Flow Titration Research Articles

Fast determination of minoxidil by photometric flow titration

A photometric flow titration based on the redox reaction between KMnO4 and minoxidil is described. The best titration results were observed at 3.20 x 10-4 mol L-1 KMnO4 and 1.00 x 10-3 mol L-1 minoxidil, using the minoxidil solutions as titrant. The flow rate was fixed at 17 mL min-1 and the titrant was added to the system in aliquots of 500 µL, the color changes were monitored at 550 nm. The method was applied to commercial samples and compared with the results from a chromatographic procedure. Recoveries from 97.6 to 102.8 % were observed depending on the sample. Comparison with the chromatographic procedure reveled relative errors of 3.5 - 4.0 %.

Development of Chemical Sensors and Their Application to Flow Analysis Systems

Flow analysis has many advantages concerning sensitivity, accuracy, repeatability and analytical throughput etc, compared with a batchwise analysis. The flow-based method makes the involved analytical processes automatic and rapid. Therefore, the combination of a chemical sensor with the flow method enhances the performances of the chemical sensor. Especially, potentiometric detection using ion-selective electrodes has been widely used in flow analysis due to its features, simplicity in analytical operation and wide dynamic range in analysis. This paper reports on several chemical sensors applied to flow systems that we have engaged so far concerning: (1) Liquid membrane type ion-selective electrodes, such as nitrate, vitamin B1, tetrafluoroborate and surfactants sensitive electrodes based on an oleophilic anion exchange resin membrane and a hydrophobic ion exchanger. (2) Flow titration by using both a buffer solution and its corresponding electrode, such as a pH glass electrode with a pH buffer, a cupper(II) ion-selective electrode with a metal ion buffer solution and a redox electrode with a potential buffer solution. (3) Surface plasmon resonance sensors for succarides and some endocrine disrupting chemicals based on a boronic acid polymer membrane and antigen or antibody immobilized membrane. (4) Sequential injection analysis combined with beads injection technique for vitellogenin as biomarker for assessing the pollution of environment in hydrosphere.

Gran method for end point anticipation in monosegmented flow titration

An automatic potentiometric monosegmented flow titration procedure based on Gran linearisation approach has been developed. The controlling program can estimate the end point of the titration after the addition of three or four aliquots of titrant. Alternatively, the end point can be determined by the second derivative procedure. In this case, additional volumes of titrant are added until the vicinity of the end point and three points before and after the stoichiometric point are used for end point calculation. The performance of the system was assessed by the determination of chloride in isotonic beverages and parenteral solutions. The system employs a tubular Ag2S/AgCl indicator electrode. A typical titration, performed according to the IUPAC definition, requires only 60 mL of sample and about the same volume of titrant (AgNO3) solution. A complete titration can be carried out in 1 - 5 min. The accuracy and precision (relative standard deviation of ten replicates) are 2% and 1% for the Gran and 1% and 0.5% for the Gran/derivative end point determination procedures, respectively. The proposed system reduces the time to perform a titration, ensuring low sample and reagent consumption, and full automatic sampling and titrant addition in a calibration-free titration protocol.

Electrochemical detection in liquid flow analytical techniques: Characterization and classification (IUPAC Technical Report)

Abstract Liquid flow analytical techniques are classified, and definitions are provided of flow-injection analysis, segmented flow analysis, flow titration, continuous monitoring, liquid chromatography, and capillary electrophoresis. Electrochemical detection and flow through detection cells are characterized with respect to the surface and bulk detection. The detector performance is discussed in terms of its principal analytical parameters, such as detection limit and dynamic concentration range, as well as its dynamic characteristics, such as the response time, sampling frequency, transport lag, and long-term stability. Moreover, different detection modes are critically evaluated, including both potentiostatic and galvano-static techniques. Factors influencing sensitivity and detection limit, which include electronic and hydrodynamic approach, are also discussed. Different detector designs are critically reviewed, and the special features of electrochemical detectors for flow analytical techniques are emphasized.

