High pressure performance of thin Pd–23%Ag/stainless steel composite membranes in water gas shift gas mixtures; influence of dilution, mass transfer and surface effects on the hydrogen flux

The hydrogen permeation and stability of tubular palladium alloy (Pd–23%Ag) composite membranes have been investigated at elevated temperatures and pressures. In our analysis we differentiate between dilution of hydrogen by other gas components, hydrogen depletion along the membrane length, concentration polarization adjacent to the membrane surface, and effects due to surface adsorption, on the hydrogen flux. A maximum H₂ flux of 1223 mL cm⁻² min⁻¹ or 8.4 mol m⁻² s⁻¹ was obtained at 400 °C and 26 bar hydrogen feed pressure, corresponding to a permeance of 6.4 × 10⁻³ mol m⁻² s⁻¹ Pa^−0.5. A good linear relationship was found between hydrogen flux and pressure as predicted for rate controlling bulk diffusion. In a mixture of 50% H₂ + 50% N₂ a maximum H₂ flux of 230 mL cm⁻² min⁻¹ and separation factor of 1400 were achieved at 26 bar. The large reduction in hydrogen flux is mainly caused by the build-up of a hydrogen-depleted concentration polarization layer adjacent to the membrane due to insufficient mass transport in the gas phase. Substituting N₂ with CO₂ results in further reduction of flux, but not as large as for CO where adsorption prevail as the dominating flow controlling factor. In WGS conditions (57.5% H₂, 18.7% CO₂, 3.8% CO, 1.2% CH₄ and 18.7% steam), a H₂ permeance of 1.1 × 10⁻³ mol m⁻² s⁻¹ Pa^−0.5 was found at 400 °C and 26 bar feed pressure. Operating the membrane for 500 h under various conditions (WGS and H₂ + N₂ mixtures) at 26 bars indicated no membrane failure, but a small decrease in flux. A peculiar flux inhibiting effect of long term exposure to high concentration of N₂ was observed. The membrane surface was deformed and expanded after operation, mainly following the topography of the macroporous support.     

Grain boundaries as barrier for oxygen transport in perovskite-type membranes

Perovskite-type membranes of (Ba_0.5Sr_0.5)(Co_0.8Fe_0.2)O_3−δ (BSCF) and (Ba_0.5Sr_0.5)(Fe_0.8Zn_0.2)O_3−δ (BSFZ) were successfully prepared via liquid-phase sintering using BN as sintering aid. The obtained membranes were examined via powder X-ray diffraction pattern (XRD), differential scanning calorimetry (DSC), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and oxygen permeation experiments. It has emerged that the use of BN as sintering aid lowers sintering temperatures in order to obtain dense membranes with relative densities in the range of 93–96% as proven by the Archimedes method. It was further shown that the perovskite structure could be maintained after sintering with BN. Additionally, BN was completely removed from the sintered membranes. Investigation of the microstructure revealed that the average grain size of the membranes was influenced by the amount of BN added prior the sintering process. It was found that large amounts of BN effectively lower the average grain size. Oxygen permeation experiments have shown that the lower the average grain size the lower the oxygen permeation performance, particularly in the case of BSCF. Transmission electron microscopy revealed that no evidence for an amorphous layer or any other interfacial phase in the grain boundary is present.     

Oxygen permeation through a Ce0.8Sm0.2O2−δ–La0.8Sr0.2CrO3−δ dual-phase composite membrane

A composite of oxygen ion conducting oxide Ce_0.8Sm_0.2O_2−δ (60 vol.%) and electron conducting oxide La_0.8Sr_0.2CrO_3−δ was prepared by sintering a powder compact at a temperature of 1550 °C. No significant reaction between the two constituent oxides was observed under preparation and oxygen permeation conditions. Appreciable oxygen permeation fluxes through the composite membrane were measured at elevated temperatures with one side of it exposed to the ambient air and the other side to a flowing helium gas stream. The oxygen flux initially increased with time, and took a long time to reach a steady value. A steady oxygen permeation flux as high as 1.4 × 10⁻⁷ mol cm⁻² s⁻¹ was obtained with a 0.3 mm thick membrane at 950 °C under a relatively small oxygen partial pressure difference of 0.21 bar/0.0092 bar. It was revealed that the overall oxygen permeation process was mainly limited by the transport in the bulk of the membrane in the range of the membrane thickness greater than 1.0 mm, and the limitation by the surface oxygen exchange came into play at reduced thickness of 0.6 mm.     

