Determination of the purity of acidimetric standards by constant-current coulometry, and the intercomparison between CRMs

The accuracy and uncertainty of the coulometric measurement results of reference materials for acidimetric titration were examined in this study. The results for amidosulfuric acid and potassium hydrogen phthalate are presented. The uncertainty 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. Acidimetric standards with the certified value linked to the SI were developed. In addition, the intercomparison of acidimetric standards was carried out by gravimetric titration, and the relationship between our coulometric results was determined. Furthermore, due to recent internationalization, not only the traceability to the SI but also the relationship and consistency of their analytical data have gained increasing importance. Our results were validated using certified reference materials (CRMs) obtained from different National Metrology Institutes (NMIs), and their relationships are presented. Presented at -- “BERM-10” -- April 2006, Charleston, SC, USA.     

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.     

Precise coulometric titration of sodium thiosulfate and development of potassium iodate as a redox standard

In this paper, we determine the effective purity of potassium iodate as a redox standard with a certified value linked to the international system of units (SI units). Concentration measurement of sodium thiosulfate solution was performed by precise coulometric titration with electrogenerated iodine, and an assay of potassium iodate was carried out by gravimetric titration based on the reductometric factor of sodium thiosulfate assigned by coulometry. The accuracy of the coulometric titration method was evaluated by examining the current efficiency of iodine electrogeneration, stability of sodium thiosulfate solutions and dependence on the amount of sodium thiosulfate solution used. The measurement procedure for gravimetric titration of potassium iodate with sodium thiosulfate was validated based on determination of a reference material of known purity (potassium dichromate determined by coulometry with electrogenerated ferrous ions) using the same gravimetric method. Solutions of 0.2 and 0.5 mol/L sodium thiosulfate were stable over 17 days without stabilizer. Investigation of the dependency of titration results on the amount of sodium thiosulfate solution used showed no significant effects, no evidence of diffusion of the sample, and no effect of contamination appearing during the experiment. Precise coulometric titration of sodium thiosulfate achieved a relative standard deviation of less than 0.005% under repeating conditions (six measurements). For gravimetric titration, the results obtained for the effective purity of potassium dichromate were sufficiently close to its certified value to allow confirmation of the validity of the gravimetric titration was confirmed. The relative standard deviation of gravimetric titration for potassium iodate was less than 0.011% (nine measurements), and a redox standard with a certified value linked to SI units was developed.     

Investigation of the drying conditions for amidosulfuric acid.

The drying conditions for primary standards of volumetric analysis have a significant effect on the titration results due to changes in the purity, stability and homogeneity. Amidosulfuric acid, a strong acid used as a reference material for volumetric analysis in Japan, was dried in a vacuum desiccator or heated at different temperatures, and then measured by Karl-Fischer titration, thermogravimetry/mass spectroscopy (TG-MS), ion chromatography and coulometric titration. The optimum drying conditions were at 50 degrees C for 24 h with crushing.     

Coulometric titration of bases in acetic acid and acetonitrile media

The working conditions and the results for coulometric titration of milligram amounts of some bases in 0.1M sodium perchlorate in a mixture of acetic acid and acetic anhydride (1:6), are given. Determinations were made both by coulometric back-titration or direct titration at the platinum anode. Back-titration was done in the catholyte, by coulometric titration of the excess of added perchloric acid. The titration end-point was detected photometrically with Crystal Violet as indicator. The direct titration of bases was done at the platinum anode, in the same electrolyte, to which hydroquinone was added as anode depolarizer and as the source of hydrogen ions, Malachite Green being used as indicator. Similarly, bases can be determined in acetonitrile if sodium perchlorate, hydroquinone and Malachite Green are added to the solvent. Errors are below 1 %, and the precision is satisfactory.     

Influence of AgCl precipitates on the precipitation titration of sodium chloride by constant-current coulometry.

The precipitation titration of sodium chloride with electrogenerated silver ion was studied. The production of a precipitate of silver chloride had a significant effect on the titration results because the precipitate involved unreacted chloride or unreacted silver ion. The accuracy of the method was investigated by changing the introduction time of a sodium chloride solution to the coulometric cell during the process of electrolysis, and examining the dependency on the sample size. The accuracy of the measurement of the precipitation titration is discussed.     