Continuous on-line feedback based flow titrations. Complexometric titrations of calcium and magnesium

The methodology of continuous feedback-based flow titrations and the principle of compensating errors [Anal. Chem. 72 (2000) 4713; Anal. Chim. Acta 435 (2001) 289] were applied to the determination of calcium and magnesium ions with EDTA. The flow of the titrant, EDTA, varied linearly in response to a controller output voltage while the total flow ( F T, the sum of the metal ion sample flow and the titrant flow) was held constant. The sample was pre-doped with a metal ion indicator; the status of the indicator color in the mixed stream was monitored by an optical detector and was used for governing the controller output as well as for interpreting the results of the titrations. The titrant flow initially ramped upward linearly. As a change in the color corresponding to the equivalence point was sensed by the detector, the controller output (instantaneous value V H) reversed its ramp direction, thus decreasing the titrant flow linearly at the same ramp rate. When the predefined absorbance corresponding to the equivalence point was sensed again, the controller voltage (instantaneous value V L) was ramped in reverse once more, going upward. Because of the lag time between a change in the controller output and its effect being sensed by the detector, the controller voltage corresponding to the actual equivalence point was the average of V H and V L. Continuous sensor-governed operation of the controller resulted in a triangular waveform. The mean of this waveform during any cycle gives the equivalence point controller voltage V E. This principle allowed true titrations with good reproducibility (0.2–0.7% R.S.D.) and throughput (33–42 s per titration).

Study and application of a new on-line titration technique

A new on-line titration method and device based on a new technique-continuous flow titration is described. By the means’ of electronically controlled switching of a solenoid valve, the main component of the system, the equivalent point of the titration is easily determined. Several kinds of mixing tools were examined, whereby a self-made mixing chamber with minimum volume gave best results and was therefore used in the device. The error of the titration is within 0.2% and the relative standard deviation (RSD) is below 1.2% . The results show no difference compared with a commercial device, meanwhile the new on-line titration system is cheaper and fully automated and thus easy to hand and less solvent consumption.

Automatic Flow System Titration Based on Multicommutation for Spectrophotometric Determination of Total Acidity in Silage Extracts

A multicommutated flow titration procedure is proposed for the spectrophotometric determination of total acidity in silage extracts. The flow network comprises a set of 3-way solenoid valves, computer-controlled to provide facilities to handle the titrand, titrant, and carrier solutions by using a single pumping channel. The procedure is based on the volumetric fraction variation approach that maintains the same volume of titrand solution and varies the titrant solution step by step, to provide determination without the use of an analytical curve. The procedure is implemented by using phenolphthalein as an external indicator; the proposed algorithm was able to titrate silage extracts presenting different color intensities. Sample solutions with concentrations ranging from 10(-3) to 10(-1) mol/L total acidity could be analyzed by using 10(-2) mol/L sodium hydroxide as the titrant solution. Other advantages include a relative standard deviation of <1.0% (n = 4) for a typical silage extract solution containing 26 mmol/L total acidity; an analytical throughput of 16 determinations per hour was also achieved. Accuracy was assessed by processing a set of silage extract solutions and comparing the results with those obtained by using the conventional potentiometric titration procedure. No significant difference at the 95% confidence level was observed.

Quantitation of Mixtures of Diprotic Organic Acids by FT-IR Flow Titrations and Multivariate Curve Resolution

The combination of FT-IR flow titrations and second-order calibration methods is assessed for the first time as a potential method for quantifying mixtures of organic acids. FT-IR spectral information is richer and more specific than that provided by other spectrometric techniques. Also, the flow titration of the sample allows the use of second-order data analysis methods, such as multivariate curve resolution, which can quantify multianalyte systems using pure analyte standards in the presence of unknown interferents. Problems linked to FT-IR flow titrations, such as the impossibility of using buffer substances and the presence of major baseline contributions in the measurements due to changes in the water structureless absorption, have been overcome through the design of an adapted experimental setup and the use of second derivative spectra. Malic and/or tartaric acid are the model substances chosen to be quantified in the presence or absence of interferents. Both substances are diprotic acids with similar p Ka values and, therefore, similar pH-dependent evolution. Samples with one or both acids with their related standards in the presence or absence of an inert interferent (sugar) are successfully resolved and quantified for a wide range of [analyte]: [interferent] ratios with no need for pH control in the process. These results suggest this methodology as an optimal and robust alternative for routine analysis.

