Construction and performance characterization of ion-selective electrodes for potentiometric determination of phenylpropanolamine hydrochloride applying batch and flow injection analysis techniques

New phenylpropanolamine hydrochloride (PPA.Cl)-selective electrodes of the conventional polymer membrane type, based on incorporation of phenylpropanolamine-tetraphenylborate (PPA-TPB) ion-pair or phenylpropanolamine-phosphotungstate (PPA-PT) ion-associate in a poly(vinyl chloride) (PVC) membrane plasticized with dioctylphethalate (DOP) or dibutylphethalate (DBP), have been constructed. The electrodes were fully characterized in terms of the membrane composition, temperature, and pH. The electrodes were applied to the potentiometric determination of PPA.Cl in pure solutions and in pharmaceutical preparations under batch and flow injection conditions. The sensors showed fast, stable, and Nernstian slope over the concentration ranges 1.0×10⁻⁵ to 8.91×10⁻³ M and 10⁻⁵ to 10⁻² M in the case of PPA-TPB applying batch and flow injection analysis (FIA), respectively, and 5.01×10⁻⁶ to 1.25×10⁻³ M and 10⁻⁵ to 10⁻² M in the case of PPA-PT for batch and FIA systems, respectively. The electrodes exhibited good selectivity for PPA.Cl with respect to a large number of inorganic cations, sugars, amino acids, and components other than phenylpropanolamine of the mixed drugs. The effect of temperature on the electrodes was also studied.     

Potentiometric flow injection analysis of dicyclomine hydrochloride in serum, urine and milk

Five plastic membrane electrodes for the determination of dicyclomine hydrochloride (DcCl) were fabricated and fully characterized in terms of composition, life span, usable pH range, working concentration range and temperature. The membranes of these electrodes consist of dicyclominium-silicotungstate (Dc-ST), silicomolybdate (Dc-SM), phosphotungstate (Dc-PT), phosphomolybdate (Dc-PM) or tetraphenylborate (Dc-TPB) ion-associations dispersed in PVC matrix with dibutyl phthalate plasticizer. The electrodes showed near-Nernstian response over the concentration range of 4.0 × 10⁻⁶ to 1.0 × 10⁻² M DcCl and applied to the potentiometric determination of dicyclominium ion in pharmaceutical preparations, serum, urine and milk in batch and flow injection (FI) conditions with average recoveries of 96.1-102.7% and relative standard deviation of 0.055-1.994%. The electrodes exhibit good selectivity for DcCl with respect to a large number of inorganic cations, organic cations, sugars and amino acids. The sensitivities of these electrodes are high enough to measure as low as 1.73 μg/ml of DcCl which permit the determination of the K_sp values of the ion-associates used. The proposed potentiometric methods offer the advantages of simplicity, accuracy, automation feasibility and applicability to turbid and colored sample solutions.     

Construction of a new Cu2+ coated wire ion selective electrode based on 2-((2-(2-(2-(2-hydroxy-5-methoxybenzylidene amino)phenyl)disufanyl)phenylimino)methyl)-4-methoxyphenol Schiff base

In this article a new coated platinum Cu²⁺ ion selective electrode based on 2-((2-(2-(2-(2-hydroxy-5-methoxybenzylideneamino)phenyl)disufanyl)phenylimino) methyl)-4-methoxyphenol Schiff base (L₁) as a new ionophore is described. This sensor has a wide linear range of concentration (1.2 × 10⁻⁷-1.0 × 10⁻¹ mol L⁻¹) and a low detection limit of 9.8 × 10⁻⁸ mol L⁻¹of Cu(NO₃)₂. It has a Nernstian response with slope of 29.54 ± 1.62 mV decade⁻¹ and it is applicable in the pH range of 4.0-6.0 without any divergence in potentioal. The coated electrode has a short response time of approximately 9 s and is stable at least for 3.5 months. The electrode shows a good selectivity for Cu²⁺ ion toward a wide variety of metal ions. The proposed sensor was successfully applied for the determination of Cu²⁺ ion in different real and environmental samples and as indicator electrode for potentiometric titration of Cu²⁺ ion with EDTA.     

