Ibrahim H. A. Badr

Flow injection analysis of sulfite ion with a potentiometric titanium phosphate–epoxy based membrane sensor

A potentiometric sensor based on the use of titanium phosphate (TP) in epoxy matrix membrane is prepared and characterized. The sensor exhibits near-Nernstian response for many anionic species over the concentration range 10(-1)-10(-5) mol l(-1). The origin of response is explained on the basis of the conversion of titanium phosphate cation exchanger into hydrated titanium oxide anion exchanger by the effect of the high pH of the epoxy matrix. The sensitivity and selectivity of the sensor for sulfite ions are optimized by conversion of sulfite into gaseous SO(x) by acidification, and diffusion of the gas through a membrane-based gas dialyzer followed by potentiometric detection of sulfite ions formed within a flowing recipient stream. No interferences are caused by many common anions and acidic gas releasing species except sulfide and nitrite ions. Determination of sulfite ion at levels as low as 10(-4) mol l(-1) or less in the presence of nitrite and sulfide ions is performed by using a modified carrier buffer stream (10(-2) mol l(-1) MES, pH 5.0 containing sulfamic acid) and pretreatment with Pb(2+). Advantages offered by the proposed gas dialyzer/flow injection system with TP-epoxy membrane based sensor over traditional ion exchange based sensors includes long life time (>8 months), excellent stability and reproducibility ( approximately 1 mV), fast response time (<30 s), wide pH working range (pH 5-9), high sample throughput ( approximately 60 samples h(-1)), low detection limit (8x10(-6) mol l(-1)) and high thermal stability (up to 80 degrees C).

Determination of Carrageenan in Food Products Using Potentiometric Polyion Sensors

Polymeric matrix tubular membrane sensors incorporating poly(vinyl chloride) matrix, o-nitrophenyloctylether (oNPOE) plasticizer and either dinonylnaphthalene sulfonate (DNNS) or tridodecylammonium chloride (TDMAC) ionophore are prepared and electrochemically evaluated for determination of carrageenan polyion. These sensors are used for monitoring potentiometric titration of carrageenan with protamine titrant. Carrageenan at the concentration levels of 0.2 ± 1.5% (m/m) in a variety of food products (e.g., cream, chocolate, caramel, ice cream, and salad dressing) are satisfactory determined. Results agree fairly well with nominal values are obtained and confirmed by recovery experiments of spiked samples. The method has several advantages over all previously reported methods in being more simple, accurate and applicable for routine analysis of real samples without prior treatment.

NITRITE-SELECTIVE OPTICAL SENSORS BASED ON ORGANOPALLADIUM IONOPHORES

Development of nitrite-selective bulk optode membranes, based on novel organopalladium ionophores, is described herein. These nitrite-selective optodes are composed of a plasticized poly(vinyl chloride) impregnated with the organopalladium ionophore, a proton-selective chromoionophore, and in some cases lipophilic additives. Two novel organopalladium ionophores (NI-I and NI-II) and different proton chromoionophores (ETH-7075, ETH-5294 and ETH-2439) were evaluated for the development of nitrite-selective bulk optode membranes. The developed nitrite optodes were found to be more selective for nitrite over other anions including the more lipophilic anions such as perchlorate, nitrate, iodide, and bromide. For instance, the optical selectivity coefficients, log K nitrite, anion opt, for NI-I, ETH-7075 optode are as follow: ClO4 −, −1.2; NO3 −, −3.4; I−, −1.5; Br−, −2.1. Further, the nitrite optical selectivity obtained using the present optode systems is greatly enhanced in comparison with that obtained using an anion-exchanger based optode membrane. Response characteristics such as, dynamic range, detection limit, response time and signal reproducibility of the prepared nitrite optode membranes are discussed as well.

Cyanex based uranyl sensitive polymeric membrane electrodes.

