Milka T. Neshkova

Piezoelectric quartz crystal humidity sensor using chemically modified nitrated polystyrene as water sorbing coating

Abstract A new coated piezoelectric crystal (CPC) humidity sensor has been developed based on surface chemically modified coatings of nitrated polystyrene (NPS). Its frequency response to relative humidity (RH) has been examined. The CPC detector has a time response of less than 3 s, exhibits high sensitivity (up to 40 Hz per %RH) and very good performance reproducibility and stability upon long-term calibration for more than 3 months. No interference with the frequency response to water vapours in the presence of corroding gases such as SO2 and NO2 has been observed. The factors contributing to optimization of selected performance characteristics have been discussed.

Electrochemically prepared chalcogenide ion-selective membranes as an alternative to conventional pressed-pellet ion-selective electrodes

Abstract Applying cathodic electrodeposition of thin metal chalcogenide (selenide, telluride or arsenide) membranes on an inert electroconductive substrate, all-solid-state ion-selective electrodes (ISEs) were developed for Cu(II), Pb(II), Ag(I) and cyanide. Technologically, the electrochemical approach offers a cost-effective means of in situ membrane preparation which suits best the demand for disposable sensors for environmental and process control. Membranes of very sophisticated composition and strongly defective structure were developed as a result of preliminary membrane composition optimization studies. A brief outline of the optimum electrodeposition conditions for each membrane is given. The major advantage of these sensors as flow-injection potentiometric detectors was demonstrated on the examples of Cu 2-x Se and Ag 2.40 Se 0.86 Te 0.14 membranes. A linear Nernstian response was observed down to n × 10 −6 M Cu (II) or Ag(I) ( n = 2−5). Interference of chloride with Cu ISEs can be eliminated with electroplated membranes in the flow-injection mode for Cl concentrations up to 0.5 M, the lower linearity limit still being 5 × 10 −6 M Cu(II). It is shown that by switching from the steady-state to the flow-injection mode, separation by rate of the Cu(II) potential-determining processes occurring at the membrane/ solution interface in the presence of Cl − becomes possible. The potential of the electrochemical approach for model response mechanism studies was demonstrated by the example of a Cu ISE. The chemical and phase composition of the Cu 2-x Se membrane was varied in the range 0.67 ⩾ x ⩾ 0 and the Cu(II) electrode function was examined for different x values. Conclusions with regard to the potential-generating reaction and the optimum membrane composition were drawn on this basis.

Chalcogenide based all-solid-state thin electroplated ion-selective membrane for Hg(II) flow-injection determinations☆

The response to Hg(II) of a thin all-solid-state Te-doped silver chalcogenide membrane, described by the general formula Ag2 + δSe1 − xTex, which has been electrochemically prepared following a previously proposed approach, has been investigated. The kinetics of formation of the membranes secondary dynamic response to Hg(II) has been successfully combined with the precise timing and transient signal, typical for flow-injection (FI) measurements, in developing a sensitive and reliable mercury FI detector. Under optimized stream conditions it exhibits a linear Nernstian response, with a double slope of the calibration graph of 59 mV dec−1, over the mercury(II) concentration range 10−6 − 10−3 M, the typical sample throughput amounting to about 70 samples per hour. The observed chemical amplification of the signal is due to the specificity of the processes dominating the initial step in formation of the steady-state signal of the membrane to mercury. The analytical performance of the Hg(II) FI detector, as regards sensitivity, reproducibility, selectivity and long-term stability has been thoroughly investigated. The exact procedure for membrane electrodeposition is given and the potential of the proposed approach as a cost-effective way for preparing chalcogenides of unique structure and properties has been outlined in the above context.

Cyanide ion-selective electrodes based on thin electroplated membranes of silver chalcogenides: Part I. Membrane preparation and characterization

Cyanide-responsive ion-selective electrodes have been developed based on thin tellurium-doped silver selenide membranes electrodeposited on platinum substrates. The chemical composition of the electroplated film membranes could be expressed by the general formula Ag2+δSe1−xTex (where 0.2 < δ < 0.8 and 0.2 ⩾ × ⩾ 0.1). These electrodes exhibit a linear response in cyanide solutions with concentrations ranging from 10−2 to 10−6 M, with a slope of the electrode function of about 90 mV (pCN)−1 (i.e., lower than the theoretically predicted double-Nernstian slope). These electrodes showed very stable behaviour during long-term investigation (several months). The conditions for the electrochemical preparation of cyanide-responsive silver chalcogenide membranes are discussed both from theoretical and practical points of view. X-ray diffraction, energy-dispersive x-ray fluorescence microanalysis (EDAX) and scanning electron microscopy (SEM) were used to examine the membrane composition, structure and surface morphology.

