Potentiometric Flow Injection Research Articles

Cacodylate Sensors and their Application in the Determination of Amino Acid Levels in Biological Samples.

The importance of recognizing and quantifying chemical anions/cations found in various types of samples, including environmental and biological samples, has been extensively studied. Recent findings suggest the possibility of health risks caused by organic compound dimethylarsinic acid (DMAs) rather than its inorganic arsenic metabolite. This article aims to fabricate polymeric-membrane electrochemical sensors with high sensitivity and selectivity for the cacodylic acid sodium salt dimethylarsinate (DMAs) based on silver diethyldithiocarbamate (AgDDTC) and CuIIphthalocyanine (CuPC) as novel neutral carriers and their applications. DMAs calibration relations and titrations were carried out using a potentiometric workstation equipped with a double-junction reference electrode, in conjunction with the fabricated working electrodes. Sensors revealed fast and stable anionic response with near-Nernstian slopes (-38.6 ± 0.9 and -31.5 ± 0.6 mV/decade), within concentration ranges (1.7 × 10-5 -1.0 × 10-2 and 3.0 × 10-5 -1.0 × 10-2 M) and detection limits (1.0 × 10-5 and 1.6 × 10-5 M) for AgDDTC- and CuPC-based sensors, respectively. Sensors are characterized by extended life-time, signal stability, high precision and short response times. Selectivity for the cacodylate anion over most common anions was tested for the proposed electrodes. Sensors were satisfactorily applied for DMAs quantification in biological matrices with recoveries ranging between 96.2 and 99.0%. Membrane sensors were interfaced with a flow-through system for continuous monitoring of DMAs. The sensors were tested for the assay of different amino acids based on their reaction with cacodylate, where reaction end points were monitored with the proposed electrodes using direct potentiometric determination and flow injection analysis (FIA). Potentiometric ion-selective PVC-membrane electrodes based on silver diethyldithiocarbamate (AgDDTC) and CuIIphthalothyanine (CuPC) provide adequate and reliable means for the determination of dimethylarsenate anion (cacodylate anion, DMAs). These membrane electrodes are easy to manufacture, they have the advantages of high selectivity and sensitivity, broad dynamic ranges, low detection limits, quick response times and cost effectiveness. Such properties make these sensors suitable for the assay of DMAs levels in aqueous solutions by direct potentiometry, flow injection and potentiometric titration, as well as in monitoring of the titration end points of the reactions between various amino acids and DMAs anion in aqueous solutions. Simple electrochemical membranes for dimethylarsinate (DMAs) were prepared, based on diethyldithiocarbamate (AgDDTC) and CuIIphthalocyanine (CuPC). - DMAs sensors were fabricated in two different modules: batch (for static) and flow-through (for hydrodynamic) approaches. - Levels of DMAs were determined in spiked biological samples. - AgDDTC-based sensors were successfully applied in the determination of several amino acids via potentiometric titration with DMAs.

Development of a robust, fast screening method for the potentiometric determination of iodide in urine and salt samples

In this work, a potentiometric flow injection method is described for the fast bi-parametric determination of iodide and iodate in urine and salt samples. The developed methodology aimed for iodine speciation with a potentially portable system (running on batteries). The iodate reduction to iodide was effectively attained in line within the same manifold. The iodide determination was accomplished in the dynamic range of 2.50×10−6–1.00×10−3M and the total iodine dynamic range, resulted from iodide plus iodate, was 3.50×10−6–2.00×10−3M. The calculated limits of detection were 1.39×10−6M and 1.77×10−6M for iodide and iodate, respectively. A determination rate of 21h−1 for the bi-parametric iodide and iodate determination was obtained for sample injection. The urine samples (RSD <5.8% for iodide and RSD <7.0% for iodate) results were in agreement with those obtained by the classic Sandell-Kolthoff reaction colorimetric reference procedure (RD <7.0%) and standard samples from Centers for Disease Control and Prevention, Atlanta, USA (CDC) international inter-laboratory EQUIP program. The developed flow method was also successfully applied to the iodide and iodate determination in salt samples (RSD <3.1% for iodate and iodide), with comparable results to conventional procedures. No significant interferences were observed interference percentage <9% for both determinations.

