A general method for coulometric titration of alkylphenols with bromine

A general method for coulometric titration of alkylphenols with anodically generated bromine is described. The reaction is carried out in a water-acetic acid medium and the reactivity is governed by varying the water content and the concentration of bromide ion and by the addition of pyridine. In that way all types of alkylphenols can be titrated quantitatively. For phenols containing more than one free ortho and para position, the titration can also be carried out either to the monobromination stage or to the full bromination stage. The mean relative error is ± 1·2% for monobromination and ±1·5% for full bromination. A method for rapid determination of the number of free ortho and para positions in alkylphenols by using two coulometric titrations is also described.     

Robust flow-batch coulometric/biamperometric titration system: determination of bromine index and bromine number of petrochemicals

A flow–batch system was constructed and evaluated to perform coulometric titrations with biamperometric end point detection. The flow section of the system is employed for sampling by injecting a sample volume (50–300 μL) in a flow injection-like system. About 1.5 mL of a suitable carrier solution is delivered by a peristaltic pump in order to quantitatively transfer the sample to the system titration cell (2.0 mL total inner volume). The carrier contains the coulometric precursor for the titrant species. The cell contains two pairs of platinum electrodes used for coulometric generation of reagent and biamperometric detection and is actively stirred. The titrant species is generated and the titration is performed by the usual batch procedure with the excess of titrant being detected by biamperometry following the analysis of the titration curve. System operation is computer controlled and all operations are automated, including titration curve analysis and cell cleaning after the titration is ended. The system is characterized by its robustness because its operation does not depend on flow rates, and the work using coulometric methods which generate gases at the counter-electrode is not troublesome. The flow–batch system has been evaluated for determination of bromine index and bromine number (relative to the total reactive olefin content) in petrochemicals according to an ASTM procedure. Typical precision (R.S.D.) is between 0.5 and 6% for different petrochemicals whose bromine number/index vary from 1000 to 10 mg of bromine per 100 g of sample, respectively. Recoveries for standard additions are between 92 and 123% for 10 mg of Br₂ per 100 g increments and 98 to 101% for 100 mg per 100 g increments. Accuracy of the proposed system was evaluated against results obtained by the standard ASTM with no significant difference detected at 95% confidence level.     

Thin-layer hydrodynamic biamperometric end-point detection: application to the coulometric titration of arsenic(III) with bromine

A sandwich-type thin-layer cell has been applied to hydrodynamic biamperometric end-point detection in coulometric titrations. The detector cell, constructed from two pieces of teflon, has platinum indicator electrodes and is placed in a flow loop attached to the coulometric titration cell. The thin-layer cavity has a volume of 6.5 μl and a thickness of 51μm, which are obtained using teflon tape as a spacer. A peristaltic pump maintains a continual flow through the loop. The titration cell is a closed vessel completely filled with the solution to be titrated. Operational amplifier circuits control the potential difference applied across the two thin-layer electrodes, and to measure the current developed in the thin-layer cell. Noise arising from the pulsed nature of the flow is reduced by a factor of 25 by using an electronic filter. The titration of arsenic(III) with electrogenerated bromine was used to study the performance of the system. Concentrations from 24 ppb to 1.6 ppm can be determined with relative inaccuracies of +4 and +0.1%, respectively.     

Automatic coulometric titration of acids

An apparatus is described for the automatic titration of acids by the constant current coulometric technique. The generator electrodes comprise a platinum cathode and a silver/silver bromide anode. The increase in pH resulting from the reduction of hydrogen ion at the cathode is indicated by a glass electrode, in conjunction with a Beckman Automatic Titrator which automatically monitors the titration and interrupts the generating current when the equivalence point pH is reached. Quantities of hydrochloric acid in the neighborhood of 0.12 millimole in 50 ml were titrated with a mean error of ^-0.07% and an average deviation from the mean of ±0.15%. The technique is applicable to any strong or weak acid, and to acid mixtures, provided that no substance is present which is either reducible at the platinum cathode or reactive in any way at the silver anode.     

Determination of the cyclodextrin inclusion constant with the constant current coulometric titration method

With fluorometry this paper has proved that alpha-cyclodextrin (CD) and gamma-CD do not form inclusion complexes with procaine, while beta-CD and HP-beta-CD do. Their molar ratios are demonstrated both 1:1 with the equimolar variation method. The constant current coulometric titration method (CCCT) is first proposed and applied in the determination of the CD inclusion constant. To compare with this method, the fluorescence experiment has been done with the satisfactory results.     

The coulometric titration of bromate with cuprous copper

A coulometric procedure has been developed by which 18 to 240 μg quantities of bromate may be determined to within 0.3 μg. The bromate is allowed to react with bromide in an acid solution and electrolytically generated cuprous copper is used for the titration. A dual platinum electrode indicator system is used in an amperometric end-point procedure     

Coulometric titration of deactivated phenols with bromine

A number of deactivated phenols containing fluorine, chlorine or bromine, formyi, acetyl, carboxyl or nitro groups have been titrated with anodically generated bromine. The reaction was carried out in a water-acetic acid-pyridine medium and the reactivity was controlled by varying the water and pyridine content and the concentration of bromide ion. Hydrogen in all free positions ortho and para to the phenolic hydroxyl group is generally exchanged for bromine, but in certain instances a partial bromination is possible. The method as developed is widely applicable for deactivated phenols. Only certain ortho-substituted phenols could not be quantitatively titrated. The mean relative error for the phenols titrated was ± 1·2%.     

