Acid-base equilibria in ethylene glycol--II: autoprotolysis constants and acid-base properties of ethylene glycol and its mixtures

The acid-base behaviour of ethylene glycol and its mixtures with water (1, 5 and 10%), methanol, ethanol, isopropanol, nitromethane (each 10%) and ethyl methyl ketone-chloroform (5 + 5%) have been investigated by potentiometric titration, in a cell without liquid junction, equipped with a glass and a silver-silver chloride electrode. The autoprotolysis as well the protolysis constants of phthalic acid were determined for each mixture. The added solvents improve the properties of ethylene glycol, decreasing the viscosity without changing the acid-base behaviour of the ethylene glycol itself, which is favourable for the titration of weak bases. Water increases the basic, and nitromethane the acidic, properties of the mixture. Small quantities of water (ca. 1%) do not impair the titration conditions.     

Coulometric titration of acids in isopropanol-I A rapid direct method for the determination of carbon in steel

A new procedure is described for the direct coulometric titration of the carbon dioxide liberated in the high-temperature combustion of steel. The carbon dioxide is absorbed in isopropanol containing 3% of ethanolamine and titrated with 100% titration efficiency with a constant current of 50 mA and spectrophotometric detection of the end-point. The accuracy and precision of the method have been established by the analysis of a wide range of standard steel samples.     

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.     

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.     

Simple and accurate method for determining dissolved inorganic carbon in environmental water by reaction headspace gas chromatography

We investigate a simple and accurate method for quantitatively analyzing dissolved inorganic carbon in environmental water by reaction headspace gas chromatography. The neutralization reaction between the inorganic carbon species (i.e. bicarbonate ions and carbonate ions) in environmental water and hydrochloric acid is carried out in a sealed headspace vial, and the carbon dioxide formed from the neutralization reaction, the self‐decomposition of carbonic acid, and dissolved carbon dioxide in environmental water is then analyzed by headspace gas chromatography. The data show that the headspace gas chromatography method has good precision (relative standard deviation ≤ 1.63%) and accuracy (relative differences ≤ 5.81% compared with the coulometric titration technique). The headspace gas chromatography method is simple, reliable, and can be well applied in the dissolved inorganic carbon detection in environmental water.     

Effect of Composition and Structure of Aqueous Monoethanolamine Solutions on Carbon Dioxide Sorption and Desorption in Purification of Gas Mixtures

Possibility of raising the efficiency of the monoethanolamine purification of gas mixtures to remove carbon dioxide is demonstrated with consideration for the real intermolecular interactions and the structuring in the absorbent solution. The composition and structure of individual aqueous monoethanolamine solutions with various concentrations and of the same solutions saturated with carbon dioxide were examined. The methods of viscometry and conductometry demonstrated that, at monoethanolamine concentrations exceeding 12 ± 2 wt %, micelles are formed on the background of the existence of associates with intermolecular hydrogen bonds. This necessitates use of high temperatures (120?140°C) in the stage of carbon dioxide desorption. It was found that using a 12 wt % aqueous solution of monoethanolamine in purification of gas mixtures makes it possible to lower the desorption temperature of carbon dioxide to 90°C. This process is more efficient than the standard technology of CO₂ removal from a 30 wt % monoethanolamine solution. This is so because, in addition to a lower expenditure of heat, the extraction of carbon dioxide grows by 16% at a simultaneous decrease in the absorbent expenditure by at least a factor of 2.5.     

Coulometric titration of carbon dioxide on the micro scale

A method is described for the coulometric titration of milligram and submilligram amounts of carbon dioxide. Carbon dioxide is absorbed and titrated in dry acetone containing about 0.5% of methanol and saturated with potassium iodide, in a two-compartment cell. Thymol Blue is used as visual indicator. Current integration is carried out by means of a low inertia motor or a solid-state integrator. The generation efficiency is 99-100%. The method has been applied to the determination of small amounts of carbon (0-1 mg) in organic and inorganic materials and in aqueous solution.     

