PRECISE COULOMETRIC TITRATION - AN APPROACH TO THE HIGHEST ACCURACY
Figures
Figures - available via license: Creative Commons Attribution-NonCommercial-NoDerivs 3.0 Unported
Content may be subject to copyright.
... e quantity of a substance produced at the electrode and the quantity of electric charge passed are linked with Faraday's laws of electrolysis. Many reports present coulometric methods of analysis of a variety of reagents [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17]. A number of original reports [3,4,11,[15][16][17] and reviews [1,2,[5][6][7][8][9][10][12][13][14] on coulometric methods were published over the past decades. ...
... Many reports present coulometric methods of analysis of a variety of reagents [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17]. A number of original reports [3,4,11,[15][16][17] and reviews [1,2,[5][6][7][8][9][10][12][13][14] on coulometric methods were published over the past decades. Coulometric method has been applied to analyze a large variety of inorganic and organic compounds [1,6,13]. ...
A gravimetric method for the quantitative assessment of the products of electrolysis of water is presented. In this approach, the electrolysis cell was directly powered by 9 V batteries. Prior to electrolysis, a known amount of potassium hydrogen phthalate (KHP) was added to the cathode compartment, and an excess amount of KHCO 3 was added to the anode compartment electrolyte. During electrolysis, cathode and anode compartments produced OH ⁻ (aq) and H ⁺ (aq) ions, respectively. Electrolytically produced OH ⁻ (aq) neutralized the KHP, and the completion of this neutralization was detected by a visual indicator color change. Electrolytically produced H ⁺ (aq) reacted with HCO 3⁻ (aq) liberating CO 2 (g) from the anode compartment. Concurrent liberation of H 2 (g) and O 2 (g) at the cathode and anode, respectively, resulted in a decrease in the mass of the cell. Mass of the electrolysis cell was monitored. Liberation of CO 2 (g) resulted in a pronounced effect of a decrease in mass. Experimentally determined decrease in mass (53.7 g/Faraday) agreed with that predicted from Faraday’s laws of electrolysis (53.0 g/Faraday). The efficacy of the cell was tested to quantify the acid content in household vinegar samples. Accurate results were obtained for vinegar analysis with a precision better than 5% in most cases. The cell offers the advantages of coulometric method and additionally simplifies the circuitry by eliminating the use of a constant current power source or a coulometer.
... Coulometry is one of the most precise and accurate techniques of chemical analysis [8,[20][21][22][23]. It was introduced in 1938 by Szebellédy and Somogyi [24], who are considered the pioneers of coulometry. ...
The use of pure materials in chemical analysis is currently in high demand due to the need to provide reliable measurement results. In the Electrochemistry Laboratory of the National Institute of Metrology from Brazil (Inmetro) is located the primary system of coulometry, which is considered the standard of the measurement for the amount of substance. The main function of this primary system is to certify primary reference materials to ensure reliability and metrological traceability for titration analyses. Some important certified reference materials (CRMs), homogeneous, stable, and characterized with an accurate property value have been developed. This work aims to present the stages for certifying the primary reference materials in a metrological approach: potassium hydrogen phthalate for acid-base titration, potassium chloride for precipitation titration, and potassium dichromate for redox titration. In addition, the key comparisons that were necessary to demonstrate the national traceability structure's equivalence for these CRMs' international recognition will be shown.
The electron transfer stoichiometry and principal oxidation products of the carbamate pesticide Aminocarb (3-methyl-4-dimethylaminophenyl N-methylcarbamate) were investigated in methanol + water and acetonitrile + water media, using flow injection coulometry,1H and 13C nuclear magnetic resonance spectrometry, and mass spectrometry. The four-electron oxidation was found to occur at aliphatic as opposed to aromatic sites, involving oxidation and nucleophilic attack at an N-methyl group to yield the hydroxy/methoxy analogs in water + methanol media, and yielding ultimately an aldehyde. The pathway in water + acetonitrile, as expected, was found to yield only the hydroxy analogs and the aldehyde.
Coulometric titrations of ammonia and sulphur dioxide can be implemented with an improved selectivity in a measuring cell combining a batch type titration cell with a gas diffusion separator. The supporting electrolyte is propelled through the coulometric titration cell and the acceptor side of the membrane separator by means of a magnetic centrifugal pump. The sample solution flows through the donor side of the membrane separator. Acceptor and donor side are separated by a microporous PTFE-membrane. The measuring cell assembly allows both selectivity enhanced coulometric titrations with enrichment of the sample substance to be determined and the continuous coulometric detection of volatile species.
The mass fraction of potassium hydrogen phthalate (KHP) from a specific batch was certified as an acidimetric standard. Two different analytical methods on a metrological level were used to carry out certification analysis: precision constant current coulometric and volumetric titration with NaOH. It could be shown that with a commercial automatic titration system in combination with a reliable software for the end-point detection it is possible to produce equivalent results with the same accuracy in comparison to a definite method handled by a fundamental apparatus for traceable precision coulometry. Prerequisite for titrations are that a high number of single measurement are applied which are calibrated with a high precision certified reference material.
The purities of sodium chloride and potassium dichromate (primary standard reagents for volumetric analysis) were directly determined and standard solutions of potassium dichromate, sodium arsenite and hydrochloric acid were standardized by the precise coulometric titration method.