Thermogravimetric study on perovskite-like oxygen permeation ceramic membranes

The oxygen permeation properties of mixed-conducting ceramics SrFeCo_0.5O_3−δ (SFCO), Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3−δ (BSCFO), La_0.2Sr_0.8Co_0.8Fe_0.2O_3−δ (LSCFO) and Ba_0.95Ca_0.05Co_0.8Fe_0.2O_3−δ (BCCFO) were studied by thermogravimetric method in the temperature range 600–900 °C. The results show that the oxygen adsorption rate constants k_a of all material are larger than oxygen desorption rate constants k_d and both k_a and k_d are not strongly dependent on temperature in the studied temperature range. The oxygen vacancy contents δ(N₂) and δ(O₂) in nitrogen and oxygen and their difference Δδ = δ(N₂) − δ(O₂) play an important role in determining the temperature behavior of oxygen permeation flux J_O2.     

Flow-injection in-line complexation for ion-pair reversed phase high performance liquid chromatography of some metal-4-(2-pyridylazo) resorcinol chelates

Flow injection (FI) was coupled to ion-pair reversed phase high performance liquid chromatography (IP-RPHPLC) for the simultaneous analysis of some metal-4-(2-pyridylazo) resorcinol (PAR) chelates. A simple reverse flow injection (rFI) set-up was used for in-line complexation of metal-PAR chelates prior to their separation by IP-RPHPLC. The rFI conditions were: injection volume of PAR 85 μL, flow rate of metal stream 4.5 mL min⁻¹, concentration of PAR 1.8 × 10⁻⁴ mol L⁻¹ and the mixing coil length of 150 cm. IP-RPHPLC was carried out using a C₁₈ μBondapak column with the mobile phase containing 37% acetonitrile, 3.0 mmol L⁻¹ acetate buffer pH 6.0 and 6.2 mmol L⁻¹ tetrabutylammonium bromide (TBABr) at a flow rate of 1.0 mL min⁻¹ and visible detection at 530 and 440 nm. The analysis cycle including in-line complexation and separation by IP-RPHPLC was 16 min, which able to separate Cr(VI) and the PAR chelates of Co(II), Ni(II) and Cu(II).     

A peroxidase-tetrathiafulvalene biosensor based on self-assembled monolayer modified Au electrodes for the flow-injection determination of hydrogen peroxide

The construction and performance under flow-injection conditions of an integrated amperometric biosensor for hydrogen peroxide is reported. The design of the bioelectrode is based on a mercaptopropionic acid (MPA) self-assembled monolayer (SAM) modified gold disk electrode on which horseradish peroxidase (HRP, 24.3 U) was immobilized by cross-linking with glutaraldehyde together with the mediator tetrathiafulvalene (TTF, 1 μmol), which was entrapped in the three-dimensional aggregate formed.

The amperometric biosensor allows the obtention of reproducible flow injection amperometric responses at an applied potential of 0.00 V in 0.05 mol L⁻¹ phosphate buffer, pH 7.0 (flow rate: 1.40 mL min⁻¹, injection volume: 150 μL), with a range of linearity for hydrogen peroxide within the 2.0 × 10⁻⁷–1.0 × 10⁻⁴ mol L⁻¹ concentration range (slope: (2.33 ± 0.02) × 10⁻² A mol⁻¹ L, r = 0.999). A detection limit of 6.9 × 10⁻⁸ mol L⁻¹ was obtained together with a R.S.D. (n = 50) of 2.7% for a hydrogen peroxide concentration level of 5.0 × 10⁻⁵ mol L⁻¹. The immobilization method showed a good reproducibility with a R.S.D. of 5.3% for five different electrodes. Moreover, the useful lifetime of one single biosensor was estimated in 13 days.

The SAM-based biosensor was applied for the determination of hydrogen peroxide in rainwater and in a hair dye. The results obtained were validated by comparison with those obtained with a spectrophotometric reference method. In addition, the recovery of hydrogen peroxide in sterilised milk was tested.     

