Automatic coulometric titration of acids

An apparatus is described for the automatic titration of acids by the constant current coulometric technique. The generator electrodes comprise a platinum cathode and a silver/silver bromide anode. The increase in pH resulting from the reduction of hydrogen ion at the cathode is indicated by a glass electrode, in conjunction with a Beckman Automatic Titrator which automatically monitors the titration and interrupts the generating current when the equivalence point pH is reached. Quantities of hydrochloric acid in the neighborhood of 0.12 millimole in 50 ml were titrated with a mean error of ^-0.07% and an average deviation from the mean of ±0.15%. The technique is applicable to any strong or weak acid, and to acid mixtures, provided that no substance is present which is either reducible at the platinum cathode or reactive in any way at the silver anode.     

Carbon Paste Electrode Modified with Functionalized Nanoporous Silica Gel as a New Sensor for Determination of Silver Ion

A new dipyridyl‐functionalized silica gel (DPSG) was synthesized. The potentiometric response of silver ion was investigated at a carbon paste electrode chemically modified with functionalized nanoporous silica gel. The electrodes with a DPSG proportions of 10.1% (w/w), showed very stable potential. Calibration plots with Nernstian slopes for Ag⁺ were observed, 58.7 (±0.9) mV decade^?1, over a wide linear range of concentration (5.0×10^?7 to 1.0×10^?1 M). The electrode has a detection limit of 1.0×10^?7 M. The selectivity coefficients measured by the match potential method in acetate buffer, pH 5.5, were investigated. The electrode has fast response time, high performance, high sensitivity in wide cation activity ranges, and good long term stability (more than 6 months). The method was satisfactory and used to determine the concentration of silver ion in waste waters contaminated by this metal.     

A comparative study of chromium(III) ion-selective electrodes based on N,N-bis(salicylidene)-o-phenylenediaminatechromium(III)

A comparative study was made between three types of Cr(III) ion-selective electrodes: PVC membrane electrode, silver coated electrode and modified carbon paste electrode based on N,N-bis(salicylidene)-o-phenylenediaminatechromium(III) complex (Crsalophen). As anticipated, electrodes with a solid contact, rather than a traditional liquid inner contact, give lower detection limits because of diminished ion fluxes. Often, however, ill-defined solid contact gives rise to instabilities and interferences by oxygen gas. The carbon paste electrode provides a more sensitive and stable device than that afforded by PVC and coated electrodes. The best performance was obtained by an electrode based on the paste containing 3.5 wt% Cr-complex, 48.5% graphite plasticized with a mixture of 24.0 wt% tris(2-ethylhexyl) phosphate (DOPh) + 24.0 wt% dioctyl sebacate (DOS). The sensor has a linear dynamic range of 7.5 × 10^-6 to 1.0 × 10^-2 M, with a Nernstian slope of 20.1 ± 0.6 mV decade^-1, and a detection limit of 1.8 × 10^-6. It has a short response time of a bout 8 s and is applicable in a pH range of 4.5-7.7. It was successfully used as an indicator electrode in potentiometric titration of Cr(III) with EDTA and in determination of Cr(III) in water samples and chromium in (Crsalophen).     

Thiocyanate-Selective PVC Membrane Electrode Based on Copper and Nickel Complexes of Para-tolualdehydesemicarbazone as Carrier

Abstract

Copper(Cu) and nickel(Ni) complexes of para-tolualdehydesemicarbazone (pTSC) were used as carrier for an thiocyanate ion–selective electrode. The Ni(II)pTSC demonstrated higher selectivity for thiocyanate ions with better performance than Cu(II)pTSC as carrier. The electrode shows a Nernstian slope of 58.8 ± 0.3 mV decade^?1 with improved linear range of 1 × 10^?2 to 1 × 10^?7 M and a low detection limit of 1.25 × 10^?7 M in the pH range of 3–10, giving a relatively fast response and reversibility within 10 s. The selectivity coefficient was calculated using matched potential method. The electrode worked well for nearly 3 months. The response mechanism is discussed by UV-visible spectroscopic technique. The electrodes were used in potentiometric titration of thiocyanate with silver nitrate. Further, the electrode was successfully applied to determine the thiocyanate content in physiological fluids.     