The behaviour of two types of copper ion-selective electrodes in different copper(II)-ligand systems.

The behaviour of two types of solid-state homogeneous sensors for copper(II), one based on pressed pellets of ternary CuAgSe and the other on thin-layer electroplated Cu(2-x)Se, in 12 different copper(II)-ligand systems, has been thoroughly investigated. Both electrodes exhibit anomalous behaviour when the ligands are of complexone type, the effect of the complexones on the deviations increasing in the order IDA < NTA < EDTA approximately DTPA, and being practically the same for the two types of sensors, thus disproving a previous suggestion that the anomaly is due to the silver in the silver-containing sensors. The experimental data do not support the specific ligand-adsorption hypothesis either. The observed deviations are tentatively explained on the basis that, as suggested by the selectivity coefficients, both sensors act as primary copper(I) ion-selective electrodes rather than copper(II)-electrodes. Thus, at very low copper(II) concentrations, according to the extended Nikolskii equation, the [Cu(I)]/[Cu(II)] ratio at the electrode surface determines the electrode sensitivity towards Cu(II). The lower detection limit could be improved by pH-control and selective complexation of Cu(I). This hypothesis has been proved experimentally. If the copper(I) activity on the electrode surface is decreased, the anomaly observed for the Cu(II)-NTA system disappears and decreases considerably for the Cu(II)-EDTA and Cu(II)-DTPA systems.

Copper(I) electrode function of two types of copper(II) ion-selective electrodes.

The response of two types of solid-state copper ion-selective electrodes with homogeneous membranes of CuAgSe and Cu(2-x)Se has been investigated in copper(I) solutions, prepared electrochemically by insitu generation from a copper anode in chloride medium. The selectivity coefficient K(pot)(Cu+, Cu(2+)) both types of electrodes has been determined. It is 10(-5.7) for the copper selenide sensor, and 10(-6.2) for the copper silver selenide one. These values are very close to that calculated for an exchange reaction proceeding on the electrode surface. The similarity in K(pot)(Cu+ ,Cu(2+)) values for different chalcogenidebased sensors suggests a common potential-generating mechanism. High chloride concentration does not interfere with the electrode response towards Cu(I), but distorts the electrode response to Cu(II).

A new generation of cyanide ion-selective membranes for flow injection application: Part III. A simple approach to the determination of toxic metal-cyanide complexes without preliminary separation.

A new flow injection approach to total weak acid-dissociable (WAD) metal-cyanide complexes is proposed, which eliminates the need of a separation step (such as gas diffusion or pervaporation) prior to the detection. The cornerstone of the new methodology is based on the highly selective flow-injection potentiometric detection (FIPD) system that makes use of thin-layer electroplated silver chalcogenide ion-selective membranes of non-trivial composition and surface morphology: Ag(2+delta)Se(1-x)Te(x) and Ag(2+delta)Se. An inherent feature of the FIP-detectors is their specific response to the sum of simple CN(-)+Zn(CN)(4)(2-)+Cd(CN)(4)(2-). For total WAD cyanide determination, ligand exchange (LE) and a newly developed electrochemical pre-treatment procedure for release of the bound cyanide were used. The LE pre-treatment ensures complete recovery only when the sample does not contain Hg(CN)(4)(2-). This limitation is overcome by implementing electrochemical pre-treatment which liberates completely the bound WAD cyanide through cathodic reduction of the complexed metal ions. A complete recovery of toxic WAD cyanide is achieved in the concentration range from 156 microg L(-1) up to 13 mg L(-1). A three-step protocol for individual and group WAD cyanide speciation is proposed for the first time. The speciation protocol comprises three successive measurements: (i) of non-treated, (ii) LE-exchange pre-treated; (iii) electrochemically pre-treated sample. In the presence of all WAD complexes this procedure provides complete recovery of the total bound cyanide along with its quantitative differentiation into the following groups: (1) Hg(CN)(4)(2-); (2) CN(-)+Cd(CN)(4)(2-)+Zn(CN)(4)(2-); (3) Cu(CN)(4)(3-)+Ni(CN)(4)(2-)+Ag(CN)(2)(-). The presence of a 100-fold excess in total of the following ions: CO(3)(2-), SCN(-), NH(4)(+), SO(4)(2-) and Cl(-) does not interferes. Thus the proposed approach offers a step ahead to meeting the ever increasing demand for cyanide-species-specific methods. The equipment simplicity makes the procedure a good candidate for implementing in portable devices for in-field cyanide monitoring.