Monosegmented flow titrator

An automatic titrator based on the monosegmented flow approach is described. The flow titrator is unique in fulfils the definition of IUPAC for the titration technique. This means that, the flow analyser produces a complete titration curve and is capable using the equivalence point concept for determination of the titrant concentration without the use of any calibration, usually employed in similar systems. The titrant is added to the sample monosegment by using a syringe driven by a step motor. The titrant is added with a precision of ±0.5 μl. The main advantage of the proposed flow titrator is in the fact that, if necessary, only one aliquot (100–200 μl) of the sample need be employed to make a complete titration with a suitable titrant solution. This feature has been demonstrated by titrating Fe(II) and H 2O 2 solutions with KMnO 4. However, a complete titration is not the fastest way to find the end point. A successive approximations algorithm is described which allows the end point to be achieved in, at most, eight steps. The novelty of this approach is that the sample solution does not need to be discarded when titrant addition do not exceed the end point. The proposed flow titrator has been evaluated for spectrophotometric, indicator based, titrations of strong and weak acids and for titration of hydrogen peroxide in commercial products using KMnO 4. The results show that the system can, on average, perform one titration each 2 min, when employing the successive approximations algorithm. The precision and accuracy for HCl or acetic acid titrations (concentration range 0.0025–0.100 mol l −1), using phenolphthalein as indicator, and H 2O 2, using the self indicating KMnO 4 titrant, was from 0.5 to 2%. Because the proposed system resembles a real titrator in all aspects, all detection techniques employed for conventional titrations can be, in principle, also employed with this flow manifold.

Automatic flow titrator based on a multicommutated unsegmented flow system for alkalinity monitoring in wastewaters

A full automatic flow system based on potentiometric titration for alkalinity monitoring in wastewater treating plants is presented. Titration to an end-point of pH 5.75 partial alkalinity (PA) and then to pH 4.3 intermediate alkalinity (IA) allows to distinguish the relative buffering contributions of both bicarbonate and volatile acids in anaerobic digesters and, thus, the attainment of a IA:PA ratio which is a sensitive parameter of digestion monitoring that increases rapidly with the process upset. The titration approach is based on a time-based sequential introduction of increasing titrant and decreasing titrand volumes into a mixing chamber. The theoretical model for the titration process already presented and discussed was used to determine the titrand concentration without prior calibration. The interface between the analytical system and the anaerobic digester was also developed, allowing completely automated on-line monitoring of alkalinity. The results obtained were reproducible (R.S.D. lower than 2.4 and 5.1% for repeatability and reproducibility, respectively) and in good agreement with those given by the comparative method.

Micro-volume flow titration and screening the dissociation constants (p Ka) of weak acids