The coulometric titration of acids and bases in dimethylsulfoxide media

The coulometric titration of 20–200 μeq of acids and bases in DMSO media is described. In the titration of bases, the electro-oxidation of hydrogen at a platinized platinum electrode is used as the source of protons. The conditions for 100 % current efficiency at this electrode are low current density to avoid passivity and regular treatment of the electrode with potassium dichromate—sulfuric acid to remove a poisoning sulfide layer. The accuracy of the titrations is better than ±1 %. Very weak acids like phenols (pK_a (DMSO) ≈16) can be titrated successfully. Tris(hydroxymethyl)aminomethane is the weakest base titrated.     

Accuracy in the precise coulometric titration of ammonia and ammonium ion with electrogenerated hypobromite

A precise and accurate method in which ammonia and ammonium ions are coulometrically titrated with electrogenerated hypobromite was studied. Through the measurement of the current efficiency for generating hypobromite and the dependencies of titration results on electrolysis current and delay time between sampling and starting electrolysis, precision and accuracy of 0.01 and 0.02%, respectively, were obtained. The accuracy of the titration was compared with that of acidimetric coulometric titration of ammonia.     

Investigation of iodine liberation process in redox titration of potassium iodate with sodium thiosulfate

Potassium iodate is often used as a reference material to standardize a sodium thiosulfate solution which is a familiar titrant for redox titrations. In the standardization, iodine (triiodide) liberated by potassium iodate in an acidic potassium iodide solution is titrated with a sodium thiosulfate solution. The iodine liberation process is significantly affected by the amount of acid, that of potassium iodide added, the waiting time for the liberation, and light; therefore, the process plays a key role for the accuracy of the titration results. Constant-voltage biamperometry with a modified dual platinum-chip electrode was utilized to monitor the amount of liberated iodine under several liberation conditions. Coulometric titration was utilized to determine the concentration of a sodium thiosulfate solution on an absolute basis. Potassium iodate was assayed by gravimetric titration with the sodium thiosulfate solution under several iodine liberation conditions. The liberation process was discussed from the changes in the apparent assay of potassium iodate. The information of the appropriate titration procedure obtained in the present study is useful for any analysts utilizing potassium iodate to standardize a thiosulfate solution.     

Production of three certified reference materials for water content based on mixed solutions of butanol, xylene and propylene carbonate

Certified reference materials (CRMs) for water content with good accuracy and homogeneity are required for calibration or validation of the Karl Fischer titration and for establishing the traceability of water content results. Three such CRMs were produced and certified: GBW 13512, 13513 and 13514 are based on solvent mixtures consisting of butanol, xylene and propylene carbonate with water contents of 10.01, 1.067 and 0.139?mg/g, respectively, certified by the Karl Fischer coulometric and volumetric methods. These CRMs were prepared, dispensed and sealed under a humidity equal to the equilibrated humidity of their headspace. In this way, the between-bottle homogeneity uncertainty (u _H,rel) could be kept as low as u _H,rel?=?0.12?% for GBW 13512. The certification methods, that is, Karl Fischer coulometric and volumetric methods, were calibrated using in-house water standards prepared by gravimetry. The results were traceable to the SI unit of mass. The relative deviation of the water contents between the two methods for GBW 13512 was only 0.05?%. The expanded uncertainty (U, k?=?2) of three CRMs was 0.12, 0.024 and 0.012?mg/g, respectively. These CRMs for water content with good accuracy can be applied in the calibration or validation of measurement procedures to ensure accurate and comparable results.     

Accurate all-purpose method for the assay of alkali and alkaline earth carbonates with perchloric acid

An accurate all-purpose method is proposed for the determination of carbonate in alkali and alkali earth carbonates. A large sample size (4–9 g depending on the carbonate) is dissolved in 100.00 ml of 1.2 M perchloric acid, the solution is boiled, and the excess of acid is titrated with standard 1.0 M sodium hydroxide solution to a methyl orange end-point. Perchloric acid is preferable to hydrochloric acid because there is no danger of loss of acid during the boiling; it is preferable to sulfuric acid since insoluble sulfates of barium, strontium, and calcium are not precipitated in analyzing alkali earth carbonates. Normal alkali and alkali earth carbonates (anhydrous and hydrated) are assayed on samples previously dried by heating in an oven at 250°C for 3 h. This treatment drives off all the water but it also converts any hydrogencarbonate present into carbonate. Hydrogencarbonates are assayed on samples dried in a desiccator containing sulfuric acid for 16 h. Twelve different carbonates were analyzed. The average standard deviation of the method was 0.050%.     