Determination of protonation constants by coulometric titration

Application of coulometric titration to the determination of the protonation constants of acids and bases offers several advantages because of its simplicity, precision and accuracy. This procedure is rapid and requires only one calibration solution of strong acid in the same solvent and at the same ionic strength as the solution of acid (or base) being investigated. The procedure seems to be especially advantageous in the case of non-aqueous or mixed solvents having amphiprotic character. The validity of the method has been checked with several substances in water, 95% ethanol and ethylene glycol.     

Automatic coulometric titration with photometric end-point detection : Part I. A coulometric titrator based on differential photometric detection

A system for automatic coulometric titrations with photometric end-point detection is described. To compensate for errors caused by gas bubbles and turbidity, the beam is split after passing the titration cell. The intensities of the respective light beams are registered by means of two photodiodes operating in the current mode. The currents are converted to voltages and the logarithm of the quotient is taken. The measured absorbance is only slightly dependent on the variation in the intensity of the incident light. A diminution of one absorbance unit causes a relative error of less than 5%. The deviations due to carbon particles (37–75 μm) were less than 0.002 absorbance units per mg of carbon present in 10 ml of solution. The improvement with respect to errors caused by gas bubbles is illustrated. The fundamental advantage of a one-lamp system over a twolamp system is shown.     

Bromine number determination by coulometric flow-injection titration

The bromine number, the number of grams of bromine which react with 100 grams of a substance under given conditions, is a widely used parameter in the chemical and petroleum industries. A method of performing the bromine number determination by coulometric flow-injection titration, using both sequential injection with sinusoidal flow and single zone techniques, is described. Titrations of olefins, known interfering compounds, and petroleum distillates were performed and compared with the results of conventional bromine number titrations. Analyses yielded a relative standard deviation of 2%. Results compared favorably with those obtained by the conventional method for both pure (97%) olefins and petroleum distillates. The method was found to be insensitive to interference from nitrogen compounds. The CFIT technique combines the advantages of coulometric titrations: controlled generation of reagent, avoidance of reagent standardization requirements and reagent storage and stability problems; with the advantages of FIA: small volume requirements which reduce cost and waste, controllable dilution through flow programming, toxic materials contained within a closed environment, and the capability of being automated.     

Investigation of manganese(III) fluoride for coulometric titration

A study has been made of the use of electrogenerated manganese(III), in an electrolyte containing fluoride, for coulometric titration. The addition of fluoride prevented the deposition of manganese dioxide on the anode, and was responsible for increase of the current efficiency and for extension of the feasible range of current density, owing to complex formation between manganese(III) and fluoride. The bonding ratio, the apparent stability constant and the redox potential of the complex were also estimated from the Nernst equation and the current-potential curves.     

A method of general applicability for the micro determination of chlorine,bromine and iodine

Summary A method is described for the combustion of halogen containing organic compounds in milligram quantities in a closed system. It has been found to have greater applicability than existing techniques. After combustion the halogen is determined by an amperometric titration, which permits the determination of Cl^–, Br^– and I^– either singly or in admixture. The accuracy is probably better than existing micro methods for single halides and comparable with these methods for mixed halides.

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Zusammenfassung Eine Methode zur Verbrennung von Milligrammengen halogenhaltiger organischer Verbindungen im geschlossenen System wurde beschrieben. Sie hat einen größeren Anwendungsbereich als bisher bekannte Verfahren. Nach der Verbrennung wird das Halogen durch eine amperometrische Titration bestimmt, womit Chlorid, Bromid und Jodid sowohl einzeln wie in Gemischen bestimmt werden können. Die Genauigkeit seheint besser zu sein als bei bekannten Mikromethoden für die Bestimmung einzelner Halogene und etwa gleich gut wie bei Methoden zur Analyse von Halogengemischen.

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Résumé On décrit une méthode de combustion en vase clos, des composés organiques contenant des halogènes en quantités de l'ordre du milligramme. Cette méthode a été trouvée comme ayant un domaine d'applications plus grand que celui des techniques existant déjà. Après combustion, l'halogène est dosé par titrage ampérométrique, ce qui permet le dosage de Cl^–, Br^– et I^– isolés ou en mélange. La précision est probablement meilleure que celle des microméthodes existant pour les halogénures isolés et comparable à celle de ces méthodes pour les mélanges d'halogénures.