Silver nanoparticle assemblies supported on glassy-carbon electrodes for the electro-analytical detection of hydrogen peroxide

Electrochemical detection of hydrogen peroxide using an edge-plane pyrolytic-graphite electrode (EPPG), a glassy carbon (GC) electrode, and a silver nanoparticle-modified GC electrode is reported. It is shown, in phosphate buffer (0.05 mol L^–1, pH 7.4), that hydrogen peroxide cannot be detected directly on either the EPPG or GC electrodes. However, reduction can be facilitated by modification of the glassy-carbon surface with nanosized silver assemblies. The optimum conditions for modification of the GC electrode with silver nanoparticles were found to be deposition for 1 min at –0.5 V vs. Ag from 5 mmol L^–1 AgNO₃/0.1 mol L^–1 TBAP/MeCN, followed by stripping for 2 min at +0.5 V vs. Ag in the same solution. A wave, due to the reduction of hydrogen peroxide on the silver nanoparticles is observed at –0.68 V vs. SCE. The limit of detection for this modified nanosilver electrode was 2.0×10^–6 mol L^–1 for hydrogen peroxide in phosphate buffer (0.05 mol L^–1, pH 7.4) with a sensitivity which is five times higher than that observed at a silver macro-electrode. Also observed is a shoulder on the voltammetric wave corresponding to the reduction of oxygen, which is produced by silver-catalysed chemical decomposition of hydrogen peroxide to water and oxygen then oxygen reduction at the surface of the glassy-carbon electrode.     

Determination of carbonate in the presence of hydroxide: Part 2. Evidence for the existence of a novel species from first-derivative potentiometric titration curves

An additional maximum has been detected in the differential potentiometric titration of mixtures of sodium hydroxide and sodium carbonate with hydrochloric acid using pH-responsive glass electrodes. This maximum is not detected under the following conditions: if ethanol (mol fraction >0.2) or t-butanol (mol fraction >0.1) is added; when the temperature is raised above 55°C; when hydrogen gas is bubbled through the solution; if a hydrogen gas electrode is used instead of the glass electrode; when carbonic anhydrase is added. The additional feature is ascribed to a new species HCO₃·H₂CO₃^- formed in solutions by a slow reaction.     

Mixed Aza‐Thioether Crowns Containing a 1,10‐Phenanthroline Sub‐Unit as Neutral Ionophores for Silver Ion

Three different recently synthesized aza‐thioether crowns containing a 1,10‐phenanthroline sub‐unit (L1–L3) and a corresponding acyclic ligand (L4) were studied to characterize their abilities as silver ion ionophores in PVC‐membrane electrodes. Novel conventional silver‐selective electrodes with internal reference solution (CONISE) and coated graphite‐solid contact electrodes (SCISE) were prepared based on one of the 15‐membered crowns containing two donating S atoms and two phenanthroline‐N atoms (L1). The electrodes reveal a Nernstian behavior over wide Ag⁺ ion concentration ranges (1.0×10^?5?1.0×10^?1 M for CONISE and 5.0×10^?8?4.0×10^?2 M for SCISE) and very low limits of detection (8.0×10^?6 M for CONISE and 3.0×10^?8 M for SCISE). The potentiometric response is independent from pH of the solution in the pH range 3.0–8.0. The electrodes manifest advantages of low resistance, very fast response and, most importantly, good selectivities relative to a wide variety of other cations. The electrodes can be used for at least 2 months (for CONISE) and 4 months for (SCISE) without any appreciable divergence in potentials. The electrodes were used as an indicator electrode in the potentiometric titration of Ag⁺ ion and in the determination of silver in photographic emulsions and in radiographic and photographic films.     