Potentiometric flow injection sensing system for determination of heparin based on current-controlled release of protamine

A flow injection system incorporated with a polycation-sensitive polymeric membrane electrode in the flow cell is proposed for potentiometric determination of heparin. An external current in nano-ampere scale is continuously applied across the polymeric membrane for controlled release of protamine from the inner filling solution to the sample solution, which makes the electrode membrane regenerate quickly after each measurement. The protamine released at membrane–sample interface is consumed by heparin injected into the flow cell via their strong electrostatic interaction, thus decreasing the measured potential, by which heparin can be detected. Under optimized conditions, a linear relationship between the potential peak height and the concentration of heparin in the sample solution can be obtained in the range of 0.1–2.0UmL−1, and the detection limit is 0.06UmL−1. The proposed potentiometric sensing system has been successfully applied to the determination of heparin in undiluted sheep whole blood.

Determination of the titratable acidity and the pH of wine based on potentiometric flow injection analysis

A FIA system using a pH-sensitive detector based on a graphite/quinhydrone/silicone composite electrode was applied to determine sequentially the titratable acidity and the pH of wine, as well as the sum of calcium and magnesium ions. For all measurements the same FIA configuration was used employing different carrier solutions. The results for the determination of acidity and pH are in good agreement with those obtained by classical potentiometric titrations and by pH measurements using a conventional glass electrode. The standard deviation was less than 1.5% for both kinds of measurements and the sample volume was 150μL. The method allows about 40 determinations of titratable acidity per hour and 30 pH measurements per hour. The titration method can be adjusted to the legal requirements in USA and Europe.

A Flow‐Injection Potentiometric System for Selective and Sensitive Determination of Serum Lithium Level

AbstractA potentiometric flow injection system was adopted by using a Li+‐selective electrode with improved selectivity for sensitive and selective determination of Li+ in human serum spiked samples. For the optimization of the offered flow injection system, multivariate method was utilized. Potentiometric characteristics of the system were investigated (linear range: 5.0×10−5−1.0×10−1 mol L−1, slope: 56.93 mV per decade, detection limit: 3.4×10−5 mol L−1, sampling‐rate: 40 injections per hour). The detection of Li+ in real serum samples was achieved. The obtained results for the spiked serum samples were in good agreement with IC results at 95 % confidence level.

Carbon paste electrode modified with chromium thiopental for the potentiometric flow injection analysis of chromium (III)

A chemically modified carbon paste electrode for chromium (III) based on the formation of ion-association complex of chromium with 5-ethyl-6-oxo-5-pentan-2-yl-sulfanyl-pyrimidin-4-olate (thiopental, THP) as electroactive ion-exchanger (Cr-THP) was prepared and investigated. The electrode has a linear dynamic range of 1.7 × 10−6–1.3 × 10−2 mol L−1, with a Nernstian slope of 19.6 ± 0.5 mV per decade and a detection limit of 9.0 × 10−7 mol L−1. It has a fast response time of <15 s and can be used for at least 7 weeks without any considerable divergence in potential. The proposed sensor revealed good selectivity for Cr(III) over a wide variety of mono-, di-, and trivalent cations and could be used in the pH range of 2.3–6.5. The proposed electrode was used in batch measurements and in flow injection analysis. The Cr-CMCPE was successfully applied for the determination of Cr(III) in real samples (wastewater of hot dip galvanizing unit and alloys). The present electrode was also used as an indicator electrode in potentiometric titration of Cr(III). The results were highly satisfactory.

Potentiometric flow injection system for determination of reductants using a polymeric membrane permanganate ion-selective electrode based on current-controlled reagent delivery

A polymeric membrane permanganate-selective electrode has been developed as a current-controlled reagent release system for potentiometric detection of reductants in flow injection analysis. By applying an external current, diffusion of permanganate ions across the polymeric membrane can be controlled precisely. The permanganate ions released at the sample-membrane interface from the inner filling solution of the electrode are consumed by reaction with a reductant in the sample solution thus changing the measured membrane potential, by which the reductant can be sensed potentiometrically. Ascorbate, dopamine and norepinephrine have been employed as the model reductants. Under the optimized conditions, the potential peak heights are proportional to the reductant concentrations in the ranges of 1.0 × 10 −5 to 2.5 × 10 −7 M for ascorbate, of 1.0 × 10 −5 to 5.0 × 10 −7 M for dopamine, and of 1.0 × 10 −5 to 5.0 × 10 −7 M for norepinephrine, respectively with the corresponding detection limits of 7.8 × 10 −8, 1.0 × 10 −7 and 1.0 × 10 −7 M. The proposed system has been successfully applied to the determination of reductants in pharmaceutical preparations and vegetables, and the results agree well with those of iodimetric analysis.

