TRIEN and TETREN as titrants in potentiometry with a silver indicator electrode

The possibility of application of triethylenetetramine (TRIEN) and tetraethylenepentamine (TETREN) in metal titrations with the silver electrode as indicator was investigated. Copper, cadmium and zinc were determined in the presence of calcium, magnesium, aluminium and iron(III) in the concentration range from 0.02 to 2mM. The errors did not exceed 1%. On a similar basis copper and iron may be successfully determined in their mixtures under carefully controlled conditions. Copper is titrated with TRIEN, and both metals with EDTA at pH 7.5-8.0 in sulphosalicylate medium. The results obtained were in good agreement with those theoretically predicted.     

Behavior of a ruthenium-titanium oxide film electrode (RTOE) in potentiometry

Polyfunctional properties of a ruthenium-titanium oxide electrode in the absence of polarization and the possibility of its application to the potentiometric detection of iron(III) titration with an ethylenediaminetetraacetic acid solution, acid-base and redox reaction types have been demonstrated. Mechanisms of reactions determining the potentials of electrodes of this type have been investigated by a number of methods.     

A photo-cured coated-wire calcium ion selective electrode for use in flow injection potentiometry

A multi-sensor cell containing a new photo-cured calcium ion selective electrode sensor is reported. Four membranes containing different components are prepared to determine the one with optimum selectivity and sensitivity. This is shown to consist of the N,N,N',N'-tetracyclohexyl-3-oxapentanediamide ligand (ETH 129) as the ionophore, 2-nitrophenyl octyl ether as the plasticiser and tetradodecyl ammonium tetrakis(4-chlorophenyl) borate as the lipophilic additive. The photo-curing process is applied after coating a thin membrane on a silver wire as substrate transducer to produce the calcium sensor. The curing process is demonstrated to be faster (1 min) than previous methods and does not require a nitrogen atmosphere for reproducible production of membrane response characteristics. Four sensors constructed with the identical optimum membrane are shown to function reproducibly in a multi-sensor flow-through cell using the steady-state mode of flow measurement, and an average calibration slope of 28.5+/-0.4 mV change per activity decade is observed over a log-linear concentration range between 0.01 and 10 mM. The sensor is also shown to respond to changes in pH. Hence, in the flow injection potentiometric mode, a constant carrier buffer composition of pH 8.3 is required for accurate potentiometric calcium determinations. The sensor is used to determine calcium in water samples by flow injection potentiometry. The accuracy of the electrode determination relative to atomic absorption spectroscopy was in the range 5-9% for three different water samples.     

Stopped-flow kinetic determination of aluminum in Chinese tea leaves by fluoride ion-selective electrode potentiometry

A stopped-flow kinetic potentiometric method for the determination of aluminum is described, based on monitoring the reaction between aluminum and fluoride at pH 3.0 using fluoride ion-selective electrode. The initial rate of the reaction is proportional to the concentration of aluminum present in the solution. The method is simple and rapid and has been applied to the determination of aluminum in Chinese tea leaves after microwave digestion.     

Determination of nitrite in animal feed by spectrophotometry or potentiometry with a nitrite-selective electrode

A method is presented for the extraction and quantitation of nitrite in animal feed. The extracting solution consists of ammonium sulfate at pH 8.5. The recovery of nitrite usually exceeds 90%. Nitrite is quantified with a nitrite-selective electrode or by a modified Griess reaction. The effects of extraction time on recovery are compared for acidic and alkaline extracting solutions.     

The decomposition of nitramide in 80% dimethyl sulfoxide-20% water solvent system

The base-catalyzed decomposition of nitramide in 80% dimethyl sulfoxide (DMSO)-20% water (by weight) has been investigated. The Brønsted catalysis law for a series of seven carboxylate ions in this mixed solvent is compared with the Brønsted relationship previously obtained in water. There is no real change in mechanism upon changing the solvent from pure water to this aqueous mixture, which is approximately equimolar in the two-solvent components.     

