Highly sensitive determination of hydroxylamine using fused gold nanoparticles immobilized on sol-gel film modified gold electrode

We are reporting the highly sensitive determination of hydroxylamine (HA) using 2-mercapto-4-methyl-5-thiazoleacetic acid (TAA) capped fused spherical gold nanoparticles (AuNPs) modified Au electrode. The fused TAA-AuNPs were immobilized on (3-mercaptopropyl)-trimethoxysilane (MPTS) sol-gel film, which was pre-assembled on Au electrode. The immobilization of fused TAA-AuNPs on MPTS sol-gel film was confirmed by UV-vis absorption spectroscopy and atomic force microscopy (AFM). The AFM image showed that the AuNPs retained the fused spherical morphology after immobilized on sol-gel film. The fused TAA-AuNPs on MPTS modified Au electrode were used for the determination of HA in phosphate buffer (PB) solution (pH = 7.2). When compared to bare Au electrode, the fused AuNPs modified electrode not only shifted the oxidation potential of HA towards less positive potential but also enhanced its oxidation peak current. Further, the oxidation of HA was highly stable at fused AuNPs modified electrode. Using amperometric method, determination of 17.5 nM HA was achieved for the first time. Further, the current response of HA increases linearly while increasing its concentration from 17.5 nM to 22 mM and a detection limit was found to be 0.39 nM (S/N = 3). The present modified electrode was also successfully used for the determination of 17.5 nM HA in the presence of 200-fold excess of common interferents such as urea, NO₂⁻, NH₄⁺, oxalate, Mn²⁺, Na⁺, K⁺, Mg²⁺, Ca²⁺, Ba²⁺ and Cu²⁺. The practical application of the present modified electrode was demonstrated by measuring the concentration of HA in ground water samples.     

A novel ion selective membrane potentiometric sensor for direct determination of Fe(III) in the presence of Fe(II)

A new PVC membrane potentiometric sensor that is highly selective to Fe(III) ions was prepared by using 2-[(2-hydroxy-1-propenyl-buta-1,3-dienylimino)-methyl]-4-p-tolylazo-phenol [HPDTP] as a suitable carrier. The electrode exhibits a linear response for iron(III) ions over a wide concentration range (3.5 × 10⁻⁶ to 4.0 × 10⁻²) with a super Nernstian slope of 28.5 (±0.5) per decade. The electrode can be used in the pH range from 4.5 to 6.5. The proposed sensor shows fairly a good discriminating ability towards Fe³⁺ ion in comparison to some hard and soft metals such as Fe²⁺, Cd²⁺, Cu²⁺, Al³⁺ and Ca²⁺. It has a response time of <15 s and can be used for at least 2 months without any measurable divergence in response characteristics. The electrode was used in the direct determination of Fe³⁺ in aqueous samples and as an indicator electrode in potentiometric titration of Fe(III) ions.     

A Coated Graphite Thorium‐Ion Selective Potentiometric Sensor Based on a Calix[4]arene Bearing Phosphoryl Groups

5,11,17,23‐Tetra‐tert‐butyl‐25,26,27,28‐tetrakis(diphenylphosphinoylmethoxy)calix[4]arene ( 1 )has been used for the preparation of a graphite coated thorium ion‐selective electrode (Th⁴⁺‐ISE). The plasticized PVC membrane containing 30% PVC, 58% ortho‐nitrophenyloctylether (NPOE), 4% sodium tetraphenylborate (NaTPB) and 8% ionophore was directly coated on a graphite rod. This sensor gave good Nernstian responses with a slope of 15.5 ± 0.1 mV/decade over a concentration range of 1 × 10^?5 ?1 × 10^?3 M of thorium ions with a limit of detection of 7.9 × 10^?6 M. The dynamic response time of the electrode to achieve a steady potential was found to be about 15 seconds. The potential of the prepared sensor was independent of the pH variation in the range 2.3–4.0. The selectivity relative to several mono‐, di‐ and tri‐valent metal ions, i.e. Li⁺, Na⁺, K⁺, Ag⁺, NH₄⁺, Sr²⁺, Mn²⁺, Fe²⁺, Co²⁺, Ni²⁺, Cu²⁺, Zn²⁺, Cd²⁺, La³⁺, Sm³⁺, Dy³⁺, Er³⁺ and Y³⁺ was examined. This electrode can be used for 6 months without any considerable divergences in the potential response. The sensor was successfully used as an indicator electrode for the potentiometric titration of a thorium solution using a standard solution of EDTA.     