Rapid and Automated Analytical Methods for Redox Species Based on Potentiometric Flow Injection Analysis Using Potential Buffers

Two analytical methods, which prove the utility of a potentiometric flow injection technique for determining various redox species, based on the use of some redox potential buffers, are reviewed. The first is a potentiometric flow injection method in which a redox couple such as Fe(III)-Fe(II), Fe(CN)6 3−-Fe(CN)(CN)6 4−, and bromide-bromine and a redox electrode or a combined platinum-bromide ion selective electrode are used. The analytical principle and advantages of the method are discussed, and several examples of its application are reported. Another example is a highly sensitive potentiometric flow injection method, in which a large transient potential change due to bromine or chlorine as an intermediate, generated during the reaction of the oxidative species with an Fe(III)-Fe(II) potential buffer containing bromide or chloride, is utilized. The analytical principle and details of the proposed method are described, and examples of several applications are described. The determination of trace amounts of hydrazine, based on the detection of a transient change in potential caused by the reaction with a Ce(IV)-Ce(III) potential buffer, is also described.

Using PVC ion-selective electrodes for the potentiometric flow injection analysis of distigmine in its pharmaceutical formulation and biological fluids

The construction and electrochemical response characteristics of poly(vinylchloride) (PVC) membrane selective electrodes for the determination of distigmine (Ds) are described. The sensing membrane comprised an ion-pair based on distigmine phosphomolybdate (Ds-PM), distigmine phosphotungstate (Ds-PT), distigmine silicomolybdate (Ds-SM), distigmine silicotungstate (Ds-ST), distigmine tetraphenylborate (Ds-TPB), and distigmine reineckate (Ds-Rein) in a plasticized PVC matrix with dioctylphthalate (DOP). The influence of membrane composition on the electrodes’ response was studied. The electrodes showed a fast, stable and Nernstian response over a wide distigmine concentration range 5.0 × 10 −7–1 × 10 −2 mol L −1 with a slope of ∼30.5 ± 1.0 mV dec −1. The response is independent of the pH of test solution within the range 3.8–10.5. The life span of the electrodes extends to at least 2 months without any considerable divergence in potential and has a fast response time of <15 s. The electrodes showed good selectivity towards distigmine with respect to large numbers of ions in batch and FIA systems. The electrodes have been applied to the determination of distigmine in pure solution, pharmaceutical compound and human urine. The dissolution profile for Ubretid tablets (5 mg/tablet) was studied.

Determination of Coenzyme A (CoASH) in the Presence of Different Thiols by Using Flow-Injection with a UV/Vis Spectrophotometric Detector and Potentiometric Determination of CoASH Using an Iodide ISE

Coenzyme A (CoA or CoASH) is one of the most important biologically active compounds, and for this reason a reliable, fast and simple determination of this species is needed. We describe a simple and fast assay of CoASH using potentiometric flow-injection analysis and spectrophotometric kinetic determination. The described methods are suitable for use over a wide CoASH concentration range (1×10-6 – 1×10-4 M).

Potentiometric flow-injection estimation of tea fermentation degree

A potentiometric flow-injection (FI) system was developed for estimating tea fermentation degrees; the method is based on the redox potentials of tea infusions. The infusions were extracted with phosphate buffer (10 mM, pH7.0), and injected (200 μL) into the FI system (1 mL min −1). The redox potentials ( vs. Ag/AgCl) of carrier stream were monitored with a home-made electrochemical flow cell equipped with a platinum indicating electrode. Flow-injection strategy successfully decreased the problem of electrode fouling with increased reproducibility (S.D. < 1.5 mV) and elevated sample throughput (>20 determination h −1). During indoor withering process (0–24 h) for making pouching tea, the redox potential of tea infusions rose (110–130 mV) along with the depletion of tea catechins (100–75 mg g −1 dry tea powder, by HPLC). The system can also be used to estimate the fermentation degrees of commercial tea samples. For commercialization, the platinum electrode can be replaced with a screen-printed carbon-paste disposable electrode with similar performance.