Novel uranyl selective polymeric membrane electrodes were prepared using three different low-cost and commercially available Cyanex extractants namely, bis(2,4,4-trimethylpentyl) phosphinic acid [L1], bis(2,4,4-trimethylpentyl) monothiophosphinic acid [L2] and bis(2,4,4-trimethylpentyl) dithiophosphinic acid [L3]. Optimization and performance characteristics of the developed Cyanex based polymer membrane electrodes were determined. The influence of membrane composition (e.g., amount and type of ionic sites, as well as type of plasticizer) on potentiometric responses of the prepared membrane electrodes was studied. Optimized Cyanex-based membrane electrodes exhibited Nernstian responses for UO₂(2+) ion over wide concentration ranges with fast response times. The optimized membrane electrodes based on L1, L2 and L3 exhibited Nernstian responses towards uranyl ion with slopes of 29.4, 28.0 and 29.3 mV decade(-1), respectively. The optimized membrane electrodes based on L1-L3 showed detection limits of 8.3 × 10(-5), 3.0 × 10(-5) and 3.3 × 10(-6) mol L(-1), respectively. The selectivity studies showed that the optimized membrane electrodes exhibited high selectivity towards UO₂(2+) ion over large number of other cations. Membrane electrodes based on L3 exhibited superior potentiometric response characteristics compared to those based on L1 and L2 (e.g., widest linear range and lowest detection limit). The analytical utility of uranyl membrane electrodes formulated with Cyanex extractant L3 was demonstrated by the analysis of uranyl ion in different real samples for nuclear safeguards verification purposes. The results obtained using direct potentiometry and flow-injection methods were compared with those measured using the standard UV-visible and inductively coupled plasma spectroscopic methods.

Epoxy Matrix Membrane Potentiometric Sensor Based on Zirconium Titanium Phosphate Ion Exchanger for Flow Injection Analysis of Nitrite

Preparation, characterization, origin of response and applications of a potentiometric sensor based on the use of a zirconium titanium phosphate (ZTP)-epoxy membrane are described. Zirconium titanium phosphate (ZTP) cationic exchanger embedded in an epoxy matrix is converted into a hydrated zirconium oxide anion exchanger which responds to many anionic species. A sensor based on this exchanger exhibits near-Nernstian response for nitrite and other anions over the concentration range 10 71 ‐10 75 M. Under optimized conditions, the gas dialyzer=flow-injection system provides a throughput of ca. 60 samples per hour with a high selectivity for nitrite ion over most common anions and acidic gas releasing species. Determination of nitrite at levels as low as 5610 74 M or less in the presence of sulfite and sulfide ions is performed by using a modified carrier buffer stream (10 72 M MES, pH 5.5 containing formaldehyde and Pb 2a ). Significant advantages are offered by the ZTP-epoxy membrane based sensor including a long 1ifetime (> 2 years), excellent stability and reproducibility (ca. 1 mV), fast response time (< 30 s), wide pH working range (pH 4.5‐8), low detection limit (4610 75 M) and high thermal stability(up to 80C).

Reduction of thrombogenicity of PVC-based sodium selective membrane electrodes using heparin-modified chitosan.

Heparin-modified chitosan (H-chitosan) membrane was utilized to enhance biocompatibility of sodium selective membrane electrode based on the highly thrombogenic polyvinyl chloride (PVC). Sodium ion sensing film was prepared using PVC, sodium ionophore-X, potassium tetrakis(chlorophenyl)-borate, and o-nitrophenyloctylether. The PVC-based sensing film was sandwiched to chitosan or H-chitosan to prevent platelet adhesion on the surface of PVC. Potentiometric response characteristics of PVC-chitosan and PVC-H-chitosan membrane electrodes were found to be comparable to that of a control PVC based sodium-selective electrode. This indicates that chitosan and H-chitosan layers do not alter the response behaviour of the PVC-based sensing film. Biocompatibility of H-chitosan was confirmed by in vitro platelet adhesion study. The platelet adhesion investigations indicated that H-chitosan film is less thrombogenic compared to PVC, which could result in enhancement of biocompatibility of sodium selective membrane electrodes based on PVC, while maintaining the overall electrochemical performance of the PVC-based sensing film.