Coulometric Karl Fischer titration of trace water in diaphragm-free cells

Factors influencing the accuracy and precision for diaphragm-free Karl Fischer coulometric determinations of low μg-amounts of water have been studied using the Metrohm 756 (pulsed current) coulometer and eight different types of commercial coulometric reagents and some modifications of these. As in the case of diaphragm-free coulometric titration of large amounts of water, the positive errors, due to the formation of oxidizable reduction products (of sulfur dioxide) in the cathode reaction (besides hydrogen), were found to be minimized by the use of highest possible pulse current (in the range 100–400 mA) in combination with the fastest possible titration rate. Most accurate (102–103%) and precise results (typical relative standard deviation 1.8%) were obtained for reagents containing very large concentrations of imidazole in combination with the presence of modifiers like hexanol, chloroform and propylene glycol (i.e. the HYDRA-POINT reagents). Similar results were obtained when this type of reagent was mixed 60/40 with xylene according to the ASTM recommendation for water determinations in petroleum products like crude and lubricating oils. Addition of decanol to this type of reagent mixture was found to reduce the influence from the oxidative reduction products significantly. A reduction of the error from 3.6% relative to 1.6% was achieved by addition of 9% (v/v) of decanol to a 60/40 reagent mixture of HYDRA-POINT Coulometric Gen (containing hexanol as modifier) and xylene. For larger concentrations of decanol the pulse current had to be lowered to 100 mA and this might explain why no further improvement was observed. An additional attempt to minimize the interference by lowering the concentration of sulfur dioxide in the reagents gave no significant effect. However, by means of a home-built computer-controlled coulometric instrumentation based on continuous instead of pulsed current (including a large cathodic current density) it was possible to achieve recovery rates close to 100% for the best reagents investigated. The reason for this improvement is discussed.     

Coulometric titrations in microscale study of ionic equilibria

Summary Coulometric titration, a well established technique in research and in routine analysis, can successfully be applied to the study of ionic equilibria in solution. Processes, where either lyate or lyonium ions serve as titrants, for determination of acids and bases, respectively, were investigated. Among the systems studied are: autoprotolytic equilibria of various amphiprotic solvents, acid-base equilibria in water, organic and mixed solvents and complexation reactions. In all these examples the titration was followed by a potentiometric pH-indicator electrode. In nonaqueous solvents coulometry overcame the limited solubility of electrolytes, the difficulty of standardization on solutions and the necessity of working in isolated systems to avoid contamination of the sample with water vapour or with carbon dioxide.Because volumes below 1 ml and concentrations in the order of 0.001 mol/l can be used, this technique can be successfully applied to the investigation of micromole amounts of compounds, when protolytic properties of substances available in minute quantities should be evaluated. Furthermore this technique was applied to the evaluation of protonation constants of biologically important buffers and the stability constants of their calcium complexes.     

Determination of Ionol by Voltammetry and Coulometric Titration

Procedures were developed for determining ionol by voltammetry and by coulometric titration with electrogenerated chlorine using the amperometric indication of the titration end point. Possible mechanisms of ionol oxidation with electrogenerated chlorine and its electrochemical oxidation at a glassy carbon and a gold electrode were discussed. Procedures were developed for determining ionol in mineral oil in analytical ranges from 1.0 × 10^–4 to 1.0 × 10^–2 M (RSD = 9%) and from 3.0 × 10^–5 to 4.0 × 10^–3 M (RSD = 9%) using a glassy carbon and a gold electrode, respectively. The detection limits for ionol at the glassy carbon and gold electrode were 2.8 × 10^–4 and 1.0 × 10^–5 M, respectively. The detection limit in coulometric titration was 20 g/mL.     

Hydration to benzo-15-crown-5, benzo-18-crown-6 and the benzo-18-crown-6-potassium ion complex in the low-polar organic solvents.

The coextraction of water with benzo-15-crown-5 (B1SC5), benzo-18-crown-6 (B18C6) and the B18C6-K+ complex into seven low-polar solvents, i.e., carbon tetrachloride (CTC), chloroform (CF), dichloromethane (DCM), 1,2-dichloroethane (1,2-DCE), benzene (BZ), chlorobenzene (CB) and o-dichlorobenzene (o-DCB), has been investigated. The mean hydration number, nH2O, of these solutes in the water-saturated organic solvents was determined. There is a trend that the nH2O values for any solutes increase with increasing the water concentration in the solvents. Those of B18C6 and B15C5 converge at almost 0.8 for B18C6 and 0.4 - 0.5 for B15C5 in the solvents with the relatively high water concentration, i.e., CF, 1,2-DCE, DCM, and nitorobenzene (NB). The nH2O value of B15C5 is about one-half of that of B18C6 for a given organic solvent. The dominant species of the B18C6-K+ complex in these solvents is non-hydrated. From these results, the hydration equilibrium constants, KH2O, in the organic solvents were estimated.     