A Novel Solid-Contact Sensor for Flow Injection Determination of Verapamil in Pharmaceutical Formulations and Urine

A novel coated-graphite selective sensor based on verapamil ion pair with phosphomolybdate (PM) for its flow injection potentiometric (FIP) determination has been described. The sensor was prepared by coating the membrane cocktail containing PVC, plasticizer, and carrier on the surface of graphite rode. Influences of the membrane composition, pH, ions and possible interfering anions on the response properties of the electrode were investigated. The sensor membrane containing 3.6 % Ver-PM ion-pair and 62.0 % dioctylphethalate (DOP) in PVC possesses the best response with quasi- Nernstian slope of 62.6 mV/ decade over a wide concentration range of 6x10-6- 1x10-2 M and a lower limit of detection (LDL) of 1.9 x10-6 M. The developed sensor has been applied for FIP determination of verapamil hydrochloride in pure solution, pharmaceutical preparations and urine. The parameters which control the FIP method have been optimized. The determination of verapamil assay in Verpamil tablets after shelf-storage for more than one year using the proposed sensor has been achieved. The percent of verapamil hydrochloride in tablets was found to decrease as the storage time increases and it reached a value of 73.5 % of its nominal value after 14 months storage. Keywords: Potentiometric flow-injection, Verapamil, CGE, Pharmaceutical preparations, Urine

A potential high-throughput method for the determination of lipase activity by potentiometric flow injection titrations

Potentiometric FIA titrations were performed to determine enzyme activities of lipase type B from Candida antarctica, CAL-B. Two substrates, triacetin and tributyrin were hydrolyzed in phosphate buffer solutions, and the concentration change of the base component of the buffer was titrated in a carrier solution containing hydrochloric acid and potassium chloride. The system was calibrated with butyric acid and acetic acid, respectively. FIA titration peaks were evaluated with respect to peak height and peak area. Butyric acid and acetic acid could be titrated in the buffer solution from 3 × 10 −3 mol L −1 to 0.1 mol L −1. The detection limit of enzyme activity was determined to be 0.07 U mL −1 (15 min reaction time) and the minimum activity was calculated to be 0.035 units corresponding to 35 nmol min −1. The specific activities of lipase B for the hydrolysis of tributyrin and triacetin were determined as 16 ± 2 U mg −1 and 2 ± 0.2 U mg −1 (per mg commercial lipase preparation), respectively.

Potentiometric Batch and Flow Injection Analysis of Betaine Hydrochloride

Novel PVC membrane electrodes for the determination of betaine ion based on the formation of betaine-tetraphenylborate (Be-TPB) and betaine-phosphotungstate (Be-PT) ion-exchangers as electroactive materials are described. The sensors show a fast, stable, near Nernstian response for 6.92 x 10(-6) to 7.94 x 10(-3) M and 1.0 x 10(-4) to 1.0 x 10(-2) M betaine hydrochloride (Be.Cl) in case of Be-TPB electrode applying batch and flow injection analysis (FIA), respectively, and 2.95 x 10(-5) to 2.26 x 10(-3) M and 3.16 x 10(-5) to 1.0 x 10(-2) M in case of Be-PT electrode for batch and FIA electrodes, respectively, at 25 degrees C over the pH range of 3.5-10 with a cationic slope of 60.2 and 59.1 mV decade(-1) and a fast potential response of < or =15 s. The lower detection limits are 7.94 x 10(-6) and 3.18 x 10(-5) M Be.Cl for Be-TPB and Be-PT electrodes, respectively. Selectivity coefficient data for some common inorganic cations, sugars, amino acids and the components other than betaine, of the mixed drug investigated show negligible interference. The electrodes have been applied to the direct potentiometric determination of betaine hydrochloride in water and in a pharmaceutical preparation under batch and FIA conditions. Potentiometric titrations of Be.Cl with NaTPB and PTA as titrants were monitored with the developed betaine electrodes as an end point indicator electrode. The determination of Be.Cl shows an average recovery of 100.8% with mean relative standard deviation of 0.61%. The effect of temperature on the electrodes was also studied.