PVC membrane electrode based on triheptyl dodecyl ammonium iodide for the selective determination of molybdate(VI)

Novel polyvinyl chloride (PVC) membrane electrodes based on triheptyl dodecyl ammonium iodide have been developed. In the presence of 12.5 mM H₂O₂, these electrodes are capable of determining molybdate(VI) ion. The electrodes exhibit near-Nernstian responses over a wide concentration range (2.0 × 10⁻⁶ to 5.0 × 10⁻³ M). The proposed electrodes demonstrate satisfying selectivity for molybdate(VI) ion in the presence of a wide variety of anions other than iodide, and can be used in the pH range 5.0-7.0. Moreover, the electrodes show an average response time of 2-3 min and can be used over a period of 2 months without any significant deviation being observed. In the light of our results, the response mechanism of the electrode is discussed and HMoO₂(O₂)₂⁻ is suggested as the response ion. The proposed electrode has been used to measure molybdenum in ore samples, and the results were in agreement with those obtained by means of ICP analysis.     

Comparative studies of mercapto thiadiazoles self-assembled on gold nanoparticle as ionophores for Cu(II) carbon paste sensors

Comparative studies of the potentiometric behavior of three mercapto compounds [2-((5-mercapto-1,3,4-thiadiazol-2-ylimino)methyl)phenol] (MTMP), [5-(2-methoxy benzylidene amino)-1,3,4-thiadiazole-2-thiol] (MBYT) and [5-(pyridin-2-ylmethyleneamino)-1,3,4-thiadiazole-2-thiol] (PYTT) self-assembled on gold nanoparticles (GNPs) as ionophores in carbon paste electrodes (CPEs) have been made. These mercapto thiadiazole compounds were self-assembled onto gold nanoparticles and then incorporated within carbon paste electrode. The self-assembled ionophores exhibit a high selectivity for copper ion (Cu²⁺), in which the sulfur and nitrogen atoms in their structure play a role as the effective coordination donor site for the copper ion. These carbon paste electrodes were applied as indicator electrodes for potentiometric determination of copper ions. The sensor based on PYTT exhibits the working concentration range of 4.0 × 10⁻⁹ to 7.0 × 10⁻² M and a Nernstian slope of 28.7 ± 0.3 mV decade⁻¹ of copper activity. The detection limit of electrode was 1.0 × 10⁻⁹ M and potential response was pH independent across the range of 3.0-6.5. It exhibited a quick response time of <5 s and could be used for a period of 45 days. The ion selectivity of this electrode for Cu²⁺ was over 10⁴ times that for other metal cations. The application of prepared sensors has been demonstrated for the determination of copper ions in spiked water and natural water samples.     

Bulk optode sensors for batch and flow-through determinations of lead ion in water samples

A sensitive optode consisting of highly lead-selective ionophore (Lead IV), proton-selective chromoionophore (ETH 5294) and lipophilic anionic sites (KTpClPB) in plasticized polyvinyl chloride (PVC) membrane was fabricated. The optode membranes were used for determination of Pb²⁺ by absorption spectrophotometry in batch and flow-through systems. The influence parameters such as pH, type of buffer solution, response time and concentration of regenerating solution were optimized. The membrane responded to Pb²⁺ by changing its color from blue to pinkish purple in Tris buffer containing different concentration of Pb²⁺ at pH 7.0. The optode provided the response range of 3.16 × 10⁻⁸ to 5.00 × 10⁻⁵ mol L⁻¹ Pb²⁺ with the detection limit of 2.49 × 10⁻⁸ mol L⁻¹ in the batch system within the response time of 30 min. The dynamic range of 1.26 × 10⁻⁸ to 3.16 × 10⁻⁵ mol L⁻¹ Pb²⁺ with detection limit of 8.97 × 10⁻⁹ mol L⁻¹ were obtained in the flow-through system within the response time of 15 min. Moreover, the proposed optode sensors showed good selectivity towards Pb²⁺ over Na⁺, K⁺, Mg²⁺, Cd²⁺, Hg²⁺ and Ag⁺. It was successfully applied to determine Pb²⁺ in real water samples and the results were compared with well-established inductively coupled plasma optical emission spectrometry (ICP-OES). No significant different value (t_critical = 4.30 > t_exp = 1.00-3.42, n = 3 at 95% of confidence level) was found.     