Highly Selective Sodium Ion Responsive Glass Electrode

Abstract

A highly selective electrode glass for the measurement of sodium ion activity is described. The corresponding glass electrodes shew a selectivity for sodium over hydrogen ions of about 10 and for sodium over potassium ions of about 10,000.     

Potentiometric determination of tetraphenylborate with silver nitrate

A simple potentiometric method for determining the tctraphenylborate ion (C₆H₅)₄B- contents of organic amine tetraphcnylborate salts has been developed. The compounds arc dissolved in 1 : 1 aqueous acetone; and the solution is buffered at a pH of 5 with 3M acetic acid and 3M sodium acetate. The resulting solution is then titrated with 0.06N aqueous silver nitrate, using a silver indicating electrode, a glass reference electrode and a Beckman pH meter to indicate EMF changes. Platinum electrodes in conjunction with the Malmstadt automatic titrator can also be used. Good end-points were obtained for all structure types except the primary aromatic amine borates. Values for (C₆H₅)₄B- found were within 2% of theory for 22 compounds so analyzed.     

Voltammetric Determination of Azidothymidine Using Silver Solid Amalgam Electrodes

A new simple and direct electroanalytical method was developed for the determination of azidothymidine in commercial pharmaceutical preparations. It is based on differential pulse voltammetry at silver solid amalgam electrode with polished surface (p‐AgSAE) or surface modified by mercury meniscus (m‐AgSAE). The electroreduction of azidothymidine in basic media at these electrodes gives rise to one irreversible cathodic peak. Its potential in 0.05 mol L^?1 borate buffer, pH 9.3 at ca. ?1050 mV is comparable to that using hanging mercury drop electrode (HMDE). Achieved limits of quantitation are in the 10^?7 mol L^?1 concentration range for both amalgam electrodes. According to the procedure based on the standard addition technique, the recoveries of known amounts of azidothymidine contained in pharmaceutical preparations available in capsules were 101.4±1.8% (m‐AgSAE), 100.3±3.5% (p‐AgSAE) and 102.0±1.0% (HMDE) (n=10). There was no significant difference between the values gained by proposed voltammetric methods and the HPLC‐UV recommended by the United States Pharmacopoeia regarding the mean values and standard deviations.     

A Membrane Sensor for Selective Determination of Bisacodyl in Tablets

A novel PVC membrane sensor for bisacodyl based on bisacodyl‐phosphotungstate ion pair complex was prepared. The influence of membrane composition (i.e. percent of PVC, plasticizer, and ion‐pair complex), pH of test solution and foreign cations on the electrode performance were investigated. The optimized membrane demonstrates Nernstian response (60.3 ± 2.1 mV per decade) for bisacodyl cations over a wide linear range from 8.0 × 10^?5 to 5.0 × 10^?3 M at 25 °C. The potentiometric response is independent from the pH of the solution in the pH range of 1.5–3.5. The proposed sensor has the advantages of easy preparation, good selectivity, fast response time, and small interferences from hydrogen ions. It was successfully used for determination of bisacodyl in tablets, and the results obtained with the electrode were in good agreement with the official chromatographic method.     

Needle-like IrO/Ag combined pH microelectrode

In this study, a combined pH microelectrode has been developed consisting of an indicator electrode made of IrO₂ prepared using the polymeric precursor method and deposited in a platinum microwire. This electrode was mounted inside a stainless steel needle, the external surface of which was painted with conductive silver ink which is used as reference electrode. This device was compared with a conventional glass electrode, and the results presented linear behavior in the pH range from 2.0 to 12.5, in Na⁺ and K⁺ solutions, exceeding glass electrodes in the alkaline range. The sensitivity was 56.9 ± 0.2 mV pH^? 1 and using ANOVA test we conclude that the electrode is not sensitive to the presence of alkaline cations such as Li⁺, Na⁺ or K⁺. Finally, the response time (t₉₅) was 4.9 to 9.0 s depending on the solution pH. The combined pH microelectrode can be used several times and, after three years, continues to have a response similar to that of a freshly produced one.     