Application of an in-line imprinted polymer column in a potentiometric flow-injection chemical sensor to the determination of the carbamate pesticide carbaryl in complex biological matrices

A flow-injection biosensor-like system based on a nonenzymatic approach has been developed to determine the carbamate pesticide carbaryl in complex biological samples without lengthy and expensive extraction steps. Molecularly imprinted polymeric beads were used to immobilize carbaryl from biological samples. pH variation permitted the elution of carbaryl from the binding cavity to the flow cell. A pH electrode was used to detect changes in the charge of carbaryl in the sample solution resulting from the protonation and deprotonation of the molecule over different pH ranges. At pH 2.0, the secondary amine group is protonated, giving a (+1) charge to the carbaryl molecule. At pH 8.0, the ionized carbaryl loses a proton to become neutral, changing the local pH of the flow cell. The pH change at the flow cell generated by the deprotonation of carbaryl ion in alkaline medium was used to determine the carbaryl concentration. Parameters influencing the performance of the system were optimized for use in the detection procedure. The validated biosensor-like system had a carbaryl detection limit of 10.0 microg/mL and a response that was linear (r2>0.98) over the concentration range of 10.0-00 microg/mL.

Validation of a flow-injection-gas diffusion method for total volatile basic nitrogen determination in seafood products

A potentiometric flow-injection method, using a gas-diffusion cell, is described for the determination of total volatile basic nitrogen (TVB-N) in seafood products. The method was based on the change of the potential of a tungsten oxide electrode when volatile basic compounds, liberated from the fish extract sample, diffuses via a permeable membrane into a phosphate buffer acceptor stream and locally shifts the pH. After being optimized the method was validated in terms of linearity, precision sensitivity and recovery. Linear calibration graph was obtained in the range 1–20 mg of N per L with a detection limit (S/N = 3) of 0.02 mg N per L. The precision (relative standard deviation, n = 10) of this method for within and between-days is better than 1.2% and 2.2%, respectively. The method was applied for the analysis of TVB-N in four fish species (sardine, red mullet, mackerel and hake) and gave results in close agreement with those obtained by the official method. Comparison of the results using student t-test and variance ratio F-test showed there was no significant difference between the methods compared. Thus the proposed method is simple, easy to handle and reliable instrument for quality control of seafood products with a high throughput (40 samples h −1).

Potentiometric Flow Injection Analysis of Bromhexine Hydrochloride and its Pharmaceutical Preparation Using Conventional and Coated Wire Ion-Selective Electrodes

Bromhexine hydrochloride ion-selective electrodes (conventional type) based on bromhexinium tetraphenyl borate (I) and bromhexinium-phosphotungstate (II) were prepared. The electrodes exhibited mean slopes of calibration graphs of 59.4 mV and 59.8 mV per decade of bromhexine concentration at 25°C for electrode (I) and (II), respectively. Both electrodes could be used within the concentration range 3.16x10-5-1.00x10-2 M bromhexine within the pH range 2.0-4.5. The standard electrodes potentials were determined at different temperatures and used to calculate the isothermal coefficients of the electrodes, which were 0.00065 and 0.00050 V°C-1 for (I) and (II), respectively. The electrodes showed a very good selectivity for bromhexine with respect to a number of inorganic cations, amino acids and sugars. The electrodes were applied to the potentiometric determination of bromhexine hydrochloride and its pharmaceutical preparation under batch and flow injection conditions. Graphite, copper and silver coated wires were prepared, characterized and successfully applied as sensors for the drug under investigation.

Carbon paste electrode modified with silver thimerosal for the potentiometric flow injection analysis of silver(I)

The utility of carbon paste electrode modified with silver ethylmercurythiosalicylate (silver thimerosal) in both static mode and flow injection analysis (FIA) is demonstrated. The electrode was fully characterized in terms of composition, response time, thermal stability, usable pH and ionic strength ranges. It has been shown that diisononyl phthalate (DINP) acts as more suitable solvent mediator for preparation of the electrode, which exhibits linear response range to Ag(I) extending from 5.0 × 10 −7 to 1.0 × 10 −3 M with detection limit of 2.5 × 10 −7 M and Nernstian slope of 59.3 ± 1.0 mV/decade. The proposed chemically modified carbon paste electrode shows a very good selectivity for Ag(I) over a wide variety of metal ions and successfully used for the determination of the silver content of silver sulphadiazine (burning cream) and developed radiological films. The electrode was also used as an indicator electrode in the potentiometric titration of thiopental and thimerosal with AgNO 3.

