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.     

Coulometric determination of the flow rate of small masses of substances II. Application to on-line measurement of the EOX-parameter

 Today’s microprocessor-controlled coulometric systems can generate complex functions obtained from the response of the microcoulometric cell. A previously developed procedure for determining the flow-rate (mass per unit time of detected species) has been accomplished by integrating, in the software, the full time dependence of the reaction of the coulometric cell and their electrodes, as well as of the titration curve. This leads to a time dependence of the measured signal, which has a fast response time without any delay. The titration curves are determined by means of various procedures. The applicability of the method is demonstrated for the case of the continuous analysis of the EOX-parameter (Extractable Organic Halogens X=Cl, Br, I) by coupling the coulometric detection with a FIA system. Results of continuous measurements of aqueous samples containing different chlorinated compounds as analytes are presented by taking into account different flow velocities in the FIA system as well as different measuring times. Recovery yields of the FIA system better than 70% have been obtained. Received: 7 November 1995/Revised: 15 May 1996/Accepted: 18 May 1996     

A cell for high-precision constant-current coulometry with external generation of titrant

A cell has been designed for the high-precision coulometric titration, with externally generated titrant, of materials which otherwise undergo undesirable reactions at the working electrodes. With this cell potassium dichromate has been titrated, via its hydrolysis reaction, with hydroxyl ion generated at the cathode, cathodic reduction of the chromium(VI) being circumvented. In this cell 99.9% of the titrant required is generated in one chamber and transferred to another for reaction; the titration is then completed with titrant generated at a second, drip-type electrode working at much lower current. By means of commercially available Leeds and Northrup coulometric titration electrical equipment, titration of NBS 136b Potassium Dichromate gave a purity of 99.976%, standard deviation 0.005%, and of NBS 84d Potassium Acid Phthalate (done as a check) 99.991%, standard deviation 0.005%, both values being in excellent agreement with other work.     

Coulometric titration of some phenolic steroids with bromine based on thin-layer hydrodynamic biamperometric end-point detection

Four phenolic steroids were titrated coulometrically with bromine in a methanol—water solvent containing hydrochloric acid and sodium bromide. End-point detection was achieved with a twin electrode thin-layer cell placed in a flow loop connected to the titration vessel. Linear-segmented biamperometric titration curves were recorded. Concentrations of 17-β-estradiol were determined in the range from 442 ppb to 8.84 ppm with relative inaccuracies of -1.3% and +2.2%, respectively. Part-per-million solutions of estrone and estriol were titrated with good accuracy and precision. Somewhat poorer precision and accuracy was found for the determination of 17-α-ethynyl estradiol.     

Flow-injection coulometric titrations

A flow-injection analysis technique based on stop flow coulometric titrations is described, utilizing a gradient chamber, reagent generation chamber, and detector flow cell integrated into a single unit. The use of stop flow allowed for automated sample dilution up to a factor of 100 times. The system has been used to titrate samples of sodium hydroxide in the range 5 x 10(-4)-4M, and nitric acid ranging from 5 x 10(-3)-15M. Analyses over the entire range of concentrations yielded a relative standard deviation of less than 3%. A correlation coefficient of 0.999 was obtained for all comparisons with manual titrations. Remote spectrophotometric detection was performed with optical fibers. No frit or membrane is required to separate the generating and counter electrodes within the system, yet the advantages of conventional coulometric titration, which eliminate the problems of reagent and calibration solution handling, storage or degradation, are retained.     

Differential scanning coulometric titrometry: Application to the system CoNiO at 1000°C

A novel technique involving solid state coulometric titration using stabilized zirconia electrolytes is presented. The method is termed differential scanning coulometric titrometry (DSCT). It is useful for the phase equilibrium studies of multicomponent oxide systems. In the DSCT, two independent and isostructural galvanic cells are used. One is a sample cell in which a sample is charged, while the other is a reference cell without a sample. Under the condition that oxygen activities in both cells are equal to each other, coulometric titration is carried out continuously in both cells. The difference between the titration currents provided to both cells corresponds to the amount of oxygen which is allowed to react with a sample per unit time. If the difference is recorded with respect to the oxygen activity, it starts to deviate from a base line when the sample starts to be oxidized or reduced. This method was successfully applied to the system CoNiO at 1000°C. The phase diagram of the system and related thermodynamic data were obtained.     

Determination of autoprotolysis constants of some non-aqueous solvents using coulometric titration

Autoprotolysis constants of acetonitrile, propionitrile, nitromethane, ethylene carbonate and dimethyl sulphoxide were determined using a coulometric — potentiometric method with a hydrogen/palladium electrode as generator. The method is based on the titration of a strong base, tetrabutylammonium hydroxide, with H⁺ ions generated by anodic oxidation of hydrogen dissolved in palladium. The titration was carried out in a galvanic cell with glass and calomel electrodes at 25°C. The pK_s values for the investigated solvents are: acetonitrile, 28.8; propionitrile, 24.6; nitromethane, 23.7; ethylene carbonate. 21.5; and dimethyl sulphoxide 29.1. These data are in accordance with those reported in the literature.     