Tuning the parameters for fast respirometry

The aerobic bacterial respiration rate is an indicator of microbial growth and metabolism, essential for monitoring the oxidation process and organic load content of samples in a diverse field of application from influent streams in wastewater treatment facilities to industrial fermentations. This paper looks at the influence of parameters, such as culture concentration and volume, sample surface area/volume ratio and headspace volume to achieve optimisation of respirometry measurement and thus design a bench-top respirometric device, based on the monitoring of the pressure changes in a closed chamber where a bacterial culture is allowed to respire in contact with a sample. Contrary to traditional respirometry, the goal is detection of bacterial respiration within 5 min in a minimal sample volume. Both qualitative and quantitative data could be derived using a simple equation and fine-tuning of the micro-manometric parameters of the device, with a most important finding being that minimal headspace volume in combination with elevated bacterial populations maximised absolute pressure change response and favoured high sensitivity at short response time, even though the conditions indicated oxygen-limitation. Furthermore, in comparison with a commercially available respirometer the typical respiration rate of stationary phase P. putida M10 gave oxygen uptake rate (OUR) and specific oxygen uptake rate (SOUR) of 38 μmol l⁻¹ min⁻¹ and 5 μmol g⁻¹ min⁻¹, respectively with the ‘classical’ system, while the μ-Warburg device designed here showed a typical response, for the culture with the same dry cell concentration, of 66 μmol l⁻¹ min⁻¹ for the OUR and 9 μmol g⁻¹ min⁻¹ for the SOUR. The remarkable outcome from this data, therefore, is that it appears that the high surface area/volume geometry of the μ-Warburg device design has achieved less respiration limitation, even though the sample is unstirred. This presents important insight regarding future respirometer design.     

Oxygen permeation study in a tubular Ba0.5Sr0.5Co0.8Fe0.2O3-δ oxygen permeable membrane

Dense tubular Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3−δ (BSCFO) membranes were successfully prepared by the plastic extrusion method. The oxygen permeation flux was determined at different oxygen partial pressures in the shell side and different temperatures between 700 and 900 °C. The oxygen vacancy diffusion coefficients (D_v) at different temperatures were calculated from the dependence of oxygen permeation flux on the oxygen partial pressure term based on the surface current exchange model. No unsteady-state of oxygen permeation flux was observed at the initial stage in our experiments. The reason is the equilibrium time is too short (less than 10 min) to observe the unsteady-state in time. The increase of the helium flow rate can increase the oxygen permeation flux, which is due to the decrease of the oxygen partial pressure in the tube side with increasing of the helium flow rate. The oxygen permeation flux can also be affected by the air flow rate in the shell side when the air flow rate is lower than 150 ml/min. But the oxygen permeation flux is insensitive to the air flow rate when the air flow is higher than 150 ml/min. The membrane tube was operated steadily for 150 h with oxygen permeation flux of 1.12 ml/(cm² min) at 875 °C. X-ray diffraction (XRD) and energy dispersive spectroscopy (EDS) analysis showed that both the surface exposed to air and the surface exposed to helium of the BSCFO membrane tube after permeation for 150 h are similar to the fresh membrane tube in composition and structure. These results indicated that the membrane tube exhibits high structure stability.     

Mechanical stability and transport properties of the Sn-promoted SrCo0.8Fe0.2O3−δ ceramic membrane

In this article, the phase compositions, thermal, mechanical and transport properties of both the SrCo_0.8Fe_0.2O_3−δ (SCF) and the SrCo_0.8Fe_0.1Sn_0.1O_3−δ (SSCF) ceramic membranes were investigated systematically. As compared with the SCF membrane, the SSCF one had a more promoted thermal shock resistance, which related to its small thermal expansion coefficient between them and an enhanced composite structure for it. For the SCF membrane, a permeation rate of 1.9 × 10⁻⁶ mol cm⁻² s⁻¹ was obtained at 1000 °C and under the oxygen partial pressure gradient of P_O2 (h)/P_O2 (l) = 0.209 atm/0.012 atm; however, the permeation rate was 2.5 × 10⁻⁶ mol cm⁻² s⁻¹ for the SSCF one in the same measuring condition. In addition, both peak values of total electrical conductivity (σ_e) for SSCF sample appeared with increasing temperature. The second peak value of σ_e for SSCF one was regarded as the contribution from its minor phase, which appeared with the mixed conducting behavior resulting from partly Co-dissolving into its lattice.     

Optimisation of ICPMS collision/reaction cell conditions for the simultaneous removal of argon based interferences of arsenic and selenium in water samples

The optimisation of ICPMS collision/reaction cell conditions for the simultaneous analysis of arsenic and selenium is described. A mixture of 3.8 mL min⁻¹ of H₂ and 0.5 mL min⁻¹ of He was found to be suitable for the removal of both ArAr⁺ and ArCl⁺ interferences. Detection limits down to 30 ng (element) L⁻¹ in total analysis, and between 81 and 230 ng (element) L⁻¹ in speciation analysis were achieved in chloride matrix (1 g L⁻¹ NaCl). After validation, the method was applied to commercially available mineral waters.     