A micro-volume, monosegmented flow analysis system has been developed for titration of weak acids and for determining dissociation constants (p K a) in the range 3–6, using pH indicators and spectroscopic detection. The system consists of a piston pump, a peristaltic pump, a detector and a 250 μm i.d. reaction silica capillary. The analytical procedure for determining the acid concentration involves the sequential introduction of air, solutions of phenolphthalein indicator, acid sample, NaOH, and finally air again, using the piston pump. The various plugs disperse axially into each other in the reaction capillary when pumping with the peristaltic pump is started. The acid sample will then be partially neutralised and the appearance of the dramatic pH change across the combined liquid segment, bracketed by the air segments, is registered by measuring the indicator absorbance at 552 nm. In the trials reported here, the acid concentration range was 35–100 mM, the sample volume less than 1 μl, the time needed less than 2 min, and the R.S.D. values obtained were in the range 0–2%. Two screening methods for the determination of p K a values were also developed. The first of these consumes less than 200 nl of the sample, but requires prior knowledge of the molar sample concentration. In this case, an indicator mixture, exhibiting a linear absorbance change as a function of pH at 598 nm is used instead of phenolphthalein. The second approach, used when no information about the acid concentration is available, involves both indicator solutions and is based on determining the pH at the half-neutralisation point. The sample volume needed, in this case, is less than 1 μl.

Oxidability Determination in Waste Waters Using an Automatic Titrator Based on a Multicommutated Unsegmented Flow System

The automatic titrator based on a multicommutated unsegmented flow system was applied to redox titrations and used for oxidability determination in waters analysis. This automatic titrator allows the attainment of complete titration curves, being the determination of titrand concentration performed without requiring any prior calibration. After sample treatment (oxidation step), the oxidability determination in waste water samples was accomplished by the automatic flow titrator (titration step). Repeated determinations of standard solutions gave a 3.5% RSD (n=10, 0.010M) for repeatability and a 3.2% RSD (n=2, 0.057M) for reproducibility. Samples results (n=9) were in good agreement (t-test) with those obtained with a reference procedure.

Development of Novel Redox Systems by Use of Ligand Effect and Its Application to Potentiometry

The redox potential of a system involving metal ions can be modified by the complexation of metal ions with a suitable ligand. By using this phenomenon, novel redox systems which are applicable to quantitative analyses of metal ions have been developed. Some potentiometric titrations of metal ions and new types of spectrophotometric determinations of metal ions and complexing agents based on the ligand effect are presented in full detail. Estimation of complexing capacity in natural water and a new kinetic-catalytic method for copper(II) are also discussed. In addition, novel potentiometric flow titration has been proposed based on the relationship of the flow rates between titrant and sample solutions.

Calibrationless determination of creatinine and ammonia by coulometric flow titration.

A precise and sensitive working microflow titration procedure was developed to determine creatinine and ammonia in urine samples. This procedure is based on enzymatic conversion of creatinine, gas diffusional membrane separation of the released ammonia into an acid acceptor stream, and coulometric titration of ammonia with hypobromite. The hypobromite is formed after the electrogeneration of bromine in an electrolyte containing 1.0 M NaBr and 0.1 M sodium borate adjusted to pH 8.5. The electrolysis current follows a triangle-programmed current-time course. An amperometric flow detector records the resulting mirror symmetrical titration curves, which show two equivalence points. The analyte concentration is calculated from the time difference between the equivalence points. For quantitative conversion of creatinine and quantitative separation of present and released ammonia no calibration is necessary to get accurate results. Both ammonia/ammonium and creatinine were determined in the range between 2 microM and 2 mM with relative standard deviations between 3.0 and 1.0% (n = 5). High recoveries were obtained for the analysis of diluted urine samples for both creatinine and ammonia.

USO DE UM DETECTOR POTENCIOMÉTRICO DE SENSIBILIDADE AUMENTADA PARA A DETERMINAÇÃO DE CLORETOS EM LEITE E PRODUTOS LÁCTEOS POR FIAPOTENTIOMETRIC FLOW TITRATION WITH INCREASED SENSITIVE DETECTOR FOR DETERMINATION OF CHLORIDES IN MILK AND DAIRY PRODUCTSUSO DE UN DETECTOR POTENCIOMÉTRICO DE SENSIBILIDAD AUMENTADA POR FIA PARA LA DETERMINACIÓN DE CLORUROS EN LECHE Y PRODUCTOS LACTEOS