New Electrode Material GCE/AgNPs-LS/Hg Based on Nanosilver Produced with the Use of Biopolymers

A method of producing nanosilver by using lignosulfonic acid derivative and a new method of the obtained AgNPs-LS electrode material for developing a chemically modified GCE/AgNPs-LS/Hg electrode is presented. The data demonstrate that the modified electrode shows enhanced performance in determining Tl⁺ ions in comparison to unmodified electrode GCE. The stripping anodic peak current of thallium characterized in linearity over its concentration range from 1.22 ⋅ 10⁻⁸ to 1.15 ⋅ 10⁻⁷ mol dm⁻³, with detection limit 4.6 ⋅ 10⁻⁹ mol dm⁻³. It is assumed that the novel AgNPs-LS/Hg²⁺ complex is suitable for preparing functionalized nanostructured material and can be used as an electrochemical sensing platform for heavy metals.     

Trace fluoride determination with specific ion electrode

A method is described for determining 10^-5–10^-4M fluoride in a variety of solutions potentiometrically with a fluoridc-specific electrode, by a standard addition method. Any change of ionic strength or the nature of the solution that might alter activity coefficients or junction potentials is minimized. The relationship between potential and fluoride concentration thus follows the Nernst equation, and the unknown concentration can be calculated. Experimental data are given for solutions of sodium choride, sodium nitrate, acidified sodium silicate and sodium hydroxide, lithium chloride, and phosphoric acid. Metal ions (e.g., Al³⁺, UO₂²⁺, Fe³⁺, Th⁴⁺) that interfere by forming complexes with fluoride can be precomplexed with phosphoric acid. The relative error is estimated at 10%, and the relative standard deviation is less than 5% over the concentration range 10^-5–10^-4M fluoride.     

Échelle d'acidité dans l'hydrogénodifluorure de potassium (KHF2) fondu à 250°C: Détermination électrochimique

In molten potassium hydrogenodifluoride (KHF₂) fully ionized into K⁺ and HF₂^?, at 250°C, the HF₂^? ion is slightly dissociated according to the equilibrium: HF₂^?HF+F^?. This is a solvent acid base equilibrium, HF being the strongest acid and F^? the strongest base in this system. By means of a voltammetric study we showed that the hydrogen electrode may be used as an acidity indicator electrode in the whole acidity range of the melt. By analysis of the equilibrium potential variation in acidic and in basic media, the HF₂^? dissociation constant (melt autoprotolysis constant): K_D = {HF} {F^?} was determined The experimental value: K_D=10^?2.05 mol² kg^?2 is compared with a calculated one, issued from thermodynamic data. Results obtained with other electrodes (LaF₃ monocrystal electrode and copper electrode) were discussed and compared with those obtained with the hydrogen electrode.     

Colorimetric and fluorometric sensing of the Lewis acidity of a metal ion by metal-ion complexation of imidazo[1,2-a]pyrazin-3(7H)-ones

Metal-ion complexation of 7-benzylimidazo[1,2-a]pyrazin-3(7H)-one derivative (2) with Li⁺, Mg²⁺, Ca²⁺, Ba²⁺, Sc³⁺, and La³⁺ in acetonitrile resulted in successive modulations of its UV/vis absorption and fluorescence spectroscopic properties. This result indicates that 2 can be used as a colorimetric and fluorometric sensor of the Lewis acidity of a metal ion in aprotic solutions. The hypothesis that the metal-ion complex has a O10(imidazopyrazinone)-metal ion bond was supported by X-ray crystallographic analysis and quantum chemical calculations for a Li⁺ complex of 7-methylimidazo[1,2-a]pyrazin-3(7H)-one derivative (1).     