Photocatalytic removal of the insecticide fenitrothion from water sensitized with TiO2

The photocatalytic removal of the insecticide fenitrothion (IUPAC name: O,O-dimethyl O-4-nitro-m-tolyl phosphorothioate), C₉H₁₂NO₅PS, from water suspension of TiO₂, was investigated by following the disappearance of the original substance along with the formation and disappearance of intermediates via recording NMR (¹H and ³¹P) and UV spectra, as well as by pH measurements. Based on the obtained data, a possible reaction mechanism was proposed. It was found that ³¹P-NMR spectrometry can be successfully used to follow the kinetics of transforming organic into inorganic phosphorus in the course of the degradation (k_a=9.2×10⁻⁷ mol dm⁻³ min⁻¹, r=0.980 for the illumination period after 15 h). The rate of fenitrothion aromatic ring decomposition was followed by UV spectrometry during the illumination (k_a=3.1×10⁻⁶ mol dm⁻³ min⁻¹, r=0.989). The complete mineralization was attained after about 66 h of irradiation.     

Hydrothermally stable silica–alumina composite membranes for hydrogen separation

A thin layer (30–40 nm) of a dual-element silica–alumina composition was deposited on a porous alumina support by chemical vapor deposition (CVD) in an inert atmosphere at high temperature. Prior to CVD, an intermediate layer of γ-alumina was coated on the macroporous alumina support. The intermediate layer was prepared by the dip-coating and calcination of boehmite sols of different sizes to give a graded structure that was substantially free of defects. The resulting supported composite membrane had high permeance for hydrogen in the order of 2–3 × 10⁻⁷ mol m⁻² s⁻¹ Pa⁻¹ at 873 K with selectivities of H₂ over CH₄, CO and CO₂ of 940, 700 and 590, respectively. The membrane operated by a hopping mechanism involving jumps of permeating molecules between solubility sites. The presence of aluminum improved the hydrothermal stability of the membranes for periods in excess of 500 h at 873 K in 16% steam, allowing the permeance to remain above 10⁻⁷ mol m⁻² s⁻¹ Pa⁻¹, although with decreased selectivities.     

Novel CO2 selectively permeating membranes containing PETEDA dendrimer

Pentaerythrityl tetraethylenediamine (PETEDA) dendrimer was synthesized from pentaerythrityl tetrabromide and ethylenediamine. Its molecular structure was characterized by elemental analysis, Fourier transform infrared resonance (FT-IR) and hydrogen nuclear magnetic resonance (¹H NMR) spectroscopy. The composite membranes for selectively permeating CO₂ were prepared by using PETEDA-PVA blend polymer as the active layer and polyethersulfone (PES) ultrafiltration membrane as the support layer and their permselectivity was tested by pure CO₂ and CH₄ gases and the gas mixture containing 10 vol.% CO₂ and 90 vol.% CH₄, respectively. For pure gases, the membrane containing 78.6 wt% PETEDA and 21.4 wt% PVA in the blend has a CO₂ permeance of 8.14 × 10⁻⁵ cm³ (STP) cm⁻² s⁻¹ cmHg⁻¹ and CO₂/CH₄ selectivity of 52 at 143.5 cmHg feed gas pressure. While feed gas pressure is 991.2 cmHg, CO₂ permeance reaches 3.56 × 10⁻⁵ cm³ (STP) cm⁻² s⁻¹ cmHg⁻¹ and CO₂/CH₄ selectivity is 19. For the gas mixture, the membrane has a CO₂ permeance of 6.94 × 10⁻⁵ cm³ (STP) cm⁻² s⁻¹ cmHg⁻¹ with a CO₂/CH₄ selectivity of 33 at 188.5 cmHg feed gas pressure, and a CO₂ permeance of 3.29 × 10⁻⁵ cm³ (STP) cm⁻² s⁻¹ cmHg⁻¹ with a CO₂/CH₄ selectivity of 7.5 at a higher feed gas pressure of 1164 cmHg. A possible gas transport mechanism in the composite membranes is proposed by investigating the permeating behavior of pure gases and the gas mixture and analyzing possible reactions between CO₂/CH₄ gases and the PETEDA-PVA blend polymer. The effect of PETEDA content in the blend polymer on permselectivity of the composite membranes was investigated, presenting that CO₂ permeance and CO₂/CH₄ selectivity increase and CH₄ permeance decreases, respectively with PETEDA content. This is explained by that with increasing PETEDA content, the carrier content increases, and the crystallinity and free volume of the PETEDA-PVA blend decrease that were confirmed by the experimental results of X-ray diffraction spectra (XRD) and positron annihilation lifetime spectroscopy (PALS).     