This paper describes the development of a flow injection analysis (FIA) system for the determination of chlorides in dairy products by potentiometric flow titration with increased sensitive detector. The potentiometric detection system used consisted of a silver ion-selective detector electrode based on a homogeneous crystalline double membrane and its potential difference was measured against a common reference electrode by an external electronic device. Milk samples were straight injected into the automatic system whereas cheese, butter and yoghurt samples were prepared according to reference methods and the solution produced was inserted without further treatment. The results obtained for chloride determinations using FIA titration are in agreement with those provided by the reference methods (R.D. < 4%). The sampling rate accomplished with the automatic system varied from 130 to 250 samples per hour, depending on the samples chloride concentration. © 2000 Altaga. All rights reserved. Resumen Este artículo describe el desarrollo de un sistema de injección en flujo (FIA) para la determianción de cloruros en productos lacteos por titulación potenciométrica con un detector de sensibilidad aumentada. El sistema de detección potenciométrico usado consiste en un electrodo selectivo a los iones plata constituido por dos membranas cristalinas homogéneas siendo la diferencia de potencial medida por un electrodo de referencia común por un dispositivo electrónico externo. Las muestras de leche fueron inyectadas directamente en el sistema y las muestras de queso, mantequilla y yogurt fueron preparadas siguiendo las indicaciones de los métodos de referencia y la solución resultante inyectada directamente. Los resultados obtenidos con el sistema FIA propuesto son concordantes con los obtenidos usando los métodos de referencia (R.D. < 4%). La velocidad de análisis del sistema automático varia entre 130 a 250 muestras por hora, dependiendo de la concentración de cloruros en las mismas. © 2000 Altaga. Todos los derechos reservados. Palabras clave: Cloruros, leche, productos lacteos, potenciometría, detector de sensibilidad aumentada. Resumo Neste trabalho descreve-se o desenvolvimento de um sistema de titulação potenciométrica, com sensibilidade aumentada, baseado na técnica de análise por injecção em fluxo (FIA), para a determinação de cloretos em vários produtos lacteos. O eléctrodo indicador de configuração tubular, sensível ao catião prata é constituido por duas membranas cristalinas homogéneas sendo a soma de potencial de cada membrana efectuada externamente através de um dispositivo construido para o efeito. As amostras de leite foram injectadas directamente no sistema e as amostras de queijo, manteiga e iogurte foram preparadas de acordo com as recomendações dos métodos de referência, sendo a solução resultante intercalada sem tratamento adicional. Os resultados obtidos com o sistema FIA proposto são concordantes com os fornecidos pelos métodos de referência (D.R.< 4%). O ritmo de amostragem fornecido pelo sistema automático variava entre 130 e 250 amostras por hora, dependendo da concentração do anião cloreto nas amostras. © 2000 Altaga. Todolos dereitos reservados. Palabras chave: cloreto, leite, produtos lácteos, potenciometria, detector de sensibilidade aumentada.

Prior assay as an approach to flow titrations. Spectrophotometric determination of iron in alloys and ores

In titrimetric procedures, the number of data points can be reduced when the analyte concentration is roughly known. This aspect is emphasised in automated titrations where a foresight of the analyte concentration can be fast and easily achieved by processing known amounts of sample and titrant, and evaluating the titrant excess. This prior assay permits a previous definition of the different titrant amounts to be further added. The feasibility of this strategy is demonstrated in designing a flow-batch titrator for spectrophotometric determination of iron in steels, iron ores and similar. The method involves titration of ferrous ions by permanganate performed inside a 400 μl reaction chamber assembled in the manifold. The proposed system handles about 20 samples per hour, requires 94 μg KMnO 4 per determination and yields precise results (R.S.D. ca. 1.5%, n=6) for 1.0–10×10 −3 mol l −1 Fe in the injectate. Accuracy was assessed by running a standard reference material and different samples already analysed by a manual titrimetric procedure.