Rapid spectrophotometric determination of fosfestrol following on-line hydrolysis by alkaline phosphatase using flow injection and chasing zones

A new flow injection (FI) method is reported for the spectrophotometric determination of fosfestrol (diethyl-stilbestrol (DES) diphosphate) in pharmaceutical formulations. The proposed method is based on the on-line hydrolysis of the analyte by alkaline phosphatase (Al-Pase) using a “chasing zones” FI manifold. The orthophosphate ions, thus, generated are determined spectrophotometrically (λ_max=690 nm) using the molybdenum blue approach. The chemical and FI variables affecting the enzymatic reaction were investigated. The proposed method is very precise (s_r=1.1% at 1×10⁻⁴ mol l⁻¹ fosfestrol, n=12), fast (allowing up to 40 samples h⁻¹ to be analyzed) and has a determination range of 2×10⁻⁵ to 2×10⁻⁴ mol l⁻¹, with a satisfactory 3σ detection limit of 5×10⁻⁶ mol l⁻¹. The method was shown to provide accurate determinations of the fosfestrol concentration in a pharmaceutical formulation, giving relative errors, e_r, of +0.6 and −0.5% compared to the value stated by the supplier (Asta Medica Inc.) and the concentration derived using a method recommended by the United States Pharmacopoeia XXI, respectively. In addition, the average recoveries of known amounts of the analyte ranged between 99.2 and 101.2%.     

Enantioanalysis of R-deprenyl based on its molecular interaction with C70 fullerenes

Two enantioselective, potentiometric membrane electrodes based on [5,6]fullerene-C₇₀ (1) and diethyl (1,2-methanofullerene C₇₀)-71-71-dicarboxylate (2) immobilized in carbon paste, were designed for the enantioanalysis of R-deprenyl. The electrodes exhibited near-Nernstian slopes: 57.90 (1) and 59.00 mV/decade of concentration (2), respectively with low limits of detection 5.9 × 10⁻¹¹ (1) and 9.6 × 10⁻¹¹ mol/L (2), respectively. The linear concentration ranges are between 10⁻¹⁰ and 10⁻⁴ mol/L (1) and between 10⁻⁹ and 10⁻⁴ mol/L (2), respectively. The different characteristics involved in the molecular interaction between R-deprenyl and C₇₀ fullerenes were explained, namely (i) the stability of each molecule and (ii) the explanation of the molecular mechanism of interaction, using restricted Hartree-Fock theory, 3-21G(*) RHF-basis set. Furthermore, two intermolecular forces of interactions confer the stability of the electrodes; electrostatic interaction and moderate hydrogen bond interaction. Stability and feasibility of all the generated structures involved in this analysis were supported by their respective fundamental frequencies and energy minima.R-deprenyl can be recovered with average recoveries higher than 99.10% (RSD < 0.03%) from synthetic mixtures between R- and S-deprenyl. The high selectivity and enantioselectivity made possible the enantioanalysis of R-deprenyl in its pharmaceutical formulations.     

Linear polyamines as carriers in thiocyanate-selective membrane electrodes

The polyamines, octyl-[2-(2-octylamino-ethylamino)-ethyl]-amine (L¹) and octyl-{2-[2-(2-octylamino-ethylamino)-ethylamino]-ethyl}-amine (L²), have been used as anion ionophores in PVC-based membrane ion-selective electrodes. Different electrodes were prepared containing L¹, or L², and o-nitrophenyl octyl ether (NPOE) or bis(2-ethylhexyl)sebacate (DOS) as plasticizers. The response of the electrodes was tested in two different buffers, HEPES-KOH (pH 7) and MES-KOH (pH 5.6). Electrodes containing L¹ and L² with NPOE (E1 and E2, respectively) showed a Nernstian response for thiocyanate with a good response time. The detection limit, linear range and slope for electrode E1 were 3.8 × 10⁻⁶ mol dm⁻³, 1 × 10⁻⁵ to 1 × 10⁻¹ mol dm⁻³ and −57.2 mV decade⁻¹ at pH 5.6 and 4.47 × 10⁻⁶ mol dm⁻³, 1.95 × 10⁻⁵ to 1 × 10⁻¹ mol dm⁻³ and −58.1 mV decade⁻¹ at pH 7.0. For electrode E2 the detection limit, linear range and slope found were 2.63 × 10⁻⁶ mol dm⁻³, 7.94 × 10⁻⁶ to 1 × 10⁻¹ mol dm⁻³ and −58.5 mV decade⁻¹ at pH 5.6 and 1.23 × 10⁻⁵ mol dm⁻³, 7.95 × 10⁻⁵ to 1 × 10⁻¹ mol dm⁻³ and −46.0 mV decade⁻¹ at pH 7. In contrast, electrodes containing DOS as plasticizers gave only response at pH 5.6 (detection limit, linear range and slope at pH 5.6 were 3.16 × 10⁻⁵ mol dm⁻³, 1 × 10⁻⁴ to 1 × 10⁻¹ mol dm⁻³ and −52.6 mV decade⁻¹). Selectivity coefficients for different anions with respect to thiocyanate were calculated. The electrode E2 at pH 5.6 was also used for the determination of SCN⁻ by potentiometric titrations with Ag⁺ ions with good results. The electrode E2 was also used to determine concentrations of thiocyanate in biological samples.     