Voltammetric study of the oxidation kinetics of 2,4,6,-tribromoaniline in acid medium

The electrochemical oxidation of 2,4,6-tribromoaniline in aqueous sulphuric acid solutions, at different concentrations, and in aqueous 60% perchloric acid solution, at a platinum electrode, has been studied by rotating disc electrode and cyclic voltammetry. An irreversible first oxidation process, controlled by diffusion is observed, in perchloric acid and in sulphuric acid concentration <12.5 M. At higher sulphuric acid concentration, this process is kinetically controlled. The electroactive species is the protonated form Ar−N⁺H₃, which is bielectronically oxidized to [Ar−NH₃]³⁺ cation. Deprotonation of this cation can produce two different species, Ar−N₂₊ H₂ (I) and Ar⁺−N₊H₃ (II). In the latter case (II), the deprotonation occurs from the benzene ring. Hydrolysis of (I) gives 3,5-dibromo-1,2,4-trihydroxybenzene, which is further oxidized to 3,5-dibromo-2-hydroxy-2,5-cyclohexadien-1,4-dione. This charge-transfer equilibrium explains an additional redox pair observed for the first process in cyclic voltammograms. An irreversible second oxidation process corresponding to an initial bielectronic oxidation of (II), in both sulphuric acid at concentration >9.0 M and perchloric acid solutions, is observed. Generally, this second process is kinetically controlled. The rate-determining step of both processes is attributed to the initial charge transfer for the electrode reaction under diffusion control or to an adsorption process, depending on the medium used.     

Adsorption of cesium using mesoporous silica gel evenly doped by Prussian blue nanoparticles

In this paper, a novel mesoporous silica gel evenly doped by Prussian blue nanoparticles (PBMSG) was successfully synthesized by using N,N-dimethylamide as template with a large Barrett-Emmett-Teller (BET) surface area of 505 m²/g and an average pore size of 2.9 nm. The static adsorption experiments showed that the equilibration time of PBMSG for Cs⁺ was about 30 min. The adsorption isotherm of PBMSG for Cs⁺ accorded with Langmuir model and the theoretical maximum adsorption capacity was 80.0 ± 2.9 mg/g. When the initial concentration of Cs⁺ was 1.00 mg/L, the adsorption partition coefficient K_d could reach 3.5 × 10⁴ mL/g After adsorption, Cs⁺ could be eluted by dilute hydrochloric acid (pH 2) with an efficiency of 89.8%, while no K⁺, Fe³⁺, Fe²⁺ was eluted. PBMSG exhibited good selectivity toward Cs⁺ and Rb⁺. In the presence of high concentration of K⁺, the selective adsorption of PBMSG could change the mass ratio of K⁺, Rb⁺ and Cs⁺ from 96.63:0.83:1.00–1.12:0.73:1.00. The separation of Cs⁺ and Rb⁺ from K⁺ with similar concentration (100 mg/g) was realized by column experiment. This indicated that PBMSG was suitable for rapid recovery of low concentration of rubidium and cesium from complex matrixes, such as wastewater and salt lake brine, etc.     

Silver(I)‐Selective PVC Membrane Potentiometric Sensor Based on a Recently Synthesized Calix[4]arene

A new PVC membrane potentiometric sensor for Ag(I) ion based on a recently synthesized calix[4]arene compound of 5,11,17,23‐tetra‐tert‐butyl‐25,27‐dihydroxy‐calix[4]arene‐thiacrown‐4 is developed. The electrode exhibits a Nernstian response for Ag(I) ions over a wide concentration range (1.0×10^?2?1.0×10^?6 M) with a slope of 53.8±1.6 mV per decade. It has a relatively fast response time (5–10 s) and can be used for at least 2 months without any considerable divergence in potentials. The proposed electrode shows high selectivity towards Ag⁺ ions over Pb²⁺, Cd²⁺, Co²⁺, Zn²⁺, Cu²⁺, Ni²⁺, Sr²⁺, Mg²⁺, Ca²⁺, Li⁺, K⁺, Na⁺, NH₄⁺ ions and can be used in a pH range of 2–6. Only interference of Hg²⁺ is found. It is successfully used as an indicator electrode in potentiometric titration of a mixture of chloride, bromide and iodide ions.     

Application de la methode polarovoltrique : Interprétation du dosage de L'acide maléique par des bases organiques en milieu n,n-diméthylformamide

The determination of the voltammetric curves, I=f(E), of maleic acid and of a basic titrant in N,N-dimethylformamide using 3 electrodes allowed interpretation of the polarovoltric titration curves of the neutralization of this acid by sodium methylate and tetrabutylammonium hydroxide. The different variations of the potential between 2 polarized platinum electrodes observed in such a neutralization reaction were identified and a notation was developed so that the origin of the observed potential variations could be established. The signal E_c^H+→S, which is related to the initial acidity, is given by the cathode and corresponds to a jump from the potential of the H⁺ reduction curves to that of the solvent reduction curves. The signal E_A^S→OH- characteristic of the second acidity, is given by the anode and is related to a jump from the potential of the solvent oxidation curves to that of the titrant oxidation curves.The effect of the use of unsymmetrical polarized electrodes (a rotating filament electrode and a large-surface, stationary electrode) on the shape of the titration curve was also examined: a rotating cathode gave rise to a curve resembling an inverted U, whereas a rotating anode gave rise to a curve resembling a deformed M.     