The end points of the titrations for sodium chloride and hydrochloric acid were determined by an aid of a pH meter with silver and glass electrodes. The standard deviations of the results were about 0.037% and 0.081%, respectively. The sensitive amperometric end point procedure (Ref. 15) was used for the titrations of potassium dichromate. The standard deviations of these results were 0.0063% for the purity test and 0.022% for the standardization of the standard solution. The end point of standardization of sodium arsenite standard solution was determined by a dead stop method. The standard deviation of the result was 0.021%.
The values of standard deviation for the standard solutions just coincided with those by pipetting. The error for the sodium chloride reagents was due to the error in weighing of sample and in the location of end point.
Advances in the technique and scope of controlled-potential coulometry during the past 30 years have made it ideally suited to the analysis of materials when high accuracy is desired. Its advantages include an inherent, routine measurement precision of better than 0.1% with 1–10 mg of analyte, minimal use of chemical calibrants, stable instrument calibrations, and excellent selectivity for elements such as the actinides and precious metals.
A method is given for the precise and accurate titration of sodium carbonate with hydrogen ions generated by the coulometer described in the preceding paper. This coulometer maintains automatically a constant current for a measured length of time. The results are in good agreement with those obtained by titration with standard acid referred to pure silver as the ultimate standard, and support proposals to establish the coulomb as a standard in volumetric analysis. Factors are discussed that affect the accuracy and precision of analysis by controlled current coulometry.
The purity of recrystallized disodium dihydrogen ethylenediaminetetraacetate dihydrate was determined accurately by titration with zinc ion coulometrically generated from an amalgamated zinc anode. The reagents used were of 99.446% ±0.002% homogeneity when the sample was dehydrated and stored in a hygrostat (relative humidity, ca. 58% at 20 °C) over 1 year. Aluminium of high purity (99.99%) was coulometrically titrated with the purified reagent. Aluminium was allowed to react with a certain amount of the reagent, excess of the reagent being titrated with zinc ion anodically generated. The end point was determined potentiometrically using a mercury electrode. The purity was found to be 99.976% with a standard deviation of 0.010% for 6 degree of freedom. Cause of the lower result (0.01%) is discussed.
Single crystals or sulphamic acid were grown from aqueous solution. The occluded and adsorbed water of the crystals prepared was measured, and the purities of the single crystals were determined by precise coulometric titration. Large single crystals more than 10 g in weight were readily prepared. The adsorbed water on the surface amounted to ca. 0.0004%. The purities of the small and the medium crystals were not satisfactory for primary standard use. However, the purity of the pieces obtained by dividing a large crystal and polishing the surfaces, was about 99.98%, with a standard deviation of about 0.015%. These large single crystals of sulphamic acid are recommended as a primary standard.
Coulometrie titration of +6 uranium with electrogenerated titanous ion ; at a platinum cathode in 8 F sulfuric acid can be performed accurately in the ; presence of ferrous ion as a catalyst. Amperometric end-point detection is ; employed. The titration is suitable for the precise assay of uranium compounds. ; Mixtures of +5 vanadium and +6 uranium can be titrated, because the reduction of ; +5 vanadium occurs at a more oxidizing potential than the reduction of +4 ; vanadium and +6 uranium. Using amperometric end-point detection, two end-points ; are observed corresponding to the reduction of +5 vanadium to the +4 state, and ; to the simultaneous reduction of +4 vanadium to the +3 state and +6 uranium to ; the +4 state. Errors are as small as plus or minus 0.3%, provided the uranium/; vanadium ratio is between 0.1 and 10. (auth);
The use of single crystals of sodium chloride as a primary standard substance for argentimetry is described. The water on the surface and the purity of the crystal are measured by coulometric titration methods. The adsorbed water amounts to 0.48 μg cm-2 when the sample is dried in a desiccator containing magnesium perchlorate at room temperature for about 3 h. The loss in weight of the crystal is negligible when it is heated at about 600°. The purities of three crystals, by precise coulometric titrations, are 99,988,99.993 and 99.995%, with standard deviations of 0.0143, 0.0138 and 0.0124%, respectively. It can be concluded that the crystal is useful as a primary standard.
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.RésuméUne méthode coulométrique a courant constant est décrite pour le dosage précis du thorium par titrage an moyen d'EDTA, formé électrochimiquement à partir de son complexe mercurique en solution tampon acétique (pH 4.5). On a obtenu des déviations standard relatives de 0.02 et 0.3% pour 30 mg et 3 μg respectivement en utilisant des électrodes de mercure polarisées pour la détection du point final. On a appliqué cette méthode à l'analyse de thorium do grande pureté.ZusammenfassungEine coulometrische Methode mit konstantem Strom wird für die genaue Bestimmung von Thorium beschriebon. Es wird mit AeDTE, das auf elcktrolytischem Wege aus seinem Quecksilberkomplex in azetatgepufferter Lösung beim pH 4.5 erzeugt wird, titriert. Mit polarisierten Quecksilberelektroden zur Endpunktsbestimmung wurden relative Standardabweichungen von 0.02 und 0.3% bei 30 mg bzw. 30 μg erhalten. Die Methode wurde für die Analyse von hochreinem Thorium verwendet, das durch Zonenschmelzen gereinigt worden war.