Schiff bases as cadmium(II) selective ionophores in polymeric membrane electrodes

The construction and performance characteristics of polymeric membrane electrodes based on two neutral ionophores, N,N′-[bis(pyridin-2-yl)formylidene]butane-1,4-diamine (S₁) and N-(2-pyridinylmethylene)-1,2-benzenediamine (S₂) for quantification of cadmium ions, are described. The influences of membrane compositions on the potentiometric response of the electrodes have been found to substantially improve the performance characteristics. The best performance was obtained with the electrode having a membrane composition (w/w) of (S₁) (2.15%):PVC (32.2%):o-NPOE (64.5%):KTpClPB (1.07%). The proposed electrode exhibits Nernstian response in the concentration range of 7.9 × 10⁻⁸ to 1.0 × 10⁻¹ M Cd²⁺ with limit of detection 5.0 × 10⁻⁸ M, performs satisfactorily over wide pH range (2.0-8.0) with a fast response time (10 s). The sensor has been found to work satisfactorily in partially non-aqueous media up to 30% (v/v) content of methanol, ethanol and acetonitrile and could be used for a period of 2 months. The analytical usefulness of the proposed electrode has been evaluated by its application in the determination of cadmium in real samples. The practical utility of the membrane electrode has also been observed in the presence of surfactants.     

Potentiometric evaluation of calix[4]arene anion receptors in membrane electrodes: phosphate detection

Ion-selective membrane electrodes doped with the urea- or thiourea-functionalised calix[4]arenes, 5,11,17,23-tetra-tert-butyl-25,27-bis[[4-N′-(phenylureido)butyl]oxy]-26,28-dipropoxy calix[4]arene (I) and 5,11,17,23-tetra-tert-butyl-25,27-bis[[4-(N′-phenylthioureido)-butyl]oxy]-26,28-dipropoxy calix[4]arene (II), were evaluated for anion sensing. Potentiometric results show that these calixarene ionophore-based membrane electrodes exhibit a good sensitivity to aqueous solutions of the monohydrogen orthophosphate species HPO₄²⁻ in the concentration range 5.0 × 10⁻⁵ to 1.0 × 10⁻¹ M, with near-Nernstian response slopes of −33.0 and −28.0 mV dec⁻¹ for ionophores I and II, respectively. Selectivity coefficient values for monohydrogen orthophosphate over a range of common anions were determined by the fixed interference and matched potential methods and indicated that these membrane electrodes exhibit a good selectivity for HPO₄²⁻ with respect to the other anions, including sulfate and nitrate.     

Host hydrated barium phenylphosphonate/guest heterocyclic amine intercalation energetics by calorimetric titration

The heterocyclic amines 2,6-lutidine, pyrazine, piperazine and piperidine were intercalated into layered crystalline hydrated barium phenylphosphonate, Ba(HO₃PC₆H₅)₂·H₂O, through a batch method in ethanolic solution, to give the maximum amounts 0.39, 0.82, 2.80 and 5.50 mmol g⁻¹, respectively. The original host interlayer distance (d) of 1532 pm increased after intercalation for piperazine (1752 pm) and piperidine (2112 pm) molecules, while for 2,6-lutidine and pyrazine molecules d values were maintained. The enthalpy of intercalation gave −5.60 ± 0.10, −1.00 ± 0.02, −9.55 ± 1.00 and −30.70 ± 0.68 kJ mol⁻¹ for the sequence of heterocyclic amines. The Gibbs free energies are negative and entropies are positive for intercalation.     