Real-time stoichiometry monitoring of electrodeposited chalcogenide films via coulometry and electrochemical quartz-crystal microgravimetry with hydrodynamic control. Ag2+δSe case

Potentiostatic coulometry is used in conjunction with electrochemical quartz-crystal microgravimetry (EQCM) with controlled hydrodynamics to develop a new electroanalytical protocol for in situ composition monitoring of metal chalcogenide thin films. The approach, its application scope and limitations, are illustrated using the example of electrosynthesized AgySe thin films, successfully used previously for developing ion-selective sensors for Ag(I), cyanide and Hg(II). Well-defined flowing electrolyte conditions are achieved by the EQCM/submerged wall jet (SWJ) cell arrangement. The electrolyte flow rate through the nozzle is maintained constant between 5.5 and 250 cm3 min−1. The hydrodynamic control makes it possible to extend the range of the combined EQCM/coulometric approach to greater layer thickness consistent with that of membranes practically used in sensor preparation. The stoichiometric coefficient (y) profile of AgySe is monitored in situ for comparatively thick electrodeposited films (up to 800 nm). Two distinct features are clearly distinguished in the electrochemical formation of non-stoichiometric layers: in the zone adjacent to the substrate (i.e. between 170 and 270 nm) and for greater thickness (up to 800 nm). A mean value for y=2.24±0.06 is determined for the deposited layers, which is in good agreement with the values previously obtained by ex situ energy dispersive X-ray fluorescent microanalysis (EDAX).

Chemical amplification of the signal in ion-selective potentiometry: indirect determination of Al(III) and U(VI) by steady-state and flow-injection potentiometry using a copper ion-selective electrode

Abstract The possibilities for the indirect steady-state and flow-injection determination of Al(III) and U(VI) with a copper ion-selective electrode wee studied. The method is based on the replacement reaction between the determinand and a CuL 2 -buffer system [L=oxalate (Ox) or malonate (Mal)] at a pH of 5.4 fixed with acetate buffer. Chemical amplification of the potentiometric signal was observed owing to the stoichiometry of the following overall replacement reaction: 2CuL 2 + M = ML 2 + 2CuL It is manifested by a double-Nernstian slope of the Cu(II) electrode function (60 mV per decade). The concentration range for Al determinations is from 1.25 to 216 mg l −1 and for uranium from 14 to 2142 mg l −1 . Good reproducibility and high precision were achieved. Metal ions that do not form carboxylate complexes or form less stable complexes than CuOx 2 (e.g., Ca, Sr, Ba, CD, Zn) do not interfere. Special masking procedures allowing the determination of Al(III) in the presence of Fe(III) and U(VI) in the presence of Th(IV) were developed and experimentally tested. A flow-injection version of this approach was elaborated which preserves the main beneficial features of the steady-state mode. The proposed approach could be extended to other indirect determinations, and seems very promising for future wider application of ion-selective electrodes as detectors in flow systems.

The coulometric generation of chromium(II) in strongly acidic media: Part I. Coulometric titration of titanium(IV)

Abstract The stability of CrBr2(H2O)4+ was investigated in strong acid medium (1.5–6 M) as a function of acid type and bromide concentration; HCl, HBr, HClO4 and H2SO4 were used. Chromium tribromide was shown to provide a suitable working solution for at least 3 days, for the coulometric generation of chromium(II). The reduction was also studied at the D.M.E. Chromium(II) catalysed the aquation process, the effect being greatest in sulphuric acid. Conditions for 100% generating efficiency of chromium(II) were established; the range of current density for >99.9% current efficiency increased in the order HCl