On the E.S.R. spectra of intermediates generated during the electrochemical oxidation of triphenylacetic acid in acetonitrile

The acidity level and the fluoride ion activity are evaluated in anhydrous HF and in water+HF mixtures of HF content higher than 70% by means of R(H) and R(F) functions. For both functions, the potentials of the ferrocene/ferricinium or perylene⁺/perylene²⁺ systems are used as reference potentials. A hydrogen electrode is used for acidity level evaluation and it is shown that the R(H) function decreases (acidity increases) when HF content increases in the mixture; the value of R(H) is ?15.5 for anhydrous hydrogen fluoride (+KF 0.1 M). A lanthanum trifluoride monocrystal electrode is built and used for F^? activity evaluations and it is shown that the R(F) function increases (activity of fluoride ion decreases) when HF content increases, up to R(F)=13.6 for basic anhydrous HF (KF 0.1 M solution).     

A Lead‐Selective Membrane Electrode Based Upon a Phosphorylated Hexahomotrioxacalix[3]Arene

Solvent extraction of a mixture of Pb^II, Mn^II, Fe^III, Co^II, Ni^II and Cd^II in aqueous perchlorate medium by a phosphorylated hexahomotrioxacalix[3]arene (calix‐3) in dichloromethane shows a significant selectivity towards lead ions. The ligand can also be incorporated into a membrane to provide a new lead ion‐selective electrode (Pb^II‐ISE). A plasticized PVC membrane containing 30% PVC, 53.5% ortho‐nitrophenyloctylether (NPOE), 4.5% sodium tetraphenylborate (NaTPB) and 12% ionophore was directly coated on a graphite rod. This sensor gave a good Nernstian response of 29.7 ± 0.7 mV decade^?1 over a concentration range of 1 × 10^?8 – 1 × 10^?4 M of lead ions, independent of pH in the range 3‐7, with a detection limit of 0.4 × 10^?8 M. The dynamic response time of the electrode to achieve a steady potential was very fast and found to be less than 7 s. The selectivity relative to Ag⁺, NH₄⁺, Li⁺, Na⁺, K⁺, Ca²⁺, Sr²⁺, Ba²⁺, Mn²⁺, Fe²⁺, Co²⁺, Ni²⁺, Cu²⁺, Zn²⁺, Cd²⁺, Fe³⁺, La³⁺, Sm³⁺, Dy³⁺, Er³⁺, Y³⁺ and Th⁴⁺ was examined. The electrode exhibits adequate stability with good reproducibility (with a slope of 29.6 ± 1.5 mV for 8 weeks). The characteristics of the sensor are compared with those of a tetraphosphorylated calix[4]arene (calix‐4) based Pb^II‐ISE, reported recently. The electrode was successfully used as an indicator electrode for a potentiometric titration of a lead solution using a standard solution of EDTA. The applicability of the sensor for lead ion measurements in various synthetic samples was also investigated.     

Catalytic effects of metal submonolayers formed by faradaic adsorption on the anodic oxidation of ascorbic acid at a platinum electrode

The catalytic effects of metal ions on the anodic oxidation of ascorbic acid on a Pt electrode in 1 M HClO₄ were studied by linear sweep voltammetry. The anodic peak due to a two-electron oxidation of ascorbic acid shifts to the negative potential side on the addition of Bi³⁺. This indicates the accelerating effect of Bi³⁺ on the oxidation of ascorbic acid. The presence of other metal ions, such as Pb²⁺, Hg²⁺, Tl⁺, Ag⁺ and Sb³⁺, also exerts similar effects. These metal ions were adsorbed on a Pt electrode at underpotentials and the adsorbed metals (denoted as M_ad) still remain on the electrode surface until the electrode potential goes up to and beyond the peak potential of the oxidation of ascorbic acid. On the other hand, metal ions forming no adsorbed layer on Pt, such as Co²⁺, Zn²⁺, Fe³⁺ and Ni²⁺, exhibit no catalytic effect. These facts suggest that the presence of a M_ad on Pt is essential for the promotion of the anodic oxidation of ascorbic acid. However, there is a difference in the catalytic action among the M_ad, for example, Cu_ad, Cd_ad, In_ad, Sn_ad and Mo_ad display no catalytic action.The catalytic activity depends on the degree of surface coverage by the M_ad. The maximal effect of the M_ad is attained in the submonolayer region. The effects of metal ions were discussed on the basis that the M_ad plays its major role in the removal of the adsorbed ascorbic acid occupying active sites on the electrode surface, and provides effective sites for the activation of adjacent water molecules. Furthermore, from the ¹³C NMR spectra for the oxidation products, the adsorbed water on the M_ad appears to function by promoting the subsequent hydration steps, following the electron-transfer step of ascorbic acid.     