Liquid-junction potential effects in measurements of sodium ion activity in unbuffered aqueous solutions

Measurements of pNa with the glass-membrane ion-selective electrode in unbuffered aqueous solutions require care in the constitution of the measuring cell, because of varialbe liquid-junction potentials at the salt bridge. Of the bridge electrolytes tested, with calomel or silver/silver chloride reference electrodes, 3 M ammonium chloride in agar-agar gel was most satisfactory. Calibration graphs were of almost theoretical slope in the range 10^?3?5 × 10^?1 M sodium ion.     

New Potentiometric Sensor for the Determination of Bromate Ion in Drinking Water

The characteristics, performance and application of ion‐selective electrodes for bromate ion based on rhodamine B and tetrahexyl ammonium bromide as electrode‐active substances are described for the first time. These electrodes respond with sensitivities of (58.0±1.0) and (61.0±2.0) mV decade^?1 over the range 1.0×10^?8–1.0×10^?2 mol l^?1 at pH 4–9 and 4–8 and a detection limit of 6.0×10^?8 and 4.0×10^?8 mol l^?1 for rhodamine B and tetrahexyl ammonium bromide sensors, respectively. The electrodes are easily constructed at a relatively low cost, have a fast response time and can be used for a period of 3 months without any considerable divergence in potential. The proposed sensors displayed good selectivity for bromate ion in the presence of several substances and inorganic anions. Sensors were used for the direct assay of bromate ion in drinking water samples.     

Investigation of Silver(I) Ion Sensing Property of Ruthenium(II) Thiosemicarbazone Complex Using Coated Graphite and Polymeric Membrane Electrodes

A ruthenium(II) complex [Ru(PPh₃)₂(pytsc)₂] {Hpytsc = pyridine‐2‐carbaldehydethiosemicarbazone, (C₅H₅N)C²(H)=N³‐N²(H)‐C¹(=S)N¹H} has been used as an ion carrier for the selective determination of silver(I) ions in solution. Silver(I) ion‐selective coated graphite based (CGE) and PVC polymeric membrane based (PME) electrodes exhibit Nernstian slope for silver(I) ions over a wide concentration range from 1.0 × 10⁻¹ M to 5.0 × 10⁻⁶ M (with CGE) and 1.0 × 10⁻¹ M to 2.0 × 10⁻⁵ M (with PME). The working pH range of these electrodes has been found to be from 1.2 to 7.2 for CGE and 2.2 to 6.5 for PME. The proposed CGE sensor exhibits better analytical features like sensitivity and selectivity towards different secondary ions in comparison to the corresponding PME with no interference from mercury(II) ions . These electrodes also act as indicator electrodes in potentiometric titration and have been successfully used for the determination of silver content in solution of real samples (1 gm dissolved in 100 mL of dilute nitric acid) such as silver ornaments and thin silver foils. Silver content determined by the use of ion selective electrode was found to vary in the concentration range from 1.20 x 10⁻² M to 7.45 x 10⁻² M and results were found to be comparable with those obtained from the traditional volumetric method of analysis. It is the first report of a metal‐ligand complex used as an ion carrier in ion selective electrode, which is selective for a metal ion other than the one used in the complex.     

Anion-selective properties of alkali metal-free lead phosphate glasses containing silver chloride and their application in an ion-selective electrode

Alkali-free lead phosphate glasses containing silver chloride have been developed for anion responsive sensors. From measurements of the final glass compositions by electron probe microanalysis, it became clear that some of chloride ions in the glass bulk were not volatilized during the glass melting process. Compared with phosphate glasses containing silver oxide, the new glass electrodes containing silver chloride could respond more rapidly, although the response behaviour for anionic species were similar. From the electrode potential vs. time curve for the anionic species, the potential rapidly reached equilibrium when these concentrations varied from 10^?5 to 10^?2 M. The response times, t₉₅, to thiocyanate of the new glass electrode and the phosphate glass electrode containing silver oxide were 30 and 110 s, respectively. Moreover, the response time required for an initial potential change with a concentration jump of thiocyanate with the new glass electrode was found to be independent of the membrane thickness within about 2 mm and of the measuring temperature between 15 and 40°C. It is concluded that the diffusion process of species such as silver ion in the glass bulk does not take part in the initial part of the response behaviour.     

