Investigation of adsorption of nanogram quantities of iron(II) tris-(1,10-phenanthrolinate) on glasses and silica by thermal lens spectrometry

Thermal lens spectrometry is used for studying adsorption equilibria in aqueous solutions at the level of nanogram quantities of iron(II) tris-(1,10-phenanthrolinate) as a model system. The kinetics of the sorption of the chelate on silica is studied and adsorption isotherms are built. Thermal lensing is used as a method for direct determination of the chelate concentration adsorbed on a quartz surface. The detected amount is 4.1×10⁻¹⁵ mol at the area irradiated by the excitation beam. The adsorption of iron(II) tris-(1,10-phenanthrolinate) on laboratory glassware at the nanogram level is characterised by measuring the residual concentration of the sorbate in solution. A procedure for handling and cleaning the laboratory glassware for determining nanogram amounts of iron in aqueous solutions is proposed. The sensitivity of thermal lensing both in measuring adsorption on silica and glass and quartz surfaces is 100-fold higher than diffuse-reflectance measurements under the same conditions.     

Studying the Adsorption of an Iron(II) Complex of 1,10-Phenanthroline on Laboratory Glassware Using Thermal Lens Spectrometry

The adsorption of iron(II) tris(1,10-phenanthroline) on the glass surface of laboratory ware from solutions has been studied by thermal lens spectrometry at a level of nanogram amounts. The effect of glass pretreatment on the fraction of the adsorbed substance has been studied, and the most appropriate variant of the pretreatment has been selected. The kinetics of iron(II) tris(1,10-phenanthroline) adsorption on the surface of laboratory glass has been studied, and an adsorption isotherm has been constructed for nanogram chelate contents of the solution. The process is most closely approximated by the Freundlich equation. Based on the obtained data, a procedure has been proposed for the preparation of solutions for constructing calibration relationships that can be used in thermal lens measurements of trace amounts of analytes. This procedure takes into account the loss of substances at the surface of laboratory glassware.     

Thermal lens spectrometry as a tool for determination of stability constants of complex compounds

The potential of thermal lens spectrometry in the determination of stability constants of complex compounds was explored using copper(I) and iron(II) complexes with 1,10-phenanthroline and 2,9-dimethyl-1,10-phenanthroline as examples. Thermal lens spectrometry offers advantages over conventional spectrophotometry in the determination of stability constants both in aqueous and nonaqueous media. The overall and stepwise stability constants of iron(II) tris(1,10-phenanthrolinate), copper(I) bis(2,9-dimethyl-1,10-phenanthrolinate), and copper(I) bis(1,10-phenanthrolinate) were determined at levels as low as 10⁻⁸–10⁻⁶ mol L⁻¹.__________Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 1, pp. 123–133, January, 2005.     

Application of Thermal Lens Spectrometry to the Study of Chemical Adsorption in a Layer of a Nafion Solid Electrolyte

Thermal lens spectrometry was used to study Langmuir–Blodgett films of a weakly absorbing Nafion polyelectrolyte membrane on the surface of inert polyethylene terephthalate (PET) and glass substrates and to estimate the amount of Nafion (number of layers) using a change in the thermal characteristics of the sample. The sensitivity of thermal lens measurements at the wavelength of the exciting radiation 532.0 nm is comparable to that of solid-state spectrophotometry in the region of the maximum absorbance of Nafion (275 nm). However, the high locality of thermal lens spectrometry (the area of the signal generation zone is 100 μm²) ensures the estimation of the uniformity of the deposition of the polyelectrolyte layer. To increase the absorbance of the layer of the applied polyelectrolyte, the latter is saturated with a colored compound (ferroin). The adsorption of ferroin into the Nafion layer on the PET surface was confirmed; the absorbance of ferroin in the Nafion layer is in the range of 1 × 10–5^–5 × 10^–4 units of absorbance, which corresponds to the surface concentration of ferroin 1 × 10^–11–4 × 10^–10 mol/cm².     

Estimation of performance parameters of a frontal variant of optoacoustic determination with a model system

Using a model system, the applicability of the frontal configuration of optoacoustic spectroscopy in analytical practice is shown. An optoacoustic cell made using layered-prism geometry was designed and tested; it ensures operation with a layer thickness of 20–100 μm and sample volumes of 50–100 μl. Conditions are proposed for determining iron(II) in the form of tris(1,10-phenanthrolinate) with the detection limit of 3 × 10⁻⁷ M, which is less that the detection limit under similar induction conditions and for the same determination procedure using thermal-lens spectrometry (4 × 10⁻⁷ M). The determination has high reproducibility (relative standard deviation for 10⁻⁶ M is 0.005) and expressivity (1 min per sample detection).     