Coulometric titration with gas diffusion separation step

Summary Coulometric titration is employed in combination with a gas diffusion step for the quantitative separation and determination of volatile substances. Complete separation is brought about by filling the sample into a closed circulating loop and transporting it repeatedly past a microporous membrane. The combination of membrane separation module with the coulometric cell allows rapid separation and exploitation of the precise absolute determinations obtainable by the coulometric titration method. The high precision and selectivity of the determination and quantitative separation are discussed using ammonia and sulphur dioxide analyses as examples.Part I: Fresenius Z Anal Chem (1989) 332:849–854     

Application of Polyaniline Incorporated Carbon Particles Coated Platinum Electrode in Coulometric Titration to Determination of Polyisoprene Alcohol

The feasibility of using electrodes modified with polyaniline incorporated carbon particles films for im-proving the precision of coulometric titration is demonstrated. The problem of large deviation produced dur-ing determining polyisoprene by coulometric titration with direct titration technique (double Pt electrodes indi-cating electrode) has been solved. In the titration process, polyisoprene alcohol, an electro-inactive species, is adsorbed on the surface of the bare Pt electrode, which inhibits the electrode reaction of Br- and Br2. Therefore, when the titration reaches the end-point, the detected current will slowly change with time, which can make the repeatability of end-point poor, The atomic force microscopic images show the morpholo-gy of the electrode surface of adsorbing polyisoprene alcohol. The application of the chemically modified elec-trode instead of the bare Pt electrode to indicating the end-point has been investigated. The results show that the Pt electrode coated with polyaniline incorporated carbon particles films is an excellent indicator electrode. This electrode has advantages that the indicating signals are sharp and repeatable at end-point. The precisionand the accuracy of the determination of polvisoprene alcohol are satisfactory.     

Potentiometric study of end-point detection in the coulometric determination of carbon dioxide

The coulometric determination of carbon dioxide is based on the alkalimetric titration of the product of absorption of carbon dioxide in an organic solution of monoethanolamine. The processes occurring in various solvents have been investigated and optimized for analytical application. The protolytic reactions of 2-hydroxyethylcarbamic acid have been investigated in 2-propanol + 2, 5 and 10% water, dimethylformamide + 2% water, dimethylsulphoxide +5, 10 and 20% water. In coulometric generation of the base, 0.1M solutions of tetraethylammonium bromide in these solvents were used. The course of the titration was followed potentiometrically with glass and antimony indicator electrodes. From the titration curves the autoprotolysis constants of the mixed solvents and protonation constants of monoethanolamine in them were calculated. These constants and the concentration of monoethanolamine influence the size of the end-point break for the titrations. On this basis, optimal conditions for analysis have been selected and the total carbon content in samples of natural waters has been determined.     

新式夹心型光透薄层光谱电化学电解池

本文设计了一种新式夹心型光透薄层紫外-可见光谱电化学池。该池采用铂网工作电极,两侧平行放置铂片为对极置于同一石英窗口夹层中,同时以聚四氟乙烯隔离膜作为边际限制器,结合池内小孔道设置内参比点进行精确的电位控制,具有理想的光谱电化学响应。利用循环伏安、循环电位-吸收、恒电位现场光谱、双电位跃-计时电量、双电位跃-计时吸收等技术,对铁氰化钾在氯化钾溶液中的行为进行了表征。     

Design and characterization of flow-through coulometric cells with porous working electrodes made of crushed vitreous carbon

Summary Three types of flow-through, three-electrode cells with porous working electrodes made of crushed vitreous carbon were tested. The electrochemical properties of the cells were studied by hydrodynamic and cyclic voltammetry using potassium ferrocyanide test solutions. Depending on the volume of the porous working electrode, Nernstian behaviour and electrochemical yields up to 100% were observed for flow rates 1 to 7 ml/min. In the stopped-flow regime the cells exhibit thin-layer cell properties. The utility of the cells is demonstrated for on-line coulometric detection, continuous removal of impurities and dissolved oxygen and for anodic stripping coulometry of trace copper.On leave from: Department of Analytical Chemistry, Slovak Technical University, CS-812 37 Bratislava, Czechoslovakia     

多元校正流动注射电位滴定法同时测定苯甲酸和山梨酸

将流动注射应用于酸碱电位滴定分析,建立了一种可同时测定混合有机酸的电位滴定新方法。在该方法中,用氢氧化钠与氯化钾的混合溶液作为滴定剂,在流通池中同时插入pH指示电极和氯离子指示电极,在滴定过程中的任一滴定点,流出液的pH值和酸碱的混合比例可由两个电极的电位测定值同时获得,从而可应用多元校正法由相应的滴定曲线求得混合酸中每一种组分的含量。该方法不仅免去了体积和时间读数,而且减少了试剂和样品的消耗量,分析速度快。应用该方法对混合样品中的苯甲酸和水杨酸进行同时测定,其相对标准偏差分别为0．19％～0．37％,回收率分别为97．3％～102．6％。     