Chemically modified carbon paste electrode for the potentiometric flow injection analysis of piribedil in pharmaceutical preparation and urine

A new carbon paste electrode selective for piribedil (PD) was prepared and fully characterized in terms of composition, usable pH range, response time and thermal stability. The electrode active recognition is by liquid ion-exchange mechanism via the use of piribedil phosphomolybdate as ion-exchanger dissolved in tricresyl phosphate as a more suitable solvent mediator for the paste. The modified electrode showed a Nernstian slope of 58.4 ± 0.6 mV over the concentration range of 7.5 × 10 −7 to 1 × 10 −3 M with an average recovery of 98.3–101.0% and R.S.D. of 0.45–1.31%. The electrode exhibits good selectivity for PD with respect to a large number of inorganic cations, organic cations, sugars and amino acids. The developed electrode was successfully used for the potentiometric determination of PD in its aqueous solutions, pharmaceutical preparation, and urine in batch and flow injection analysis (FIA).

Sensitive Potentiometric Flow Injection Analysis of Redox Species Based on the Detection of an Intermediate Produced by a Redox Reaction with the Potential Buffer

This review summarizes a highly sensitive potentiometric flow injection method for the determination of various types of oxidative species, where a large transient potential change due to chlorine or bromine, generated during the reaction of the oxidative species with an Fe(III)-Fe(II) potential buffer containing chloride or bromide as an intermediate, is utilized. The analysis of trace amount of hydrazine based on the detection of a transient change in potential caused by the reaction of hydrazine with a Ce(IV) -Ce(III) potential buffer is also described. 1. Introductionprevious minireview (1), we described a potentiometric flow injection analysis (FIA) method for the determination of redox species using a simple redox reaction of a sample with a potential buffer solution consisting of a redox couple such as Fe(III)-Fe(II), Ce(IV)-Ce(III) and Fe(CN)6 3- -Fe(CN)6 4- . This method is based on the detection of the change in potential of a redox electrode caused by a change in the composition of the potential buffer solution as the result of a reaction of the sample with the potential buffer. One of advantages of the method is that samples in a wide concentration range can be determined by selecting an appropriate concentration of the potential buffer. The excellent characteristics of the FIA method permits its use in detecting a final product at an equilibrium state as well as of an intermediate in transient reactions before chemical equilibrium is reached, where an appropriate chemical reaction was used. Among the many redox reactions, there are several reactions where an intermediate compound with a short lifetime is generated. In such a case, the FIA method would be useful for detecting of the intermediate. During our research on the use of potentiometri c FIA for the determination of redox species using the potential buffer, we discovered an interesting phenomena in which a large transient potential change of the redox electrode appeared, when a bromate solution was added to a Fe(III)-Fe(II) potential buffer containing bromide (2). The large change in potential was found to be due to the fact that bromine, generated as an intermediate, was reduced to bromide by Fe(II) in the potential buffer. In the case of potentiometric FIA, if the electrode potential of the intermediate generated during the redox reaction of an analyte with the potential buffer is much higher than that of the potential buffer, a highly sensitive determination of the analyte could be achieved, if the intermediate in the FIA system could be detected. No report on a highly sensitive potentiometric FIA method for the determination of oxidative species in which the detection of an intermediate formed during a redox reaction has appeared, so far. In this minireview, we describe a highly sensitive potentiometric FIA method based on the detection of a large transient potential change in the redox electrode, which appears a short period after mixing an analyte with the potential buffer. In the case where a Fe(III)-Fe(II) potential buffer is used, it was found that the addition of bromide or chloride to the potential buffer enhances the sensitivity to various types of oxidizing species such as bromate, chrominum(VI), hydrogen peroxide and oxychlorine, etc. A high sensitivity to such oxidative species was obtained by the potentiometric detection of a large transient potential change due to the generation of intermediate bromine or chlorine during the reaction of the analyte with chloride or bromide in the potential buffer.