Argentomctric coulomelric titration of thioacetamide

The coulometric determination of thioacetamide (TAA) with electrogenerated silver is described. The titration is done in a solution 0.1M in both ammonia and sodium hydroxide, and the end-point is detected potentiometncally with a silver-silver sulphide electrode. On repeat analyses of approx. 2-mg samples of TAA an average error of -04% (relative standard deviation 0.25%) was obtained. Important steps in the procedure include cleaning the silver generating electrode in nitric acid before each titration, purging well with nitrogen to remove oxygen, and not using too large a sample.     

The determination of organic substances by the oxidation with permanganate: XVIII. The oxidation of malonic acid

The oxidation of malonic acid with permanganate was studied under various acidity conditions. Analytical procedures, based on oxidation with excess reagent in a sodium carbonate medium and titration of the oxalate or manganese dioxide formed or of unconsumed permanganate, are proposed. On the basis of successive oxidation in sodium carbonate and sulfuric acid media, a titration determination, involving complete oxidation of malonic acid to carbon dioxide and water, was developed.     

Coulometric titration with air transported sample

Air transported samples of a mixture containing the analyte and a supporting electrolyte are consecutively propelled into a microconduit system through a titration cell, a coil and a detector cell. Different amounts of electrochemically generated titrant are obtained in the separate samples while they are passing through the titration cell. Various quantities of the titrant are achieved by changing the duration of the titrant generating process that is done by various time for the sample solution staying in the titration cell (different sample movement rate). The titrated sample is then homogenised in the coil and detected in an electrochemical detector cell that supplies necessary data for the analyte determination. The declared method is experimentally verified on coulometric titration of hydrochloric acid (HCl) with electrogenerated sodium hydroxide and a titration of aniline with bromine generated in a bromide solution.     

Arsenopyrite mineral based electrochemical sensor for acid–base titrations in γ-butyrolactone and propylene carbonate

Abstract  

A novel acid–base sensor based on the natural mineral arsenopyrite for titrations in γ-butyrolactone and propylene carbonate is validated and studied. This sensor, which requires only small sample volumes, was employed for the titrations of some important organic acids (benzoic, anthranilic, and salicylic acid) with potassium hydroxide. A stable stationary potential was attained at the arsenopyrite electrode in γ-butyrolactone and propylene carbonate in less than 5 min. The change in the electrode potential from the acidic to basic region was 379 to −160 mV in γ-butyrolactone and 434 to −67 mV in propylene carbonate. The arsenopyrite electrode showed a relatively fast response time in the investigated solvents (11 s in γ-butyrolactone and 14 s in propylene carbonate). It can be used without any time limit or without considerable divergence in the potential. The investigated electrode showed a linear dynamic response for p-toluenesulfonic acid concentrations in the range 0.1–0.001 M, and a sub-Nernst dependence in γ-butyrolactone as well as in propylene carbonate. Unlike traditional pH titration, an end-point color indicator is unnecessary in this method, and real-time monitoring can be realized. The relative standard deviations for measurements of benzoic, anthranilic, and salicylic acids were 0.10–0.31%, which show that the repeatability and accuracy of measurements taken with the sensor are satisfactory.     

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 by a controlled-potential pulsed-charge potentiometric technique-II Acid-base, precipitation and homogeneous redox systems

An unconventional coulometric method, based on electrolysis at controlled potential by means of discrete charge-pulses, with end-point detection obtained by mathematical linearization of potentiometric data, has been applied to various systems. The results appear very satisfactory: very weak acids can be determined with high accuracy; in the case of precipitation reactions the selectivity is high enough to allow titration of the components of halide mixtures; for homogeneous redox systems some problems caused by irreversibility at the indicator electrode are overcome.     

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.     

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     

Ultramicrocoulometric determination of ammonia in serum, plasma, and blood

A method has been developed for the ultramicrodetermination of ammonia in as little as 0.01 ml of serum, plasma, or blood by coulometric titration with electrogenerated hypobromite using a sensitive amperometric endpoint. The only accurate volumetric measurement required is the pipetting of the sample. The sample is added to a 33-mm microdiffusion cell for 10 min to separate the ammonia; the ammonia is collected in 0.1 ml of 0.0025 M sulfuric acid in the center compartment. The separated ammonia is added to a coulometric generation cell where an excess amount of hypobromite has been generated. The decrease in amperometric current due to the utilization of the hypobromite by the ammonia is taken as a measure of the ammonia content; comparison is made with diffused standards. An optimum pH of 8.6 for the titration has been found to give reproducible results at all levels encountered. One-tenth-milliliter blood samples are routinely analyzed. When elevated ammonia levels are encountered, 0.01-ml samples can be used with accurate results. The time for the analysis is 10–15 min after initiating the diffusion.