Non-linear calibration in single point flow titration of protolytes: A comparison of model options by using Delta Test

The complete description of the non-linear absorbance/concentration relationship obtained for systems used in single point photometric flow injection titrations of acids and bases is presented. The applicability of different models for fitting calibration data, as well as the more frequently used data processing options, are investigated under normal and restraining conditions by using experimental and simulated data sets. Theoretical and empirical models are examined in order to choose the simplest and the more efficient ones to provide good recoveries. The comparison among models is based on a novel statistical criterion (Delta Test) for contrasting the prediction efficiency by detecting systematic errors in estimated concentrations.

An Automatic Spectrophotometric Titration Procedure for Ascorbic Acid Determination in Fruit Juices and Soft Drinks Based on Volumetric Fraction Variation

A spectrophotometric flow titration procedure for ascorbic acid in fruit juices and soft drinks based on volumetric fraction variation employing a reaction with 2,6-dichloroindophenol is described. A flow network was designed to implement the multicommutation process and a set of microcomputer controlled three-way solenoid valves was employed to handle the insertion of the titrand and titrant solutions into the analytical path. All the steps involved in the titration procedure were carried out automatically without any operator assistance. Fruit juices and soft drinks could be analyzed without any prior chemical treatment. Accuracy was assessed by comparing results with those obtained by the reference method (AOAC). No significant differences at 95% confidence level were observed. Other profitable features were achieved such as a percentual standard deviation of results of 1.1% (n = 5) for a typical juice sample containing 1.2 mmol L–1 ascorbic acid, a low sample consumption (6 μL per run), and an analytical throughput ranging from 5 to 30 determinations per hour for sample concentrations ranging from 0.6 to 6.0 mmol L–1.

Coulometric microflow titrations with chemiluminescent and amperometric equivalence point detection: Bromimetric titration of low concentrations of hydrazine and ammonium

A coulometric flow titration with electrogenerated bromine was combined with chemiluminescent and amperometric flow detection to determine low concentrations of hydrazine and ammonium. Hydrazine can be precisely titrated in the range between 2 μM and 1.1 mM with chemiluminescence detection of the equivalence points with relative standard deviations between 1.9 and 0.4% ( n=5, α=0.05). 60 nM is the lowest concentration of hydrazine, which can be titrated. Despite quenching the chemiluminescence ammonia concentrations lower than 0.5 mM do not influence the titration results. Between pH 1 and 7 hydrazine can be selectively titrated in the presence of ammonium. The sum of hydrazine and ammonia/ammonium was titrated at pH 8.3 with amperometric detection. Both hydrazine and ammonia/ammonium can be determined without calibration. The amperometric detection of the equivalence points is to be preferred in the presence of ammonia. Hydrazine and ammonia were titrated precisely in their mixtures and in concentration ranges between 20 μM and 0.5 mM. Micromolar concentrations of hydrazine and ammonia/ammonium were determined precisely and accurately in spiked water samples.

Coulometric microflow titrations with chemiluminescent and amperometric detection of the equivalence points — basic investigations and bromimetric titration of low concentration solutions of arsenite

Coulometric flow titrations were combined with chemiluminescent detection of the equivalence points. The flow channel system consists of a miniaturized flow electrolytic cell, a knotted type tubular reactor and a capillary wall-jet chemiluminescence detector with an outer tube for the addition of luminol reagent solution. The chemiluminescence detector can be used to determine bromine in the range between 0.1 μM and 1 mM opening up a sensitive way to follow flow titrations with linear concentration gradients of bromine. Very low concentrations of arsenite can be titrated in a sample flow channel system with electrogenerated bromine. The chemiluminescence detection was compared with the amperometric monitoring of the titration curves. To perform such gradient controlled flow titrations without calibration, it was necessary to minimize the so-called tailing. At gradient generation times ≥120 s, arsenite can be titrated almost without calibration in the range between 0.05 and 100 μM. The accessible determination range is determined by the maximum electrolysis current and the sample flow rate. Added micromolar concentrations of arsenite were accurately determined in river water and phosphate solutions.