     

Evaluation of certified reference materials for oxidation-reduction titration by precise coulometric titration and volumetric analysis

The accuracy and uncertainty of coulometric titration of Japanese certified reference materials (CRMs) for oxidation-reduction titration were examined in this study. The results for potassium dichromate, sodium oxalate, and potassium iodate are presented. Potassium dichromate was directly determined by coulometric titration, and sodium oxalate and potassium iodate were determined by volumetric analysis using potassium dichromate assigned by coulometry.The uncertainty of the method was investigated by examining the dependency on the sample size and on the electrolysis current. Changes in the titration parameters did not result in any significant effects on the titration results. The coulometric system used primarily consists of a constant current source, timer, switching circuit, indicator unit, and voltmeter. They were controlled using the coulometry software by a PC/AT compatible computer. A highly automated coulometric system achieves repeatabilities of less than one part in 30,000 (k = 2) and uncertainties of less than one part in 15,000 (k = 2). In addition, using volumetric method, SI units traceable sodium oxalate and potassium iodate (purity standards for redox reaction) CRMs were developed.Reference materials for volumetric analysis are the most basic substances used in analytical chemistry. These materials are analyzed by several analytical methods and are produced globally; however, their purities have not been compared at the international level. Therefore, the relationship between the purity and reliability of these materials has not yet been established. In this paper, we determine the relationship between these parameters by titrating each material obtained from different laboratories.     

The indirect coulometric titration of cytochrome c oxidase with cytochrome c

The indirect coulometric titration of cytochrome c oxidase and dioxygen using cytochrome c as a mediator is described. Results of both the indirect coulometric titrations and the cyclic voltammetric experiments reported herein verify that the reaction mechanism involves the catalytic regeneration of the electroactive species, the cytochrome c mediator, with the selective reduction of cytochrome c oxidase alone. During the indirect coulometric titrations dioxygen is reduced to water only by cytochrome c oxidase and not by either direct reduction at the electrode surface or reaction with cytochrome c. This system utilizes the electron transfer selectivity of cytochrome c for cytochrome c oxidase over dioxygen and offers a means by which the reaction of cytochrome c oxidase and dioxygen can be examined.     

Potentiometric and coulometric titration of 2-thiobarbituric acid

A new method for the determination of 2-thiobarbituric acid, using its reaction with iodine in an alkaline medium is presented. In the volumetric titration with potentiometric end-point detection, the determinability range is 10-400 micromol (1.4-58 mg). In coulometric titration using the biamperometric end-point detection, 0.1-20 micromol (1.4 x 10(-2)-2.9 mg) of 2-thiobarbituric acid was successfully determined.     

Applications of coulometric titration for studies of oxygen non-stoichiometry in oxides

This article gives a brief overview of the modern methods for in situ studying of oxygen non-stoichiometry in oxides. The emphasis is on the techniques that are based on the use of transport properties of solid electrolytes with unipolar oxygen ion conductivity, as well as on the experimental implementation of coulometric titration. The possible sources of experimental errors are considered, and methods are discussed of their accounting and minimization. Several examples are given of the using of the data on non-stoichiometry for the determination of thermodynamic properties and defect structure in oxides.     

Thermodynamic study of lithium-vanadium bronzes by coulometric titration

Thermodynamic study has been made of lithium-vanadium bronzes β-Li_xV₂O_5?y in reversible cells with a lithium-conducting solid electrolyte using coulometric titration. The linear dependence of $\text{Δ}\text{G}{}_{\mathrm{<mtext>Li</mtext>}}$ in the bronze on parameters x and T was found. It is concluded that the dependence of $\text{Δ}\text{G}{}_{\mathrm{<mtext>Li</mtext>}}$ on the P_O2 value is weak. The limits of the homogeneity region of β-Li_xV₂O_5?y have been more clearly defined.     

Further developments in the high-precision coulometric titration of uranium

An experimental study of the current efficiency in the coulometric generation of Ti(III), as a function of electrolyte composition, current density and electrode material, has been performed. The cathodes investigated include platinum, mercury and graphite. The first two are suitable for high-precision determination of uranium. The graphite surface is readily poisoned, rendering it useless for high-accuracy work. The use of mercury requires thorough removal of chloride from the system. The precision and error obtained are comparable for both the mercury and platinum cathodes, and are of the order of 50 ppm.     

Electrogenerated bromine as a coulometric reagent for the estimation of the bioavailability of polyphenols

A method is proposed for the estimation of the bioavailability of polyphenols using electrogenerated bromine as a coulometric titrant. The titration of model solutions of casein and bovine serum albumin (BSA) shows that casein does not interact with electrogenerated bromine, while BSA reacts with the titrant in the ratio 1: 63. The proteins bind rutin and quercetin (from 14 to 90%) at a high rate and thus reduce the bioavailability of polyphenols. The concentration of free polyphenol is reduced with an increase in the concentration of protein in the mixture. The total antioxidant capacity (AOC) of tea is determined. Green tea is shown to possess higher AOC than the black one because of the partial oxidation of polyphenols to respective thearubigins and theaflavins at the fermentation step in the production of green tea. The total AOC of tea drops from 7 to 85%, in proportion to the increase in the amount of milk in the mixture. Milk proteins bind tea polyphenols into complexes because of intermolecular interactions and thus reduce their bioavailability. The observed effect of milk is independent of the brand of black tea. The degree of reduction of the total AOC of tea in going from one tea to another remains virtually constant.