A simple photo-electric carbon dioxide titrator

A simple photo-electric apparatus is desscribed for the automatic titration of small amounts of carbon dioxide The apparatus is based on automatic titrant shut-off controlled by a photoresistor and is used for the non-aqueous titration proposed by Blomet al. (pyridine/monoethanolamine medium, sodium methanolate titrant).     

Low pressure solubility and thermodynamics of solvation of oxygen,carbon dioxide,and carbon monoxide in fluorinated liquids

The solubility of oxygen, carbon dioxide, and carbon monoxide in three fluorinated liquids – perfluorohexylethane, perfluorooctane and bromoperfluorooctane – is presented. Mole fraction solubilities were calculated from new experimental Ostwald coefficient data for CO₂ and CO, and from previously published values for O₂, associated with original values of density and vapour pressure for the pure solvents. Carbon dioxide is the most soluble gas with mole fraction solubilities of the order of 10⁻². Oxygen and carbon monoxide are one order of magnitude less soluble. The measurements were done as a function of temperature between (288 and 313) K and from the variation of the calculated Henry’s law constants with temperature, the thermodynamic properties of solvation such as the Gibbs free energy, the enthalpy and the entropy were calculated. The precision of the experimental data, considered as the average absolute deviation of the Henry’s law constants from appropriate smoothing equations is of 1% for carbon dioxide and oxygen and of 3% for carbon monoxide. The data obtained here are judged accurate to within ±5%.     

以多孔碳毡为工作电极的流动注射库仑检测器的研究及应用

本文设计了一个以多孔碳毡为工作电极的库化检测器并将其于用流动注射分析,以抗坏血酸、氢醌,没食子酸及二氧化硫为电活性物质检验该检测器的电化学特性。详细地研究了该检测器的库仑转换效率,选择性,线性范围,检测限及质量转移系数,并将该检测器作为双检测器流动注射分析系统中的“清除”电解池,用于酒中二氧化硫的测定,其结果与常规的吹气－氧化法非常吻合。     

Determination of dissociation constants of weak acids by deconvolution of proton binding isotherms derived from potentiometric data

Potentiometric titration of six carboxylic acid analytes were measured in aqueous and semiaqueous solvents to determine the effect of the solvent composition on the dissociation constants of the acids. The analytes studied were monoprotic (formic acid, acetic acid), diprotic (maleic and succinic acid) and triprotic (1, 2, ranging in composition from 0 to 80% by volume dioxane. The methodology used to assess the acidity constants was deconvolution of the proton binding isotherm of each analyte in each of the solvents. The dissociation constants agreed with those reported in the literature for titration in water. In the presence of the organic component, correlations relating the acidity constants with properties of the media were also found. In particular, affinity distributions derived from potentiometric titration data were used to assess the solvent media for resolving dissociation constants of weak acids and bases.     

非水库仑滴定法测定天然水体中溶解的无机碳

介绍用非水库仑滴定法测定天然水体中溶解的无机碳，对非水库仑滴定方法的关键技术－非水电解液进行了组分选择及优化，并对实验条件进行了优化。用该方法对碳酸钠标准溶液进行测定，测定结果的相对标准偏差为0.18%，测定值与理论值的平均相对误差为0.48%，表明该方法具有较高的精密度与准确度。     

The effect of aqueous organic solvents on the dissociation constants and thermodynamic properties of alkanolamines

The dissociation constants of protonated monoethanolamine and N-methyldiethanolamine have been determined in methanol–water, ethanol–water, and t-butanol–water solvents. The alcohol mole fractions were ranging from 0.2 to 0.95 and the temperatures from 283 to 323 K, 283 to 333 K, and at 298.15 K, respective to the different solvents. The experimental results are reported with the standard state thermodynamic properties. The basic strength of the protonated alkanolamine decreases with decreasing dielectric constant and increasing temperature of the solvent.     