Enhanced electrochemical performance at screen-printed carbon electrodes by a new pretreating procedure

A new method for the pretreatment of screen-printed carbon electrodes (SPCEs) by two successive steps was proposed. In step one, fresh SPCEs were soaked into NaOH with high concentration (e.g. 3 M) for tens to hundreds of minutes, and the resulted electrodes were called as SPCE-I. In step two, SPCE-I were pre-anodized in low concentration of NaOH, which were designated as SPCE-II. The pretreated electrodes showed remarkable enhancement in heterogeneous electron transfer rate constant (k⁰) increased from 1.6 × 10⁻⁴ cm s⁻¹ at the fresh SPCE to 1.1 × 10⁻² cm s⁻¹ at SPCE-I for Fe(CN)₆^3−/4− couple. The peak to peak separation (ΔE_p) in cyclic voltammetry was reduced from ca. 480 to 84 mV, indicating that the electrochemical reversibility was greatly promoted, possibly due to the removing of polymers/oil binder from the electrode surfaces. The electroactive area (A_ea) of the electrode was increased by a factor of 17 after pretreatment in step one. Further analysis by the electrochemical impedance method showed that the electron transfer resistance (R_ct) decreased from ca. 2100 to 1.4 Ω. These pretreated electrodes, especially SPCE-II, exhibited excellent electrocatalytic behavior for the redox of dopamine (DA). Interference from ascorbic acid (AA) in the detection of DA at SPCE-II could be effectively eliminated due to the anodic peak separation (190 mV) between DA and AA, which resulted from the functionalization of the electrode surface in the pretreatment of step two. Under optimum conditions, current responses to DA were linearly changed in two concentration intervals, one was from 3.0 × 10⁻⁷ to 9.8 × 10⁻⁶ M, and the other was from 9.8 × 10⁻⁶ to 3.3 × 10⁻⁴ M. The detection limit for DA was down to 1.0 × 10⁻⁷ M.     

11-Molybdobismuthophosphate—A new reagent for the determination of ascorbic acid in batch and sequential injection systems

The guanidinium salt of the new heteropolymolybdate 11-molybdobismuthophosphate Gua₆PBiMo₁₁O₄₀ (11-MBP) was synthesized, characterized and used as a reagent for batch spectrophotometric (SP) and sequential injection determination of ascorbic acid (AsA). When compared to other Keggin's heteropolyanions, the reduction of 11-MBP with AsA is both fast and maximal within a pH range of 1.6-2.0. The stoichiometry of the reaction was determined using molar ratio and continuous variation methods and was shown to be 1:1. The molar absorptivity of the reduced form of 11-MBP was 6.0 × 10³ L mol⁻¹ cm⁻¹ at 720 nm. The reaction is also specific for AsA. Only cysteine, hydroquinone and hydroxyacids were found to interfere with the reaction, while no interference was observed with the common reducing agents, including reducing sugars, catecholamines, nitrite, sulfite and iron(II) ions. Batch SP and sequential injection analysis (SIA) systems were developed for the determination of AsA, with calibration ranges of the SP methods at 2 × 10⁻⁶-8 × 10⁻⁵ M for a 10 mm cell and 5 × 10⁻⁷-3 × 10⁻⁵ M for a 50 mm cell and a limit of detection at 3 × 10⁻⁷ M. The linear range of the SIA method was 6 × 10⁻⁶-5 × 10⁻⁴ M, with a detection limit of 2 × 10⁻⁶ M and a sample throughput of 15 h⁻¹. The proposed methods were successfully used for the determination of AsA in both pharmaceuticals and fruit juices, and the results were consistent with those provided by the 2,6-dichlorophenolindophenol method.     

Investigation on enhanced chemiluminescence reaction systems with bis(hydrogenperiodato) argentate(III) complex anion for fluoroquinolones synthetic antibiotics