Flow injection potentiometry for enzymatic assay of cholesterol with a tungsten electrode sensor

Flow injection potentiometry (FIP) for the enzymatic determination of cholesterol is reported. The assay utilises a combination of three enzymes: cholesterol esterase (CE), cholesterol oxidase (COD) and peroxidase (POD). The method is developed by the use of a tungsten wire electrode as a sensor vs. Ag/AgCl in conjunction with a redox mediator ferrocyanide. CE converts esterified cholesterol to free cholesterol, which is then oxidised by COD with hydrogen peroxide as product. Ferrocyanide is converted to ferricyanide by hydrogen peroxide, catalysed by POD, and the tungsten electrode responds to the ratio of ferricyanide to ferrocyanide. Flow injection potentiometry gave well-defined peaks for cholesterol samples with a fast response (30 s). Linear calibration was obtained from 0.05 to 3.0 mM cholesterol, with a slope of 60.2 mV/decade change in cholesterol concentration, and detection limit 0.01 mM cholesterol (S/N=3). Repeatability was 3% (CV). Interferences from commonly found species were shown to be negligible. The sensor cell is simple to construct, and it was free from surface contamination problems over long periods of use. The application of the sensor for the determination of serum cholesterol was demonstrated.     

Determination of fluoride in natural waters by ion-selective electrode potentiometry after co-precipitation with aluminium phosphate

The most effective conditions for masking aluminium in the determination of mug/1. levels of fluoride in water by ion-selective electrode potentiometry after co-precipitation with aluminium phosphate have been re-examined. The effectiveness of citrate for masking aluminium increases with pH, and up to 1.5 x 10(-2)M aluminium can be masked quantitatively at pH 8.5. Fluoride (5-100 mug in 500 ml of sample solution) is quantitatively co-precipitated at pH 4.7 with approximately 90 mg of aluminium phosphate. After dissolution of the precipitate and adjustment of the solution to pH 8.5 with TISAB, the fluoride content can be measured with a fluoride ISE. The method is simple and rapid, and is suitable for the determination of trace amounts of fluoride in various water samples.     

Determination of cadmium in indium phosphide by electrothermal atomic absorption spectrometry and ion-selective electrode potentiometry

 Two independent methods for the determination of cadmium in cadmium-doped indium phosphide have been developed. Electrothermal atomic absorption spectrometry (ETAAS) utilized both platform atomization and a chemical modifier composed of magnesium nitrate and orthophosphoric acid. As the matrix mass was found to influence the cadmium sensitivity, matrix matched calibration standards were necessary. The detection limit (3s_B) is 0.20 μg/g for a 100 mg sample. The electrochemical method employed a solid-state cadmium sulfide-silver sulfide electrode as potentiometric sensor. An excess of indium (III) influenced the electrode response. A preliminary chelation-extraction of indium with acetylacetone at pH 5.0 in acetate buffer overcame the interference. The detection limit of the ISE-potentiometric method is 10 μg/g for a 200 mg sample. Two indium phosphide single crystals grown from melts doped with cadmium sulfide or cadmium telluride were analyzed for their cadmium content. Received: 27 August 1996/Revised: 27 January 1997/Accepted: 30 January 1997     

The electrode responses of a tungsten bronze electrode differ in potentiometry and voltammetry and give access to the individual contributions of electron and proton transfer

A tungsten wire covered with Na_0.75WO₃ acts in potentiometry as a reversible pH electrode having a pH dependent open-circuit potential E_ocp with nernstian slope. The mid-peak potential E_mp of cyclic voltammograms also depends on pH. At low pH (e.g., pH 2) and slow scan rates (e.g., 2 mV s^–1) the voltammetric response is almost completely reversible. At higher pH and faster scan rates, the voltammetric systems exhibit features of increasing irreversibility. Under the conditions of reversibility, the E_ocp and E_mp differ significantly. E_ocp is determined by the proton transfer at the electrode surface; whereas E_mp is determined by the electron transfer equilibrium tungsten(VI)/tungsten(V) and the proton transfer at the electrode surface. The difference between E_ocp and E_mp provides the individual thermodynamic contributions of electron and proton transfer to the overall pH dependent redox electrode. This is the first time that both contributions can be separated for an insertion electrochemical system (thin surface layer). It is also shown for the first time that the mechanism of an ion-sensitive electrode can differ in potentiometry and voltammetry.     