Optimisation of flow-injection-hydride generation inductively coupled plasma spectrometric determination of selenium in electrolytic manganese

Flow-injection-hydride generation procedure for Se in electrolytic manganese was optimized by means of the experimental design approach. Instrumental variables like power supplied (P), sample (F) and argon (G) flow rates together with chemical variables like NaBH₄ and HCl concentrations were studied. In case of the chemical variables, it was concluded that sodium tetrahydridoborate concentrations higher than 1.0% extinguished the plasma while HCl concentration should always be higher than 0.6 mol dm⁻³. The analysis of effects suggested that all the instrumental variables are significant factors, and the optimum conditions were P = 1550 W, F = 4.75 mL min⁻¹ and G = 0.6 mL min⁻¹. The influence of Mn was specially studied and it was concluded that the interferences were negligible if Mn is below 2.0 g L⁻¹. In the same sense, the interferences of antimony(III), arsenic(V) and mercury(II) were also considered negligible. Finally, a detection limit of 0.0005% (w/w) was obtained (a repeatability R.S.D. <2.0% for all Se concentrations tried). Some manganese samples were also spiked with different concentrations of Se(IV) and the results demonstrated to be in good statistical agreement with expected values.     

Oxygen permeation and oxidative coupling of methane in membrane reactor: A new facile synthesis method for selective perovskite catalyst

A dense membrane of La_0.6Sr_0.4Co_0.8Fe_0.2O_3−δ (LSCF) perovskite was prepared by a new chelating agent, ethylene diamine N,N,N′,N′-tetra N-acetyl diamine (EDTNAD). As a potent ligand, EDTNAD provided a facile one-step method to form complexes of the four metal ions, simultaneously. The oxygen permeation flux through the pure perovskite LSCF dense membrane was measured over temperature range of 1073–1223 K, thickness of 0.7–1.0 mm and oxygen partial pressure of 0.1–1.0 bar. Oxidative coupling of methane (OCM) reaction using LSCF disk in the atmospheric membrane reactor and over the temperature range of 1073–1173 K showed a C₂ selectivity of 100% and C₂ yield of 5.01% at 1153 K. Furthermore, OCM reaction data of the membrane reactor were discussed and compared with those of the fixed bed using the same perovskite powder as the catalyst.     

Survey of the quasi-ternary system La0.8Sr0.2MnO3–La0.8Sr0.2CoO3–La0.8Sr0.2FeO3

An overview on the variation of the thermal expansion, the electrical conductivity as well as non-stoichiometry of the oxide content as a function of composition within the quasi-ternary system La_0.8Sr_0.2MnO_3−δ–La_0.8Sr_0.2CoO_3−δ–La_0.8Sr_0.2FeO_3−δ in air is presented. The various powders were synthesized under identical conditions. The DC electrical conductivity values of the compositions at 800 °C in air vary in a wide range from 15 to 1338 S cm⁻¹. The magnitude of electrical conductivity of the perovskites is mainly determined by the percentage of cobalt in the compositions. A similar behaviour was observed for the measured thermal expansion coefficients between room temperature and 1000 °C in air, increasing from 10.9 to 19.4 × 10⁻⁶ K⁻¹ as a function of cobalt content. Changes in the oxygen stoichiometry of the materials were characterized by temperature-programmed oxidation measurements.     

Determination of folic acid by chemiluminescence based on peroxomonosulfate-cobalt(II) system

Based on the chemiluminescence (CL) phenomena of folic acid in peroxomonosulfate-cobalt(II) system, a rapid and sensitive CL method was developed for determination of folic acid in pharmaceutical preparations and its urinary metabolism processes. Under the optimum conditions, the relative CL intensity was linear over the concentration ranging from 10⁻⁹ to 8 × 10⁻⁷ mol L⁻¹ (R² = 0.9991) with a detection limit as low as 6 × 10⁻¹⁰ mol L⁻¹ (S/N = 3) and relative standard deviation was 2.63% for 2 × 10⁻⁸ mol L⁻¹ folic acid (n = 11). This method has been successfully applied to the determination of folic acid in tablets and human urine. The blank CL emission was yielded owing to the formation of singlet oxygen molecular pair from the quenching experiment of 1,4-diazabicyclo[2.2.2]octane, and pterine-6-carboxylic acid might be the degradation intermediate in this system and it also acts an energy acceptor and sensitizes the chemiluminescence based on the studies of the CL and fluorescence spectra.     