A novel chemiluminescence (CL) reaction system with bis(hydrogenperiodato) argentate(III) complex anion (Ag(III) complex, [Ag(HIO₆)₂]⁵⁻), for the first time, is developed for the determination of lomefloxacin (LMFX), enrofloxacin (ENLX) and pefloxacin (PFLX). The possible CL emission mechanism was discussed by comparing the fluorescence emission with CL spectra. The CL conditions of [Ag(HIO₆)₂]⁵⁻-H₂SO₄-LMFX/ENLX/PFLX systems were investigated and optimized. Under the optimized experimental conditions, the CL intensity is proportional to the concentration of the drugs in the range 0.2994-36.80 × 10⁻⁷ g mL⁻¹ for LMFX, 4.00-30.0 × 10⁻⁷ g mL⁻¹ for ENLX and 1.54-27.64 × 10⁻⁷ g mL⁻¹ for PFLX. The limit of detection (s/n = 3) was 9.1 × 10⁻⁹ g mL⁻¹ for LMFX, 3.1 × 10⁻⁹ g mL⁻¹ for ENLX and 4.4 × 10⁻⁹ g mL⁻¹ for PFLX. The recovery of LMFX, ENLX and PELX from the spiked pharmaceutical preparations was in the range of 92.3-105% with the RSDs of 0.5-2.7%. For urine, serum and milk samples the recoveries of the three drugs were in the range of 85.1-107% for LMFX with the RSDs of 2.3-3.4%. 80.2-112% for ENLX with the RSDs of 1.4-2.8%, and 87.8-114% for PFLX with the RSDs of 1.6-2.7%. The proposed method was applied successfully to the determination of these compounds in real samples.     

A method for rapid screening of interactions of pharmacologically active compounds with albumin

We determine the association constants for ligand–protein complex formation using the flow injection method. We carry out the measurements at high flow rates (F = 1 mL min⁻¹) of a carrier phase. Therefore, determination of the association constant takes only a few minutes. Injection of 1 nM of the ligand (10 μL of 1 μM concentration of the ligand solution) is sufficient for a single measurement. This method is tested and verified for a number of complexes of selected drugs (cefaclor, etodolac, sulindac) with albumin (BSA). We obtain K = 4.45 × 10³ M⁻¹ for cefaclor, K = 1.00 × 10⁵ M⁻¹ for etodolac and K = 1.03 × 10⁵ M⁻¹ for sulindac in agreement with the literature data. We also determine the association constants of 20 newly synthesized 3β- and 3α-aminotropane derivatives with potential antipsychotic activity – ligands of 5-HT_1A, 5-HT_2A and D₂ receptors with the albumin. Results of the studies reported here indicate that potential antipsychotic drugs bind weakly to the transporter protein (BSA) with K ≈ 10²–10³ M⁻¹. Our method allows measuring K in a wide range of values (10²–10⁹ M⁻¹). This range depends only on the solubility of the ligand and sensitivity of the detector.     

Metallo-tetraazaporphyrin based anion sensors: regulation of sensor characteristics through central metal ion coordination

The iron(III) and cobalt(III) complexes of 2,3,7,8,12,13,17,18-octakis(benzylthio)-5,10,15,20-tetraazaporphyrin, (OBTAP) were synthesized and incorporated into PVC matrix as ionophores to fabricate anion selective membrane electrodes that exhibit selective potentiometric response to azide and nitrite ions, respectively. The membrane of [Fe(OBTAP)]⁺ (III) with a composition of 6:190:200 (III:DBP:PVC) (w/w), and of [Co(OBTAP)]⁺ (IV) with a composition of 10:148:200 (IV:DOP:PVC) (w/w), i.e. 1a and 2b, respectively (where DBP:dibutylphthalate and DOP=dioctylphthalate) gave the best performance. The membrane 1a showed a slope of 29.2±0.2 mV per decade of activity for N₃⁻ in the working concentration range of 8.9×10⁻⁶ to 1.0×10⁻¹ M. The membrane 2b showed a slope of 30.0±0.2 mV per decade of activity for NO₂⁻ in the working concentration range of 1.1×10⁻⁵ to 1.0×10⁻¹ M. The membranes worked satisfactorily in the pH range of 4.3-10.5 (1a) and 2.8-6.4 (2b) and had fast response time of 12±2 and 13±2 s, respectively. Electrodes exhibited a high degree of selectivity for N₃⁻ and NO₂⁻, respectively, over several other monovalent and bivalent anions. Only SCN⁻ and S²⁻ (at >1.0×10⁻⁴ M) cause moderate interference for electrode 1a and Cl⁻ and S²⁻ (at >1.0×10⁻⁵ M) for electrode 2b. They gave reproducible results with the relative standard deviation in the observed values of potentials (σ) of 1.96 and 1.80 mV for electrodes 1a and 2b, respectively, from the least-squares fit line. The 90% confidence limit lies within ±0.2 mV per decade of activity. Reproducible results were obtained over a period of 5 months. Their performance in non-aqueous solvent mixtures having up to 50% (v/v) methanol, ethanol and acetone were evaluated and were found satisfactory. The proposed sensors are superior in terms of detection limit and response time in comparison to the reported ones.     

