cycles from an initial value of ca. 360 mAh g⁻¹ to stabilise at 340 mAh g⁻¹. The replacement of a classical electrolyte in molecular liquids (cyclic carbonates) with an electrolyte based on the MePrPyrrNTf₂ ionic liquid highly increases in the cathode/electrolyte non-flammability.     

Adsorption of antimony using amino-functionalized magnetic MIL-101(Cr): Optimization by response surface methodology

Amino-functionalized magnetic MIL-101(Cr) was prepared via a one-step solvothermal method, characterized, and applied in adsorptive Sb(III) removal. The effects of solution pH, adsorbent dosage, and coexisting substances on the adsorption of Sb(III) by MIL-101(Cr)–NH₂/MnFe₂O₄ were studied. The adsorption kinetics were analyzed using pseudo-first order, pseudo-second order, intraparticle diffusion, and Elovich models, while Freundlich and Langmuir isotherm models were used to fit the experimental data. The pseudo-second-order kinetic model provided the best fit for the kinetic data. The maximum adsorption capacity of MIL-101(Cr)–NH₂/MnFe₂O₄ for Sb(III) was 91.07 ​mg/g, as calculated using the Langmuir adsorption isotherm model. Thermodynamic analysis revealed that the adsorption of antimony onto MIL-101(Cr)–NH₂/MnFe₂O₄ is spontaneous and endothermic, while response surface optimization revealed that the optimal conditions for Sb(III) adsorption by MIL-101(Cr)–NH₂/MnFe₂O₄ are an adsorbent loading of 222.55 ​mg/L, a pH of 4.5, and a temperature of 294.59 ​K. The predicted adsorption capacity of MIL-101(Cr)–NH₂/MnFe₂O₄ for Sb(III) is only a 1.8% deviation from the actual value. Furthermore, MIL-101(Cr)–NH₂/MnFe₂O₄ exhibits strong magnetism, allowing it to be separated from wastewater using a magnet. Finally, a preliminary economic analysis showed that the cost of treating a ton wastewater containing 25 ​mg/L antimony using this composite would be 26.24 USD. Thus, MIL-101(Cr)–NH₂/MnFe₂O₄ is promising for treatment of Sb(III)-containing wastewater.     

Effect of graphite pore former on oxygen electrodes prepared with La0.6Sr0.4CoO3−δ nanoparticles

This study aimed at fabricating porous crack-free and delamination-free La_0.6Sr_0.4CoO_3?δ electrodes using nanopowders and investigating oxygen reduction (occurring at solid oxide fuel cell cathodes) and oxygen evolution (occurring at solid oxide electrolysis cell anodes) at 600 °C in air. The electrodes were deposited by screen-printing on Ce_0.8Gd_0.2O_1.9 substrates. The pastes were prepared with nanoparticles synthesised by flame spray synthesis and graphite pore former. Without graphite, the electrodes sintered at 1000 °C exhibit relatively low porosity and significant densification which led to partial delamination and large overpotentials. The addition of graphite, which was removed by combustion at ca. 650 °C during sintering, markedly improves electrode performance by increasing porosity and reducing densification. A minimal overpotential for both the oxygen reduction and oxygen evolution was reached for a layer porosity of ca. 50–60 vol.%.     

Degradation of p-aminophenol wastewater using Ti-Si-Sn-Sb/GAC particle electrodes in a three-dimensional electrochemical oxidation reactor

In this study, Ti-Si-Sn-Sb/GAC particle electrodes were prepared by sol–gel method. The particle electrodes were characterized by SEM, XRD, EDX, and BET then used to carry out three-dimensional (3D) electrocatalytic oxidation degradation on simulated refractory p-aminophenol (PAP) wastewater. The effects of initial pH, cell voltage, aeration flow rate and initial PAP concentration on degradation experiments were investigated. Under the optimal conditions, the PAP and COD removal rates were 89.45% and 75.17% respectively. In addition, the possible degradation mechanism of PAP was further investigated by UV–Vis and HPLC. Finally, it was found that the Ti-Si-Sn-Sb/GAC particle electrodes with high catalytic activity and excellent stability could significantly improve the PAP wastewater removal efficiency.     