Behaviour of glass and silver/silver chloride electrodes in some non-aqueous media

The stability of glass electrodes and silver/silver chloride electrodes in isopropanol, methyl ethyl ketone and a mixture of equal volumes of these has been measured, and found to be of the order of 1 mV. Changes in the electrode potential due to addition of water and to addition of supporting electrolyte have been investigated. The glass electrode responded reversibly to hydrogen ion activity changes in buffers of picric acid-tetraethylammonium picrate, and perchloric acid-di-isopropylamine. The autoprotolysis constants at 25 degrees were calculated to be 2 x 10(-19) in isopropanol, 2 x 10(-26) in the ketone and 8 x 10(-19) in the mixture.     

Sodium Ion-Selective Chalcogenide Glass Electrodes

Abstract

Analytical characteristics and sensing mechanism of sodium ion-selective electrodes based on NaCl-Ga₂S₃-GeS₂ glasses have been investigated. Chalcogenide glass electrodes containing 10 mol.% NaCl in the membrane showed near-Nernstian response in the concentration range from 10-³ to 1 M sodium nitrate solution. These sensors were superior to the conventional pNa oxide glass electrodes in selectivity in the presence of hydrogen ions and in Na⁺ ion sensitivity in fluoride media. Prolonged solution treatment for several days reduces, however, the detection limit of the sensors and the slope of the electrode response. Ionic processes at the membrane surface have been investigated using XPS technique and ²²Na tracer measurements. It was shown that sodium ion-exchange governed Na⁺ ion response of chalcogenide     

Determination of betamethasone sodium phosphate in pharmaceuticals by potentiometric membrane sensor based on its lidocaine ion pair

Betamethasone sodium phosphate (BMNaP) has been employed as an electroactive material in the design of an ion-selective electrode (ISE). The electrode incorporates PVC membrane with betamethasone sodium phosphate-lidocaine ion pair complex. The influences of membrane composition, temperature, pH of the test solution, and the interfering ions on the electrode performance were investigated. The sensor exhibits a Nernstian response for betamethasone sodium phosphate ions over a relatively wide concentration range (1.0 × 10^?1 to 1.0 × 10^?5 M) with a slope of 28.4 ± 0.9 mV per decade at 25°C. It can be used in the pH range 4.0–10.0. The isothermal temperature coefficient of this electrode amounted to ?0.0008 V/°C. The membrane sensor was successfully applied to the determination of betamethasone sodium phosphate in pharmaceutical products.     

Determination of Hydrogen Single Ion Activity Coefficients in Aqueous HCl Solutions at 25°C

The single ion activity coefficients of hydrogen and chloride ions in aqueous HCl solutions have been estimated at 25°C at concentrations up to 1 mol-kg^–1, using potentiometric measurements with ion-selective electrodes and appropriate calibration procedures. Two methods are described for an internal calibration of the electrodes in the extended Debye–Hückel concentration range. The results are compared to the conventional pH calibration with external buffer solutions. Since the latter calibration method does not account for the liquid junction potential E _J which arises at the reference electrode, the resulting activity coefficients are quite different in HCl solutions of higher concentration. These differences between internal and external calibration decrease significantly, when a correction for E _J is introduced into the conventional pH calibration. Hence, in solutions of higher ionic strength the accuracy of the conventional pH electrode calibration using buffer solutions is very limited, when exact H⁺ activities are required. The consistency of the results indicates that the liquid junction potentials in the examined systems calculated by the Henderson/Bates approximation are of reasonable precision.