Interaction of the Fungal Metabolite Harzianic Acid with Rare-Earth Cations (Pr3+, Eu3+, Ho3+, Tm3+)

Rare-earth elements (REEs) are in all respect a class of new contaminants that may have toxic effects on organisms and microorganisms and information on their interactions with natural ligands should be of value to predict and control their diffusion in natural environments. In the current study, we investigate interactions of tripositive cations of praseodymium, europium, holmium, and thulium with harzianic acid (H₂L), a secondary metabolite produced by selected strains of fungi belonging to the Trichoderma genus. We applied the same techniques and workflow previously employed in an analogous study concerning lanthanum, neodymium, samarium, and gadolinium tripositive cations. Therefore, in the current study, HPLC-ESI-HRMS experiments, circular dichroism (CD), and UV-Vis spectrophotometric absorption data, as well as accurate pH measurements, were applied to characterize bonding interactions between harzianic acid and Pr³⁺, Eu³⁺, Ho³⁺, and Tm³⁺ cations. Problems connected to the low solubility of harzianic acid in water were overcome by employing a 0.1 M NaClO₄/(CH₃OH + H₂O 50/50 w/w) mixed solvent. For Pr³⁺, Ho³⁺, and Tm³⁺, only the mono complexes PrL⁺, HoL⁺, and TmL⁺ were detected and their formation constant determined. Eu³⁺ forms almost exclusively the bis complex EuL2− for which the corresponding formation constant is reported; under our experimental conditions, the mono complex EuL⁺ is irrelevant. Combining the results of the present and previous studies, a picture of interactions of harzianic acid with rare-earth cations extending over 8 of the 17 REEs can be composed. In order to complement chemical information with toxicological information, a battery of bioassays was applied to evaluate the effects of praseodymium, europium, holmium, and thulium tripositive cations on a suite of bioindicators including Aliivibrio fischeri (Gram-negative bacterium), Raphidocelis subcapitata (green alga), and Daphnia magna (microcrustacean), and median effective concentration (EC50) values of Pr³⁺, Eu³⁺, Ho³⁺, and Tm³⁺ for the tested species were assessed.     

Synthesis and characterisation of a novel bi-nuclear copper2+ complex and its application as electrode-modifying agent for simultaneous voltammetric determination of dopamine and ascorbic acid

A new binuclear complex of copper²⁺, [LCu²⁺(CH₃COO)₂Cu²⁺L](CH₃COO)₂ where L is N,N-bis(phthalimide)ethylenediamine, was synthesised and characterised. The complex ion [LCu²⁺ (CH₃COO)₂Cu²⁺L]²⁺ was encapsulated into ZSM-5 zeolite and used to modify the surface of the glassy carbon electrode. This modified electrode, in a phosphate buffer solution at pH 7.0, exhibited an oxidation potential for dopamine (DA) and ascorbic acid (AA) at electrode potentials of 0.230 V and ?0.090 V vs. Ag/AgCl respectively, a separation of 0.320 V. The electro-oxidation of DA or AA on the modified electrode is independent of each other. No interference was observed from Na⁺, K⁺, Cl^?, SO ₄ ^2? , Mg²⁺, Ca²⁺, Zn²⁺, Fe²⁺, and glucose. The detection limits obtained were 2.91 × 10^?7 M for DA and 3.5 × 10^?7 M for AA.     