Thermal lens studies of the reaction of iron(II) with 1,10-phenanthroline at the nanogram level

The determination of iron(II) with 1,10-phenanthroline in aqueous solutions was carried out exemplarily by thermal lens spectrometry. The peculiarities of analytical reactions at the nanogram level of reactants can be studied using this method. Under the conditions of the competing reaction of ligand protonation, the overall stability constant for iron(II) chelate with 1,10-phenanthroline was determined at a level of n × 10^–7 mol L^–1, logβ ₃ = 21.3 ± 0.1. The rates of formation and dissociation of iron(II) tris-(1,10-phenanthrolinate) at a level of n × 10^–8 mol L^–1 were found to be (2.05 ± 0.05) × 10^–2 min^–1 and (3.0 ± 0.1) × 10^–3 min^–1, respectively. The conditions for the determination of iron(II) with 1,10-phenanthroline by thermal lensing were reconsidered, and ascorbic acid was shown to be the best reducing agent, which provided minimum and reproducible sample pretreatment. Changes in the conditions at the nanogram level improved both the selectivity and sensitivity of determination. The optimum measurement conditions for thermal lensing were determined not only by the absorption of the analyte and reagents, but also by the background absorption of the solvent. The limits of detection and quantification of iron(II) at 488.0 nm (excitation beam power 140 mW) are 1 × 10^–9 and 6 × 10^–9 mol L^–1, respectively; the reproducibility RSD for the range n × 10^–8–n × 10^–6 mol L^–1 is 2–5%.     

Thermal lensing spectrophotometric analysis with ion-pair solvent extraction

Thermal lensing spectrophotometry is applied to the determination of iron(II) with 4,7-diphenyl-1,10-phenanthroline disulfonic acid in aqueous solution, and in chloroform by ion-pair extraction with trioctylmethylammonium chloride. A phase-sensitive detection system with digital processing was used, the optimum modulation frequency being 5–10 Hz. A baseline drift of 0–03% was achieved. In water, the enhancement factor (sensitivity relative to conventional spectrophotometry) was 70 at an exciting power of 800 mW, and 2 × 10^-9 M iron(II) was determined. In chloroform 2 × 10^-10 M iron(II)—complex could be detected, the enhancement factor being 1200.     

Investigations of Cobalt, Nickel, and Copper Diethyldithiocarbamates Using Thermal Lens Spectrometry

Thermal lens spectrometry was used to study the dissociation kinetics of diethyldithiocarbamate complexes of copper(II), cobalt(III), and nickel(II) as a function of pH in the presence of chloride and sulfate ions. It is shown that, as distinct from conventional spectrophotometric and potentiometric measurements, the reversible dissociation of the test complexes and the irreversible oxidation of the ligand can be studied separately (at a level of n × 10^–8–n × 10^–6 M) using thermal lens spectrometry. Because of work in more dilute solutions and due account of the kinetic features of the systems in question, thermal lens spectrometry provides a higher accuracy of the determination of stability constants for diethyldithiocarbamate complexes of copper(II), cobalt(III), and nickel(II). The adsorption of the diethyldithiocarbamate complexes in question from water–ethanol solutions (1 : 3) on Silasorb C₁₈ silica is studied, and the adsorption constants are determined. The limits of detection of copper(II), cobalt(III), and nickel(II) diethyldithiocarbamates obtained in extraction–thermal-lens determination are n × 10^–8 M.     

Spectrophotometric determination of aluminum and iron in calcium

A simple, rapid, and accurate method is described whereby micro amounts of aluminum and iron can be determined in calcium metal. The iron is determined spectrophotometrically by measuring the absorbency of the reddish-orange color of the tris(1,10-phenanthroline)iron(II)ion, [(C₁₂H₈N₂)₃Fe]⁺⁺. This complex effectively removes the iron interference, and the aluminum is determined spectrophotometrically in the solution remaining from the iron determination by extracting it with a chloroform solution of 8-quinolinol and measuring the absorbency of the yellow color of the tris(8-quinolinolo)aluminum(III) in the chloroform solution.     