Determination of polar organic compounds by potentiometry at an anodically pretreated nickel oxide electrode

The open-circuit potential drop of an oxidatively pretreated nickel electrode in 0.1 M NaOH was used to develop a technique for the determination of alcohols, amino acids, carbohydrates, etc., in aqueous solution. The electrode pretreatment consisted of the formation of nickel hydrated oxides on the electrode surface with an oxidation state > 2. Both electrochemical and chemical pre-oxidation of the electrode surface were examined. The analytical signal was the enhancement of the potential drop corresponding to analyte concentration. The analytical signal was linearly related to the logarithm of the analyte concentration. The limits of determination ranged from 1 mM for low-molecular-weight to 0.02 mM for alkyl polyether alcohols. The flow-injection technique allows convenient pretreatment and direct observation of the analytical signal. Interferences from chloride and calcium can be readily eliminated by excluding them during electrode pretreatment. Potentiometric measurements were correlated with amperometric anodic responses at the nickel oxide electrode, allowing an unusual direct comparison of the two methods.     

Simple and rapid determination of iodide in table salt by stripping potentiometry at a carbon-paste electrode

A simple and rapid procedure, utilising constant-current stripping analysis (CCSA) at a carbon-paste electrode containing tricresyl phosphate as a pasting liquid (TCP-CPE), has been developed for the determination of iodide in table salt. Because of a synergistic accumulation mechanism based on ion-pairing and extraction of iodide in combination with electrolytic pretreatment of the TCP-CPE, the method is selective for iodide and enables direct determination of iodide in samples of table salt containing anti-caking agents such as K(4)[Fe(CN)(6)] (food additive "E 536") or MgO. The iodide content (calculated as KI) can be determined in a concentration range of 2 to 100 mg kg(-1) salt, with a detection limit (S/N=3) of 1 mg kg(-1), and a recovery from 90 to 115%. The proposed method has been used to determine iodide in several types of artificially iodised table salt and in one sample of natural sea salt. The results obtained agreed well with those obtained by use of three independent reference methods (titration, spectrophotometry, and ICP-MS) used to validate the CCSA method, indicating that the developed method is applicable as a routine procedure for rapid testing in salt production process control and in the analysis of marketed table salts.     

Nanostructured materials in potentiometry

Potentiometry is a very simple electrochemical technique with extraordinary analytical capabilities. It is also well known that nanostructured materials display properties which they do not show in the bulk phase. The combination of the two fields of potentiometry and nanomaterials is therefore a promising area of research and development. In this report, we explain the fundamentals of potentiometric devices that incorporate nanostructured materials and we highlight the advantages and drawbacks of combining nanomaterials and potentiometry. The paper provides an overview of the role of nanostructured materials in the two commonest potentiometric sensors: field-effect transistors and ion-selective electrodes. Additionally, we provide a few recent examples of new potentiometric sensors that are based on receptors immobilized directly onto the nanostructured material surface. Moreover, we summarize the use of potentiometry to analyze processes involving nanostructured materials and the prospects that the use of nanopores offer to potentiometry. Finally, we discuss several difficulties that currently hinder developments in the field and some future trends that will extend potentiometry into new analytical areas such as biology and medicine.     

Determination of chlorine in silicate rocks by ion-exchange chromatography and direct potentiometry with an ion-selective electrode

The method has been developed for determination of chlorine in silicate rocks by ion-exchange chromatography and an ion-selective electrode.     