Performance improvement in chemical oxygen demand determination using carbon fiber felt/CeO<Subscript>2</Subscript>-β-PbO<Subscript>2</Subscript> electrode deposited by cyclic voltammetry method

A three-dimensional (3D) structured electrode in which a compact CeO₂-β-PbO₂ particle layer on each carbon fiber in the felt (denoted as CF/CeO₂-β-PbO₂) was fabricated using cyclic voltammetry (CV) method in the presence of CeO₂ nanoparticles in the electrolyte and supposed to be used as a sensor for in situ chemical oxygen demand (COD) detection. It was found that CeO₂ was codeposited with PbO₂ onto the anode, and the deposited crystals were tiny and compacted with each other. The electrochemical behaviors demonstrate that the fabricated CF/CeO₂-β-PbO₂ electrode possesses larger effective surface area, higher electrochemically catalytic activity, and better mechanical stability as compared with the anode without CeO₂ deposited by CV method or constant potential (CP) method. The results of COD determination by the fabricated CF/CeO₂-β-PbO₂ electrode show a sensitivity of (3.0 ± 0.02) × 10^?3 mA cm^?2/mg L^?1, a detection limit of 3.6 mg L^?1 (S/N = 3) and a linear range of 30–8500 mg L^?1 with correlation coefficient (R) of 0.9985 and RSD within 5 %.

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Graphical abstract A 3D CF/CeO₂-β-PbO₂ electrode with CeO₂-β-PbO₂ particle layer on each carbon fiber in the felt was supposed to be used as a sensor for in situ chemical oxygen demand (COD) detection. It was fabricated by cyclic voltammetry (CV) method in the presence of CeO₂ nanoparticles in the electrolyte containing Pb²⁺. It was found that CeO₂ was codeposited with PbO₂ onto the anode and the deposited particles became tinier and more compact. The addition of CeO₂ enhances the electrochemical catalytic activity. Tinier and more compact crystals enlarge the effective electrode area and improve the mechanical strength, which makes the CF/CeO₂-β-PbO₂ electrode possess higher detection sensitivity, wider linearity range, and longer service life in COD detection as compared with the anodes without CeO₂ fabricated by CV method or constant potential (CP) method.

     

Removal of dithioterethiol (DTT) from water by membranes of cellulose acetate (AC) and AC doped ZnO and TiO2 nanoparticles

Dithioterethiol (DTT) is a typical example of substances that contain sulfur with adverse effects on human health. Membranes-based cellulose acetate is used for the separation processes of thiols after the addition of ZnO and TiO₂ nanoparticles. The measurement of permeability allows us to estimate the efficiency of membrane cleaning. The permeability increases from 8.82 L.h^?1.m^?2.bar^?1 for CA membrane to 20.77 L.h^?1.m^?2.bar^?1 for CA-TiO₂ and 21.96 L.h^?1.m^?2.bar^?1 for CA-ZnO membranes. For the permeability values of DTT, we noted that the CA-ZnO membrane has the highest permeability (50.66 L.h^?1.m^?2.bar^?1). The CA-ZnO membrane changes from nanofiltration to ultrafiltration membrane. On the other hand, for the CA-TiO₂ modified membrane, the permeability decreases to 6.00 L.h^?1.m^?2.bar^?1. The CA-TiO₂ membrane is in the category of reverse osmosis membranes. This variation is explained by the interaction between nanoparticles and DTT. The contact angles of the incorporated membranes decrease progressively with the addition of TiO₂ or ZnO-NPs. The low contact angle with water means high hydrophilicity, indicated that the addition of TiO₂ and ZnO improved the hydrophilicity of the membranes. The CA membrane had the highest contact angle with water of 92.64 ± 1.5°. After the addition of 0.1 g of TiO₂ or ZnO, the contact angle of CA-TiO₂ and CA-ZnO was reduced to 86.7 ± 0.2° and 70.51 ± 1.5°, respectively. Both TiO₂ and ZnO caused strong hydrophilicity of membranes. From the elimination rates of DTT, it is concluded that there are optimal conditions of (1) Pressure P = 2 bars, (2) pH = 10 and (3) DTT concentration = 2 mM.     

Determination of formal potential of NADH/NAD+ redox couple and catalytic oxidation of NADH using poly(phenosafranin)-modified carbon electrodes

The electrochemical regeneration of NADH/NAD⁺ redox couple has been studied using poly(phenosafranin) (PPS)-modified carbon electrodes to evaluate the formal potential and catalytic rate constant for the oxidation of NADH. The PPS-modified electrodes were prepared by electropolymerization of phenosafranin onto different carbon substrates (glassy carbon (GC) and basal-plane pyrolytic graphite (BPPG)) in different electrolytic solutions. The formal potential was estimated to be ? 0.365 ± 0.002 V vs. SHE at pH 7.0. As for the bare carbon electrodes, the oxidation of NADH at the BPPG electrode was found to be enhanced compared with the GC electrode. For the PPS-modified electrodes, it was found that the electrocatalysis of PPS-modified electrodes for the oxidation of NADH largely depends on the carbon substrate and electrolyte solution employed for their preparation, i.e., the PPS-modified BPPG electrode prepared in 0.2 M NaClO₄/acetonitrile solution exhibits an excellent and persistent electrocatalytic property toward NADH oxidation in phosphate buffer solution (pH 7.0) with a diminution of the overpotential of about 740 and 670 mV compared with those at the bare GC electrode and the PPS-modified GC electrode prepared in 0.2 M H₂SO₄ solution, respectively. A quantitative analysis of the electrocatalytic reaction based on rotating disk voltammetry gave the electrocatalytic reaction rate constants of the order of 10³–10⁴ M^?1 s^? 1 depending on the preparation conditions of the PPS-modified electrodes.     