Preconcentration of metal ions on aluminum oxide modified with tiron

It is found that iron(III), titanium(IV), vanadium(V), copper(II), lead(II), and zinc(II) ions are adsorbed on aluminum oxide modified with Tiron. The adsorption of iron(III) is accompanied by the violet coloration of the adsorbent (560 nm), which indicates the formation of a 1: 2 complex on the adsorbent surface; copper forms a 1: 1 yellowish green complex (430 nm). The group preconcentration of metal ions on this adsorbent in a dynamic mode is possible; the preconcentration rate is 2.5 × 10³. The method for the determination of metal ions is proposed based on the adsorption recovery of the elements followed by the direct determination by X-ray fluorescence spectroscopy on the adsorbent surface.     

Electrocatalysis at chemically modified electrodes : Detection/determination of redox gaseous species in continuous-flow systems

The electrocatalytic response of chemically modified carbon paste and glassy carbon electrodes with anchored Fe(II)/Fe(III) centers is presented for gaseous species such as NO₂, SO₂, H₂S, and Cl₂. Carbon paste electrodes are modified by direct admixing of tris[4,7-diphenyl-1,10-phenanthroline]iron(II) perchlorate, and glassy carbon electrodes are modified by oxidative electropolymerisation of tris[5-amino-1,10-phenanthroline]iron(II) perchlorate. The electrocatalytic cycle operating in the integrated reaction/pre-electrode layer of chemically modified electrodes results in significant signal enhancement. Chemical modification of the conducting surfaces protects the electrode surface from poisoning. evaluation was done in a continuous-flow system using gravitational flow fo supporting electrolyte carrier propulsion, direct injection of air containing parts per billion or parts per million (v/v) of the gaseous species, and amperometric detection with the chemically modified electrodes as part of a thin-layer cell.     

Real-time thermal lens absorption measurements with application to flow-injection systems

Flow-injection methods are shown to reduce effectively the blank-related uncertainties in trace-level spectrophotometry. When thermal lens detection is used with efficient, real-time data processing, sensitive determinations are possible with response times adequately fast for flow-injection monitoring. Absorbance detection limits of 8.5 × 10^?7 in a 1?cm pathlength flow cell were obtained for injections into carbon tetrachloride solvent. Thermal lens detection of iron with 1,10-phenanthroline as reagent produced detection limits of 37 pg of iron in the 100-μl injection volume. The advantages of flow injection for minimizing problems with photochemically unstable analytes in thermal-lens measurements are demonstrated.     

THE KINETICS OF ELECTRON TRANSFER FOR A SERIES OF IRON(II) CYCLIDENE COMPLEXES

Abstract

The kinetics of oxidation of a series of iron(II) cyclidene complexes by tris(1,10-phenanthroline)cobalt(III) in methanol have been measured by stopped flow spectrophotometry. The reactions obey a first order rate law when the cobalt(III) complex is present in large excess. The corresponding second order rate constants fall in the range 5.0–130 × 10⁵ M^?1s^?1 (25°C). A linear correlation between the logarithm of the rate constant and the iron(III)/(II) redox potential indicates that the reactions behave as simple outer-sphere electron transfer processes. The self-exchange rate constants for the iron cyclidene complexes have been estimated from the Marcus equation and found to vary between 0.7 × 10⁷ M¹s¹ and 9 × 10⁷ M¹s¹. The dependence of the self-exchange rate constant on ligand structure is discussed.     

Anion selectivity studies on liquid membrane electrodes

Selectivity coefficients of liquid-membrane electrodes for common inorganic anions were measured in electrodes containing tris(l,10-phenanthroline)iron(II), tris(4,7-diphenyl-1,10-phenanthroline)iron(II) or tetraheptylammonium ion in nitrobenzene, and tris(4,7-diphenyl-1,10-phenanthroline)iron(II) ion in nitrobenzene, chloroform or n-amyl alcohol as the liquid membrane. With the exception of the amyl alcohol electrode, selectivity coefficients were relatively independent of membrane composition and followed a common sequence of decreasing selectivity: PF₆^?> ClO₄^?>SCN^?~I^?~BF₄^?>NO₃^?>Br^?>Cl^?. This sequence parallels the order of increasing anion hydration energy, suggesting that aqueous phase solvation energies play a predominant role in determining electrode selectivity for these ions. Time-dependent behavior of liquid-membrane electrodes on transfer between solutions containing different ions also is described. Instantaneous e.m.f. readings were used to determine selectivity coefficients.     