Influence of post-annealing temperature on properties of ZnO:Li thin films

Li-doped ZnO thin films were prepared on glass substrates by DC reactive magnetron sputtering. The influence of post-annealing temperature on the electrical, structural, and optical properties of the films was investigated. A conversion from p-type conduction to n-type in a range of temperature was confirmed by Hall measurement. The optimal p-type conduction is achieved at the annealing temperature of 500 °C with a resistivity of 57 Ω cm, carrier concentration of 1.07 × 10¹⁷ cm⁻³ and Hall mobility of 1.03 cm² V⁻¹ s⁻¹. From the temperature-dependent PL analysis, the energy level of Li_Zn acceptor was determined to be 140 meV above the valence band.     

An NMR study of methanol diffusion in polymer electrolyte fuel cell membranes

Methanol diffusion in two polymer electrolyte membranes, Nafion 117 and BPSH 40 (a 40% disulfonated wholly aromatic polyarylene ether sulfone), was measured using a modified pulsed field gradient NMR method. This method allowed for the diffusion coefficient of methanol within the membrane to be determined while immersed in a methanol solution of known concentration. A second set of gradient pulses suppressed the signal from the solvent in solution, thus allowing the methanol within the membrane to be monitored unambiguously. Over a methanol concentration range of 0.5–8 M, methanol diffusion coefficients in Nafion 117 were found to increase from 2.9 × 10⁻⁶ to 4.0 × 10⁻⁶ cm² s⁻¹. For BPSH 40, the diffusion coefficient dropped significantly over the same concentration range, from 7.7 × 10⁻⁶ to 2.5 × 10⁻⁶cm² s⁻¹. The difference in diffusion behavior is largely related to the amount of solvent sorbed by the membranes. Increasing the methanol concentration results in an increase in solvent uptake for Nafion 117, while BPSH 40 actually excludes the solvent at higher concentrations. In contrast, diffusion of methanol measured via permeability measurements (assuming a partition coefficient of 1) was lower (1.3 × 10⁻⁶ and 6.4 × 10⁻⁷ cm² s⁻¹ for Nafion 117 and BPSH 40 respectively) and showed no concentration dependence. The differences observed between the two techniques are related to the length scale over which diffusion is monitored and the partition coefficient, or solubility, of methanol in the membranes as a function of concentration. For the permeability measurements, this length is equal to the thickness of the membrane (178 and 132 μm for Nafion 117 and BPSH 40 respectively) whereas the NMR method observes diffusion over a length of approximately 4–8 μm. Regardless of the measurement technique, BPSH 40 is a greater barrier to methanol permeability at high methanol concentrations.     

Repetitive determination of ascorbic acid using iron(III)-1.10-phenanthroline-peroxodisulfate system in a circulatory flow injection method

Ascorbic acid (AA) could be determined in large quantities of a co-existing oxidant. The incorporation of an on-line reagent regeneration step based on redox reaction eliminates the baseline drift in the procedure. This makes it possible to adopt a circulatory flow injection method (cyclic FIA) and to determine AA repetitively. The method is based on the reduction of iron(III) to iron(II) by the analyte, the reaction of the produced iron(II) with 1,10-phenanthroline (phen) in a weak acidic medium to form a colored complex, and the subsequent oxidation reaction of iron(II) to iron(III) by the co-existing peroxodisulfate. A solution (50 ml) of 3.0×10⁻⁴ mol l⁻¹ ferric chloride, 9.0×10⁻⁴ mol l⁻¹ phen and 5.0×10⁻² mol l⁻¹ ammonium peroxodisulfate in acetate buffer (0.2 mol l⁻¹, pH 4.5) is continuously circulated at a constant flow rate of 1.0 ml min⁻¹. Into this stream, an aliquot (20 μl) of the sample solution containing AA is quickly injected by means of a six-way valve. The complex formed is monitored spectrophotometrically (at 510 nm) in the flow system. The stream then returns to the reservoir after passing through a time-delay coil (50 m). The iron(II)–(phen)₃ complex is oxidized to iron(III)–(phen)₃ complex by peroxodisulfate which exists excessively in the circulating reagent solution. The proposed method allows as many as 300 repetitive determinations of 15 mg l⁻¹ AA with only 50 ml reservoir solution. The contents of AA in commercial pharmaceutical products were analyzed to demonstrate the capability of the developed system.