A new approach to flow-batch titration. A monosegmented flow titrator with coulometric reagent generation and potentiometric or biamperometric detection

Monosegmented flow analysis (MSFA) has been used as a flow-batch system to produce a simple, robust, and mechanized titrator that enables true titrations to be performed without the use of standards. This paper also introduces the use of coulometry with monosegmented titration by proposing a versatile flow cell. Coulometric generation of the titrand is attractive for titrations performed in monosegmented systems, because the reagent can be added without increasing the volume of sample injected. Also, biamperomeric and potentiometric detection of titration end-points can increase the versatility of the monosegmented titrator. The cell integrates coulometric generation of the titrand with detection of end-point by potentiometry or biamperometry. The resulting titrator is a flow-batch system in which the liquid monosegment, constrained by the interfaces of the gaseous carrier stream, plays the role of a sample of known volume to be titrated. The system has been used for determination of ascorbic acid, by coulometric generation of I₂ with biamperometric detection, and for determination of Fe(II), by coulometric generation of Ce(IV) with potentiometric detection of the end-point, both in feed supplements.     

Coulometric determination of serum iron

The iron in 500 μl of serum is determined by a coulometric titration developed for ferroin. The titration step is preceded by chloroform extraction from a protein-free filtrate of serum iron as ferroin perchlorate. Evaporation of chloroform in the presence of an acetate buffer of pH 4.6 causes the ferroin perchlorate to back-extract into the aqueous layer. The ferroin is introduced into a titration cell and titrated with electrogenerated cerium(IV), a modified amperometric end-point detection system being used.     

High-precision analysis of nuclear materials by constant-current coulometry : Part II. Determination of thorium

A constant-current coulometric method is described for the precise determination of thorium by titration with EDTA, elcctrogenerated from its mercury complex in acetate-buffered solution at pH 4.5. With polarised mercury electrodes for end-point detection, relative standard deviations of 0.02 and 0.3% for 30 mg and 30 μg respectively were obtained; the method was applied to the analysis of zone-refined thorium of high purity.     

Über die Reinheitsbestimmung von Polythionaten

In the preparation of individual polythionates contamination by others is possible. Purity determination is then problematic. The classical analytical procedures in the literature have been critically examined for a special example and compared with the micromethod based on coupling of thin-layer high-voltage electrophoresis with coulometric titration.     

A coulometric method for the assay of some easily oxidized organic substances with iodine as an oxidizing agent

A coulometric method for the assay of easily oxidized organic substances, using iodine as an oxidizing agent, has been developed. The principle of the method is to oxidize the substance with an excess of iodine in a water-acetic acid medium and then titrate the iodide formed, with anodically generated silver ion. The titration is followed by dead-stop indication using two platinum electrodes. The method has been applied to the assay of dihydric phenols with hydroxyl groups in the ortho or para positions and to the determination of ascorbic acid in various pharmaceutical preparations. The absolute relative error varied between 0.2 and 2.8% for the 8 phenols investigated and the relative standard deviation between 0.2 and 1%. For standard solutions of ascorbic acid the absolute relative error was 0.9% and for the 6 pharmaceutical preparations investigated the relative standard deviation lay in the range 0.3-0.9%. The method is easy to apply and has a number of advantages over current iodimetric methods.     

Rapid coulometric titrations with a new design of electrolytic cell

The construction of a compact coulometric titration cell with integrated injection valve and photometric and biamperometric flow-through detectors is reported. The lower part of the cell is designed as a magnetic centrifugal pump by means of which the supporting electrolyte is propelled through the other functional parte. Various rapid titration procedures were developed. The favourable flow conditions allow titration times between 10 and 100 s. The principle of continuous titration up to a preselected set-point is applied for both the photometric and biamperometric detectors. Microgram amounts of cyanide and thioglycolic acid are titrated directly with electro-generated iodine. Full advantage of the cell design is obtained in the implementation of back-titration procedures during which the sample and the reagent in excess are injected simultaneously into the circulating electrolyte. Back-titrations are implemented to determine microgram amounts of iodine, copper (II) and iron (III).     

A new measuring device for the rapid coulometric determination of adsorbable organic halogen compounds

Summary A device for AOX determination with high precision and reproducibility is described. It permits great sensitivity at low electrolyte volumes and high gas flow rates. After incineration, the halide ions are titrated against electrolytic silver(I). Computer-controlled square-wave polarization of the indicator electrodes allows extremely low noise titration curves. Control of the coulometric titration and incineration temperature by a personal computer enables the entire analysis to be automated. The device permits the determination of the AOX fraction in liquid samples, the elementary analysis of halogen in organic compounds, and the determination of AOX in air.