Assessment and assurance of quality in water measurement by coulometric Karl Fischer titration of petroleum products

The precise and accurate determination of water in petroleum products—gasoline, diesel oil and aromatic hydrocarbons—is of significant importance for their normal functional and operational characteristics for economics as well as in meeting international requirements. The standard method ISO 12937 for the measurement of water by the coulometric Karl Fischer titration method was subject to within-laboratory assessment. Thereby it was established that the analytical procedure could not always be relied upon, especially when low levels of water were determined. The aim of this work is to assess each stage of the test procedure depending on the particular problems defined by the samples, using high-reactive Karl Fischer reagents and coulometric cells with and without a diaphragm, and to indicate the major factors causing the uncertainty. A reduced combined uncertainty was achieved by modifying the sample preparation step, the administration of the moisture in the coulometric cell, and sample injection manipulation. Experiments showed that the cell without the diaphragm ensures lower and uniform uncertainty of the measurements in the range of 0.01 to 10 mg H₂O in comparison to the cell with a diaphragm and therefore is more appropriate for the determination of low water levels in the petroleum products. A procedure that utilized a mixture of the reagent and toluene (10:6) was able to resolve the problem of two phases of formation and reduced conductivity of the reagent in direct titration of diesel oils. The results suggest that the modified procedure ensures relative expanded uncertainty equal to or less than 2.0% (n=5, confidence interval close to 95%) and 99.4% recovery for petroleum products investigated in repeatability conditions.     

Acid-base dissociation constants of 2,2'-bipyridyl in mixed protic solvents

The acid dissociation constants (K_a), base dissociation constants (K_b) and the autoprotolysis constants (K_s) for 2,2′-bipyridyl in water and in water+alcohol(methanol, ethanol, iso-propanol) mixed solvents have been determined at 25°C and an ionic strength of 0.1 mol l⁻¹, from a direct potentiometric method based on the treatment of the data of a single pH titration. It has been shown that K_a increases, whereas K_b and K_s decrease, with increasing proportion of the alcohol in the mixed solvents. Linear relations between pK_a, pK_b, pK_s and the mole fraction of the alcohol were observed in the composition range investigated. These results are discussed in terms of the properties of solvent and the interactions of the different species existing in dissociation equilibrium with solvents. It is concluded that the higher stabilization of both 2,2′-bipyridyl and its protonated form by dispersion forces and of the proton by its interaction with solvent molecules in the mixed solvents compared with that in water are largely responsible for the observed changes of pK_a with composition. On the other hand, the low stabilization of OH⁻ in the mixed solvents relative to that in water and the electrostatic effect are the main factors in determining the solvent effect on pK_b.     

Coulometric microdetermination of carbon and hydrogen in organic compounds

Summary An apparatus for accurate determination of carbon and hydrogen has been developed, based on a previously described coulometric Karl Fischer method for determination of small amounts of water in solids. Hydrogen and carbon in a sample are oxidized to water and carbon dioxide, respectively. The water is trapped first, and the carbon dioxide is converted into water by lithium hydroxide and determined in a coulometric cell. Then the water from the hydrogen is released and determined in the same cell. An alternative method for the determination of hydrogen alone with very high precision has also been developed. Samples weighing 0.1–5 mg can be analysed and the standard deviations obtained for a complete C, H determination are ±0.17% and ±0.08%, respectively. That for determination of hydrogen alone is ±0.009%.

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Zusammenfassung Eine Apparatur zur exakten Bestimmung von Kohlenstoff und Wasserstoff wurde auf der Basis einer früher beschriebenen coulometrischen Karl-Fischer-Methode zur Bestimmung geringer Wassermengen in Feststoffen entwickelt. Wasserstoff und Kohlenstoff werden zu Wasser bzw. Kohlendioxid oxydiert. ZunÄchst wird das Wasser absorbiert und das Kohlendioxid anschlie\end mit Lithiumhydroxid in Wasser übergeführt und in einer coulometrischen Zelle bestimmt. Das vom Wasserstoff herrührende Wasser wird in derselben Zelle bestimmt. Eine alternative, sehr genaue Bestimmungs-methode nur für Wasserstoff wurde ebenfalls entwickelt. Bei Probenmengen von 0,1–5 mg betrÄgt bei einer vollstÄndigen CH-Bestimmung die Standardabweichung ±0,17% bzw. ±0,08%; für die alleinige Wasserstoff-Bestimmung ist die Standardabweichung ±0,009%.