Determination of chemisorbed metals on planar fused-silica surfaces using thermal lens spectrometry

Thermal lens spectrometry was used to determine the surface concentrations of metals on planar fused-silica surfaces modified with the use of vanadium(V) oxochloride, tin(IV) tetrachloride, and titanium(IV) oxochloride after complexation reactions with 4-(2-pyridylazo)resorcinol and 2-thiazolylazopyrocatechol from aqueous solutions. The surface concentrations of vanadium and titanium found from thermal lens experiments (8 × 10^?11 and 6 × 10^?10 mol/cm², respectively) correlate well with those theoretically calculated based on monolayer and bilayer approximations for metal-containing groups, respectively. The coverage uniformity and the concentration of a chemisorbed substance on a glass surface were evaluated.     

Spectrophotometric determination of hexafluoroarsenate with ferroin

A spectrophotometric method for the hexafluoroarsenate anion is described. The determination involves selective extraction of the tris(1,10-phenanthroline)iron(II) salt of the anion into n-butyronitrile, followed by measurement of the absorbance of the organic layer. The extracts give good Beer's law plots over a wide range of concentrations and more than 98% of the hexafluoroarsenate is removed in a single extraction. Interference studies were made on a number of anions.     

A Sensitive Fluorescence Quenching Method for the Determination of Iron( Ⅱ） with 1,10-Phenanthroline

A sensitive and selective fluorescence quenching method for the determination of Fe²⁺ with 1,10‐phenanthroline was developed. The fluorescence intensity of 1,10‐phenanthroline at λ_ex of 266 run and λ_em of 365 nm was constant in the range of pH 4.0 to 10.0 and decreased linearly upon addition of Fe²⁺ to its solution. This decrease was mainly due to a static quenching effect caused by the formation of a non‐fluorescent complex of Fe²⁺ with 1, 10‐phenanthroline. The total amount of iron was determined by using hydroxylamine hydrochloride to reduce Fe³⁺ to Fe²⁺. The linear range was from 5.0 × 10–⁷ to 2.0 × 10–⁵ mol/L with a detection limit of 2.4 × 10–⁸ mol/L at 3s?. The quenching constant of Fe²⁺ to 1,10‐phenanthroline was calculated to be (5.70 × 0.05) × 10⁴ L/mol at 25 °C. Effects of foreign ions on the determination of Fe²⁺ were investigated. The results of the new method for the determination of iron in tap water and natural water samples were in good agreement with those obtained by graphite atomic absorption spectrometry.     

Improvement of iron determination by use of ferroin-organic dye ion-association complexes

Extraction of ion-associates of tris(1,10-phenanthroline)-iron(II) cation with monosulphonated azo dye anion into 1,2-dichloroethane is described. The method increases the sensitivity of the colorimetric determination of bivalent iron with 1,10-phenanthro-line.     

Hexacyanoferrate(III) as a mediator in the determination of total iron in potable waters as iron(II)-1,10-phenanthroline at a single-use screen-printed carbon sensor device

Hexacyanoferrate(III) was used as a mediator in the determination of total iron, as iron(II)-1,10-phenanthroline, at a screen-printed carbon sensor device. Pre-reduction of iron(III) at −0.2 V versus Ag/AgCl (1 M KCl) in the presence of hexacyanoferrate(II) and 1,10-phenanthroline (pH 3.5-4.5), to iron(II)-1,10-phenanthroline, was complete at the unmodified carbon electrode surface. Total iron was then determined voltammetrically by oxidation of the iron(II)-1,10-phenanthroline at +0.82 V, with a detection limit of 10 μg l⁻¹.In potable waters, iron is present in hydrolysed form, and it was found necessary to change the pH to 2.5-2.7 in order to reduce the iron(III) within 30 s. A voltammetric response was not found at lower pH values owing to the non-formation of the iron(II)-1,10-phenanthroline complex below pH 2.5.Attempts to incorporate all the relevant reagents (1,10-phenanthroline, potassium hexacyanoferrate(III), potassium hydrogen sulphate, sodium acetate, and potassium chloride) into a modifying coated PVA film were partially successful. The coated electrode behaved very satisfactorily with freshly-prepared iron(II) and iron(III) solutions but with hydrolysed iron, the iron(III) signal was only 85% that of iron(II).     

New method for the determination of pentachlorophenol by atomic absorption spectrophotometry

A new application of atomic absorption is reported for the determination of pentachlorophenol. The method is based on solvent extraction into nitrobenzene of the ion-pair formed between tris(1,10-phenanthroline) iron(II) and the anion of pentachlorophenol, and the subsequent determination of the iron concentration m the extract by atomic absorption at the 2483 A iron line.