Rapid Legionella pneumophila determination based on a disposable core–shell Fe3O4@poly(dopamine) magnetic nanoparticles immunoplatform

A novel amperometric magnetoimmunoassay, based on the use of core–shell magnetic nanoparticles and screen-printed carbon electrodes, was developed for the selective determination of Legionella pneumophila SG1. A specific capture antibody (Ab) was linked to the poly(dopamine)–modified magnetic nanoparticles (MNPs@pDA-Ab) and incubated with bacteria. The captured bacteria were sandwiched using the antibody labeled with horseradish peroxidase (Ab-HRP), and the resulting MNPs@pDA-Ab-Legionella neumophila-Ab-HRP were captured by a magnetic field on the electrode surface. The amperometric response measured at −0.15 V vs. Ag pseudo-reference electrode of the SPCE after the addition of H₂O₂ in the presence of hydroquinone (HQ) was used as transduction signal. The achieved limit of detection, without pre-concentration or pre-enrichment steps, was 10⁴ Colony Forming Units (CFUs) mL⁻¹. The method showed a good selectivity and the MNPs@pDA-Ab exhibited a good stability during 30 days. The possibility of detecting L. pneumophila at 10 CFU mL⁻¹ level in less than 3 h, after performing a membrane-based preconcentration step, was also demonstrated.     

Removal, preconcentration and determination of Mo(VI) from water and wastewater samples using maghemite nanoparticles

Removal and recovery of Mo(VI) from aqueous solutions were investigated using maghemite (γ-Fe₂O₃) nanoparticles. Combination of nanoparticle adsorption and magnetic separation was used to the removal and recovery of Mo(VI) from water and wastewater solutions. The nanoscale maghemite with mean diameter of 50 nm was synthesized by reduction coprecipitation method followed by aeration oxidation. Various factors influencing the adsorption of Mo(VI), e.g. pH, temperature, initial concentration, and coexisting common ions were studied. Adsorption reached equilibrium within <10 min and was independent of initial concentration of Mo(VI). Studies were performed at different pH values to find out the pH at which maximum adsorption occurred. The maximum adsorption occurred at pHs between 4.0 and 6.0. The Langmuir adsorption capacity (q_max) was found to be 33.4 mg Mo(VI)/g of the adsorbent. The results showed that nanoparticle (γ-Fe₂O₃) is suitable for the removal of Mo(VI), as molybdate, from water and wastewater samples. The adsorbed Mo(VI) was then desorbed and determined spectrophotometrically using bromopyrogallol red as a complexation reagent. This allows the determination of Mo(VI) in the range 1.0–86.0 ng mL⁻¹.     

Green and facile electrode modification by spark discharge: Bismuth oxide-screen printed electrodes for the screening of ultra-trace Cd(II) and Pb(II)

We report that highly effective electrode modification can be achieved by sparking process between a flat electrode substrate and a tip counter electrode. The concept is introduced by the development of Bi₂O₃-modified graphite screen printed electrodes (SPEs). SPEs were sparked with a bismuth wire at 1.2 kV under atmospheric conditions. The effect of polarity on the morphology of the sensing surface, bismuth loading and the sensitivity of the resulting sensors for the simultaneous anodic stripping voltammetric determination of Cd(II) and Pb(II) was investigated. Compared with electroplated and various bismuth precursors bulk-modified SPEs, the developed sparked electrodes exhibited considerably lower limit of detection (0.2 μg L^− 1, S/N = 3) for each target ion. Therefore, sparking technique offers a facile and green approach for the development of highly sensitive bismuth-based electrodes, and a wide-scope of applicability in the development of metal-modified sensing surfaces.     