DDB liver drug as a novel ionophore for potentiometric barium (II) membrane sensor

Dimethyl-4,4-dimethoxy-5,6,5′,6′-dimethylene dioxy biphenyl-2,2-dicarboxylate (DDB) liver drug is used as a novel ionophore in plasticized poly (vinyl chloride) (PVC) matrix membrane sensors for barium ions. Optimum performance characteristics are displayed by membrane sensor incorporating DDB ionophore, potassium tetrakis(4-chlorophenyl)borate as lipophilic salt, and o-nitrophenyloctyl ether as plasticizer. The sensor exhibits a linear response over the concentration range 10⁻¹-10⁻⁵ mol l⁻¹ BaCl₂ with a Nernstian slope of 30 mV per decade and high selectivity towards Ba²⁺ with respect to Li⁺, Na⁺, K⁺, Rb⁺, NH₄⁺, Mg²⁺, Ca²⁺, Sr²⁺, Mn²⁺, Co²⁺, Ni²⁺, Cd²⁺, Al³⁺, La³⁺, and Ce³⁺ ions. The sensor response is stable over a wide pH range (4-9) and the lifetime is about 2 months. The proposed sensor is successfully applied to the determination of Ba²⁺contents of some rocks.     

Bismuthiol II as an analytical reagent

Summary The estimation of bismuth by the reagent Bismuthiol II is studied critically. The effect of acidity, reagent concentration and interfering ions are given in detail. The maximum acidity that may be tolerated for the complete precipitation of bismuth is 0.3 N in nitric acid, 0.5 N in hydrochloric acid and 1N in sulphuric acid. Higher acidity than 0.1 N decomposes the reagent present in excess. In 0.1 N nitric acid bismuth has been separated from a number of ions like Al³⁺, Cr³⁺, Th⁴⁺, rare earths, Zr⁴⁺, Ti⁴⁺, UO₂ ²⁺, Be²⁺, Mn²⁺, Co²⁺, Ni²⁺, Mg, alkalis and alkaline earths, SO₄ ^2–, Cl^–, C₂O₄ ^2–- and from Fe²⁺ and Ce³⁺ in 0.1 N hydrochloric acid. In presence of a citrate or a tartrate it can be separated from As³⁺, Ce⁴⁺, MoO₄ ^2–- and WO₄ ^2–-at p_H 1.5 to 2.5. When Hg²⁺, Pb²⁺, Pd²⁺, Cd²⁺, Cu²⁺, Ag⁺ and Tl⁺ are present they are first precipitated by the reagent at p_H 6 to 8 in presence of a citrate or a tratrate and the bismuth is estimated gravimetrically in the acidified filtrate. Ions as F^– and PO₄ ^3– that form insoluble compounds with bismuth, Sb³⁺ and Sn²⁺ that form less soluble compounds with the reagent and Fe³⁺, VO₃ ^–, CrO₄ ^2–, AsO₄ ^3– that act as oxidising agents, interfere.     

Electrochemical reduction of uranium(VI) in nitric acid-hydrazine solution on glassy carbon electrode

Electrochemical reduction of U(VI) in nitric acid-hydrazine solution is greatly influenced by the concentration of nitric acid. In low acidity nitric acid solution such as 0.1M (M=mol/dm³) HNO₃, U(VI) was firstly reduced to U(V) and then partially reduced to U(IV). In high acidity nitric acid solution, e.g., 3-6M HNO₃, an electrode process of two-electron transfer was involved in the reduction of U(VI). A higher U(IV) yield could be achieved in nitric acid solution with higher concentration. Hydrazine was very effective in suppressing the reduction of concentrated nitric acid, and the optimal concentration of hydrazine added was 0.075 to 0.15M in 6M HNO₃ This revised version was published online in July 2006 with corrections to the Cover Date.     

Evaluation of electrodes coated with metal hexacyanoferrate as amperometric sensors for nonelectroactive cations in flow systems

Two electrodes modified with either nickel or cupric hexacyanoferrate films were evaluated and compared as sensors for nonelectroactive cations in a flow-injection system. Both gave responses for group 1A and ammonium ions, but only the electrode modified with cupric hexacyanoferrate was sufficiently stable for use in flowing solutions. This electrode responded to K⁺, NH, Rb⁺, and Cs⁺ ions rather selectively. Within this group, the selectivity could be controlled from general to almost specific toward Cs⁺ by the potential at which the electrode was poised. The electrode was compatible with a mobile phase of dilute nitric acid commonly used in ion chromatography, and chromatographic detection limits of 2 × 10⁻⁷ M and linear responses over two decades were obtained. The electrode was applied to the ion chromatographic analysis of K⁺ and NH in urine and K⁺ in blood serum samples.