Uncertainty of measurement for the determination of fluoride in water and wastewater by direct selective electrode potentiometry

A procedure to estimate the uncertainty of measurement applied to the fluoride determination of waters and wastewaters matrices by selective electrode potentiometry was implemented based on Eurachem Guide. The major sources of uncertainty were identified as the calibration standard solutions, fluoride concentration obtained by potential interpolation of the regression line and the precision. However the relative uncertainties depend on the anion concentration levels. The methodology proposed was presented to two fluoride concentration levels that are in the range of surface water samples (C _sample=1.12 mgF l⁻¹) and of wastewater matrices (C _sample=101.4 mgF l⁻¹). The expanded uncertainties calculated were 0.40 and 9.1 mg l⁻¹ for low and high concentration levels, respectively, using the reproducibility uncertainty as precision evaluation. The relative expanded uncertainty was around ±10% for the highest concentration, which can be considered acceptable for the ion selective electrode potenciometric methods and ±36% for the lowest concentrations. In this case the sample fluoride content is very close to the limit of quantification which has a relative uncertainty of about ±30%. If the repeatability was used in spite of duplicate analysis the same conclusions were obtained (C _sample=1.12 ± 0.39 mgF l⁻¹ and C _sample=101.4 ± 7.0 mgF l⁻¹). Although the calculated expanded uncertainties and consequently the combined uncertainty, do not vary significantly in the cases where it was used the repeatability or reproducibility for evaluating the precision, each relative variances uncertainty contributions do. When the repeatability is used to determine the combined uncertainty, the CSS and uncertainties contributions are the most dominant ones. However, if reproducibility is used, relative uncertainty variance contributions are distributed among CSS, C _F, and precision. In both cases, the contribution increases and r _CSS contribution decreases with the increasing of the concentration level. The precision variance contribution is only significant in the case where the reproducibility is used, and increases with the increasing of the concentration level. The uncertainty in the result calculated using the proposed methodology (C _sample ± U _sample = 2.17 ± 0.42 mgF l⁻¹) is in satisfactory agreement with the estimated expanded uncertainty obtained using the relative reproducibility standard deviation obtained in interlaboratory studies ().     

Determination of phosphate in hydroponic nutrient solutions using flow injection potentiometry and a cobalt-wire phosphate ion-selective electrode

The direct flow injection potentiometric (FIP) analysis of phosphate in hydroponic nutrient solution has been carried out using a cobalt-wire ion-selective electrode (ISE). Synthetic hydroponic nutrient solution, commercial hydroponic nutrient solution and working hydroponic farm nutrient solution were analysed for phosphate using the FIP technique. It is shown that FIP results compare favourably to standard methods of analysis such as spectrophotometry and indirect photometric ion-pair chromatography. Reproducible FIP response curves with a slope of −(47.57±0.03) mV per decade and intercept of −(169.7±0.1) mV were obtained for four separate calibrations in the concentration range 5.0×10⁻⁴–1.0×10⁻² M H₂PO₄⁻. Anion corrections for interferences by Cl⁻, NO₃⁻ and SO₄²⁻ were applied to all samples using the selectivity coefficients determined independently using a fixed interference method. Nevertheless, it was found that anion corrections were not necessary, as the deviations fell within the bounds of experimental error for the cobalt-wire ISE technique (i.e.±2–5% R.S.D.). The proposed FIP method enables the direct determination of phosphate in hydroponic nutrient solutions.     

The standard electrode potential of the silver-silver chloride electrode in 5 wt. % tetramethylurea-water mixture at 30°C

Electromotive force measurements were made at 30°C with the cell $$H_2 (g)|Pt(s)|HCl(m){\text{ }}TMU(x){\text{ }}H_2 O(100 - x)|AgCl(s)|Ag(s)$$ wherex=5 wt. % tetramethylurea (TMU). The standard electrode potential of the silver-silver chloride electrode, the mean molal activity coefficient of hydrogen chloride, the primary medium effect, and the free energy of transfer of hydrogen chloride from the aqueous standard state to the standard state in the mixed solvent were derived from the measurements. The acquisition of data was limited to this single composition and temperature because of the difficulty of preparing hydrogen electrodes for this solvent medium. The results obtained for hydrogen chloride in 5 wt. % tetramethylurea-water mixture are discussed, relative to other organic-aqueous mixtures of the same composition, in terms of structural effects and hydrogen bonding.     

Simultaneous determination of bromide and chloride in natural waters by ion-exchange chromatography and direct potentiometry with an ion-selective electrode

A method has been developed for the sequential determination of bromide and chloride in natural waters by use of ion-exchange chromatography and an ion-selective electrode. Hydrous zirconium oxide has successfully been used as the ion-exchanger.