Synthesis and application of silica and calcium carbonate nanoparticles in the reduction of organics from refinery wastewater

Oil refinery is one of the fast growing industries across the globe and it is expected to progress in the near future. The worldwide increase in the generation of refinery wastewater along with strict environmental regulations in the discharge of industrial effluent, persistent efforts have been devoted to recycle and reuse the treated water. The wastewater from the refining operation leads to serious environmental threat to the ecosystem. Therefore, this study aimed to synthesize silica (SiO₂) and calcium carbonate nanoparticles (CaCO₃) in the reduction of organics from refinery wastewater. The synthesized nanoparticles were employed in the reduction of chemical oxygen demand (COD) from refinery wastewater by studying the influence of solution pH, contact time, dosage of nanoparticles and stirring speed on adsorption performance. From the batch experimental studies, the optimized processing conditions for the reduction of COD using SiO₂ nanoparticles are pH 4.0, dosage 0.5 g, stirring speed 125 rpm and 90 min stirring time, and the corresponding values for CaCO₃ nanoparticles are pH 8.0, dosage 0.4 g, stirring speed 100 rpm and 90 min stirring time. The study demonstrates that SiO₂ and CaCO₃ nanoparticles have a promising future in the reduction organics from refinery wastewater in different pH regimes.     

Phosphomolybdate-doped-poly(3,4-ethylenedioxythiophene) coated gold nanoparticles: Synthesis,characterization and electrocatalytic reduction of bromate

Phosphomolybdate, H₃PMo₁₂O₄₀, (PMo₁₂)-doped-poly(3,4-ethylenedioxythiophene) (PEDOT) coated gold nanoparticles have been synthesized in aqueous solution by reduction of AuCl₄⁻ using hydroxymethyl EDOT as a reducing agent in the presence of polystyrene sulfonate and PMo₁₂. The resulting PMo₁₂-doped-PEDOT stabilized Au nanoparticles are water soluble and have been characterized by UV–visible spectroscopy, scanning electron microscopy and electrochemistry. Glassy carbon electrodes modified with these Au nanoparticles show excellent stability and catalytic activity towards the reduction of bromate in an aqueous electrolyte solution containing 10 mM H₂SO₄ and 0.1 M Na₂SO₄.     

Investigation of different alcoholic modifiers for the separation and determination of two isomers of dinitrotoluene (2,4 and 2,6) by ion mobility spectrometry

Some alcoholic modifier gases were applied to separate isomer peaks in ion mobility spectrometry (IMS). Different mechanisms have been investigated on the separation, such as collision cross-section and analyte-modifier cluster formation. In this regard, some parameters that affected the cluster formation, such as dipole moment, electron affinity, the position of functional groups, and the modifier structure, were evaluated. On the other hand, some effective experimental parameters, including cell temperature and the flow rates of the drift and modifier gases, were also optimized. The combination of dispersive liquid–liquid microextraction with thin-film evaporation (DLLME-TFE) was used as a sample preparation method for the extraction of 2,4-dinitrotoluene (2,4-DNT) and 2,6-dinitrotoluene (2,6-DNT) isomers (as the target analytes). Isobutanol was selected as the alcoholic modifier to separate the ion molecular peaks of these isomers. The limit of detection and the limit of quantification obtained were 15 and 50 μg L⁻¹, and the linear dynamic range (50–700 μg L⁻¹) with coefficient of determination of 0.9941 and 0.9914 were obtained for 2,4-DNT and 2,6-DNT, respectively. The intra- and inter-day relative standard deviations were obtained between 3% and 5%. For validation of the method, determination of the isomers was accomplished for a red wastewater field sample, resulting in relative recovery values of about 96%.     

Determination of trace amounts of mercury(II) in water samples using a novel kinetic catalytic ligand substitution reaction of hexacyanoruthenate(II)

A simple, sensitive, selective and rapid kinetic catalytic method has been developed for the determination of Hg(II) ions at micro-level. This method is based on the catalytic effect of Hg(II) ion on the rate of substitution of cyanide in hexacyanoruthenate(II) with nitroso-R-salt (NRS) in aqueous medium and provides good accuracy and precision. The concentration of Hg(II) catalyst varied from 4.0 to 10.0 × 10⁻⁶ M and the progress of reaction was followed spectrophotometrically at 525 nm (λ_max of purple-red complex [Ru(CN)₅NRS]³⁻,  = 3.1 × 10³ M⁻¹ s⁻¹) under the optimized reaction conditions; 8.75 × 10⁻⁵ M [Ru(CN)₆⁴⁻], 3.50 × 10⁻⁴ M [nitroso-R-salt], pH 7.00 ± 0.02, ionic strength, I = 0.1 M (KCl), temp 45.0 ± 0.1 °C. The linear calibration curves, i.e. calibration equations between the absorbance at fixed times (t = 15, 20 and 25 min) versus concentration of Hg(II) ions were established under the optimized experimental conditions. The detection limit was found to be 1.0 × 10⁻⁷ M of Hg(II). The effect of various foreign ions on the proposed method has also been studied and discussed. The method has been applied to the determination of mercury(II) in aqueous solutions.