PVC Membrane electrodes for manual and flow-injection determination of tetraphenylborate: Applications to separate and sequential titrations of some metal ions

Three novel poly (vinyl chloride) matrix membrane electrodes, highly sensitive and selective for tetraphenylborate anion (TPB), are developed and electrochemically evaluated. They are based on the use of iron(II) bathophenanthroline, nickel(II) bathophenanthroline-and nitron-TPB ion-pair complexes as electroactive materials with dioctylphthalate (DOP) and 2-nitrophenyl phenyl ether (NPPE) as plasticizing solvent mediators. The electrodes exhibit stable and rapid near-Nernstian response for 10(-2)-10(-6)M TPB over the pH range 4-10. Use of these electrodes for direct potentiometric determination and potentiometric titration of as low as 1 mug of TPB/ml and 0.6 mg of TPB/ml give results with average recoveries of 99.3% (mean standard deviation 0.5%) and 99.4% (mean standard deviation 0.2%), respectively. Incorporation of nitron-TPB PVC sensor in a flow-through sandwich cell provides an efficient flow-injection detector for determining TPB with an input rate of at least 60 samples/hr. The limit of detection is 1.6 mug TPB/ml in a 20-mul sample. The electrodes are also used to monitor separate and sequential titrations of some metal ions with TPB. Alkaline earth and transition-metal ions upon reaction with polyethylene glycol and ethylenediamine, respectively, form cationic complexes readily titrated with TPB. Optimum conditions are outlined for sequential titrations of various combinations of metallic species.

Enhancing biocompatibility of some cation selective electrodes using heparin modified bacterial cellulose

Bacterial cellulose (BC) and heparin-modified bacterial cellulose (HBC) were utilized to enhance the biocompatibility of highly thrombogenic PVC-based potassium and calcium membrane electrodes. Three types of membrane electrodes were prepared: (1) conventional PVC electrode (control), (2) PVC-based electrode sandwiched with bacterial cellulose membrane (BC-PVC), and (3) PVC-based electrode sandwiched with heparin-modified bacterial cellulose membrane (HBC-PVC). The potentiometric response characteristics of the modified potassium and calcium membrane electrodes (BC-PVC and HBC-PVC) were compared with those of the control PVC-based potassium and calcium selective electrode, respectively. Response characteristics of the modified membrane electrodes were comparable to the control PVC membrane electrode. The platelet adhesion investigations indicated that (BC) and (HBC) layers are less thrombogenic compared to PVC. Therefore, use of BC or HBC would enable the enhancement of the biocompatibility of PVC-based membrane electrodes for potassium and calcium while practically maintaining the overall electrochemical performance of the PVC sensing film.

Kinetics of chlorine isotope exchange reaction between sodium chloride-36 and triphenyltin chloride in mixed solvents

Isotope exchange reaction between NaCl-36 and triphenyltin chloride in dioxane-water (80∶20% w/w) and ethanol-water (90∶10% w/w) mixed solvents has been studied at 25, 35 and 50 °C. The exchange reaction was found to proceed via a bimolecular SN2, limiting mechanism with reaction rates depending on the solvent used. Inhibition of the exchange in ethanol-water is probably due to solvation of chloride ion through hydrogen bond formation. The rate laws for the exchange reactions are: Re=3.24×109 e−65550/RT [Rh3SnCl] α [NaCl] in dioxanewater and Re=6.61×108 e−69600/RT [Ph3SnCl] α [NaCl] in ethanol-water, where α is the degree of dissociation of NaCl and Re is the rate of exchange in mol l−1 s−1. The activation parameters ΔH*, ΔS* and ΔG* are reported.