Flow injection determination of thyroxine in pharmaceutical preparations using tris(2,2′-bipyridyl)ruthenium(III)-NADH chemiluminescence detection

A flow injection (FI) method is reported for the determination of thyroxine based on its enhancement of chemiluminescence (CL) from the Ru(bpy)₃³⁺-NADH system. The calibration graph was linear over the range 2.0-10 × 10⁻⁸ mol L⁻¹ (r² = 0.9989) with relative standard deviations (R.S.D.) in the range 2.0-4.5% (n = 4). The limit of detection (3σ blank) was 1.0 × 10⁻⁹ mol L⁻¹ with sample throughput of 120 h⁻¹. The effect of some organic compounds, anions and cations were studied for l-thyroxine determination. The method was applied to pharmaceutical preparations and the results obtained were in reasonable agreement with the amount labeled. The method was statistically compared with the results obtained by RIA; no significant disagreement at 95% confidence limit was observed. A calibration graph of NADH over the range 1.3 × 10⁻⁸-1.3 × 10⁻⁶ mol L⁻¹ was also established (r² = 0.9992) with R.S.D. in the range1.0-3.5% (n = 4). The limit of detection (3σ) was 1.0 × 10⁻¹⁰ mol L⁻¹ NADH.     

Construction of novel simple phosphate screen-printed and carbon paste ion-selective electrodes

The construction and performance characteristics of different phosphate ion-selective electrodes are described. Three types of electrodes are demonstrated, namely screen-printed, carbon paste and the conventional PVC membrane electrodes. The cited electrodes are based on bisthiourea ionophores and show a considerable selectivity towards hydrogenphosphate with Nernstian slopes depending on the type of the electrode and the ionophore used. Matrix compositions of each electrode are optimised on the basis of effects of type and concentration of the ionophore as well as influence of the selected plasticizers. The screen-printed electrodes work satisfactorily in the concentration range 10⁻⁵ to 10⁻² mol L⁻¹ with anionic Nernstian compliance (32.8 mV/decade activity) and detection limit 4.0 × 10⁻⁶ mol L⁻¹. The screen-printed electrodes show fast response time of about 2.2 s and exhibit adequate shelf-life (4 months). The fabricated electrodes can be also successfully used in the potentiometric titration of HPO₄²⁻ with Ba²⁺.     

Electrochemical,magnetic, catalytic and DNA cleavage studies of binuclear copper(II) complexes derived from pendant substituted tetraaza macrobicyclic compartmental ligands

A series of macrobicyclic unsymmetrical binuclear copper(II) complexes of compartmental ligands were synthesized from the Schiff base condensation of 1,8[N,N′-bis{(3-formyl-2-hydroxy-5-methyl)benzyl}]-1,4,8,11- tetraaza-5,5,7,12,12,14-hexa methylcyclotetradecane with diamines like 1,2-diamino ethane, 1,3-diamino propane, 1,4-diaminobutane, 1,2-diaminobenzene and 1,8-diaminonaphthalene. The complexes were characterized by elemental and spectral analysis. Electrochemical studies of the copper(II) complexes show two irreversible one-electron reduction processes around E¹_pc = −0.70 to −1.10 V and E²_pc = −0.98 to −1.36 V. ESR spectra of the binuclear copper(II) complexes show a broad signal at g = 2.10 and μ_eff values in the range 1.46–1.59 BM, which convey the presence of antiferromagnetic coupling. Cryomagnetic investigation of the binuclear complexes [Cu₂L³(ClO₄)](ClO₄) and [Cu₂L⁴(ClO₄)](ClO₄) show that the observed −2J values are 144 and 216 cm⁻¹, respectively. The observed initial rate (V_in) for the catalytic hydrolysis of p-nitrophenyl phosphate by the binuclear copper(II) complexes were in the range 1.8 × 10⁻⁵ to 2.1 × 10⁻⁵ Ms⁻¹. The initial rate (V_in) for the catalytic oxidation of catechol to o-quinone by the binuclear copper(II) complexes were in the range 2.7 × 10⁻⁵ to 3.5 × 10⁻⁵ Ms⁻¹. The copper(II) complexes have been found to promote cleavage of plasmid pBR 322 DNA from the supercoiled form I to the open circular form II.