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.     

Investigation on drying conditions and assays of amidosulfuric acid and sodium carbonate by acidimetric coulometric titration and gravimetric titration

Amidosulfuric acid and sodium carbonate as standards for acid–base titrimetry were assayed by coulometric titration and gravimetric titration. Amidosulfuric acid was directly assayed by coulometric titration with electrogenerated hydroxide ions, and sodium carbonate was assayed by gravimetric back-titration. For sodium carbonate, excess amount of sulfuric acid, whose concentration was determined by coulometric titration, was added to sodium carbonate, and then gravimetrically back-titrated using a sodium hydroxide solution whose concentration was determined by gravimetric titration using the sulfuric acid. The accuracy of the coulometric titration for amidosulfuric acid and sulfuric acid was evaluated by examining the current efficiency of pulse electrolysis, the amount of the electrolysis current used, and the time spent for a titration. In addition, the drying conditions for high purity primary standards have a significant effect on the titration results due to changes in the acid–base assay. The suitable drying conditions for amidosulfuric acid and sodium carbonate were evaluated by mass-change measurements, coulometric titration and gravimetric titration. The measurement uncertainties were estimated from the uncertainties on the titration processes. Finally, the assays of amidosulfuric acid and sodium carbonate were 99.986% ± 0.010% (k = 2) after drying at 50 °C for 2 h, and 99.970% ± 0.016% (k = 2) after drying at 280 °C for 4 h, respectively. In addition, the international consistency was confirmed by measuring certified reference materials (CRMs) available from different National Metrology Institutes, and the compatibility of values among CRMs was experimentally ascertained.     

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.     

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.     

Purity of potassium hydrogen phthalate, determination with precision coulometric and volumetric titration--a comparison

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 preparation, assay and certification of aqueous ethanol reference solutions

Internationally, certified ethanol reference materials are required to calibrate breathalysers and blood-alcohol measurement instruments. The CSIR National Metrology Laboratory of South Africa provides certified aqueous ethanol solutions with traceability to the SI. Ethanol solutions in the concentration range 10 mg/100 g to 20 g/100 g are prepared gravimetrically by mixing ethanol and reagent quality water. To verify the concentration of the ethanol it is oxidized to acetic acid with potassium dichromate in the presence of sulphuric acid. The unreacted potassium dichromate is back-titrated with sodium thiosulphate. The method utilizes gravimetry and titrimetry, which are both primary methods. This paper addresses aspects of ethanol-certified reference materials that have not been previously published: traceability, stability of unpreserved ethanol solutions, homogeneity, quality control measures and the effect of reproducibility on the measurement uncertainty. Presented at -- “BERM-10” -- April 2006, Charleston, SC, USA     

Preparation and certification of creatinine and urea reference materials with certified purity as a traceability source in clinical chemical measurements

Purity certified reference materials (CRMs) are playing a key role in metrological traceability, because they form the basis for many traceability chains in chemistry. Recently, the National Metrology Institute of Japan (NMIJ) has developed two purity CRMs for creatinine (NMIJ CRM 6005-a) and urea (NMIJ CRM 6006-a), because the concentrations of these two compounds are frequently measured in clinical laboratories for monitoring the renal functions. In the certification of purity CRMs, it is essential that the materials have been thoroughly characterized for purity, and the purity should preferably be determined directly by a primary method of measurements. In the development of these two CRMs, we used the purified materials as candidates. The certified values were assigned based on the results of two different methods; acidimetric titration and nitrogen determination by the Kjeldahl method. Since both methods cannot distinguish some impurities from the target compounds, major impurities in the candidate materials were also identified, quantified, and subtracted. These CRMs can provide a traceability link between routine clinical methods and SI units. Presented at BERM-11, October 2007, Tsukuba, Japan.     

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.     

Matrix reference materials – provision in Australia

 Until quite recently, Australia has not been much involved in the preparation and certification of matrix reference materials for chemical testing. Even today, the vast bulk of chemical reference materials used in Australia are imported from other world producers. Increased international focus on the accuracy, traceability and comparability of chemical measurements has led to the establishment of the National Analytical Reference Laboratory (NARL) within the Australian Government Analytical Laboratories. Part of the work of NARL will be to supply matrix reference materials, not available from existing sources, to meet specific Australian requirements. This need has been addressed in the past by a combination of industry and government initiatives. Examples include a series of certified matrix reference materials for chemical testing of iron ore, coal and mineral sands produced by Standards Australia and a series of three animal fat matrix reference materials certified for a range of pesticide residues produced by the Australian Chemical Standards Laboratory (now part of NARL). To make effective use of limited resources, it will be important for NARL to focus on identified priorities and to maximise the use of available Australian resources and expertise through technical collaboration for reference material production. An important part of this process should be input on needs and priorities from reference material "users" such as government legislators, regulatory authorities, standards setting bodies, industry and the analytical community. The aim will be to produce matrix certified reference materials that are traceable to SI or other international standards at a stated level of measurement uncertainty.     

Metrological traceability is not always a straight line

The two most important concepts in metrology are certainly “traceability to standards” and “measurement uncertainty evaluation”. So far the questions related to these concepts have been reasonably solved in the metrology of “classical quantities”, but for the introduction of metrological concepts in new fields, such as chemistry and biology, a lot of problems remain and must be solved in order to support international arrangements. In this presentation, the authors want to develop the strategy implemented at Laboratoire national de métrologie et d’essais (LNE) in metrology in chemistry and biology. The strategy is based on: (1) pure solutions for calibration of analytical instruments, (2) use of certified reference materials (matrix reference materials), and (3) participation to proficiency testing schemes. Examples will be presented in organic and inorganic chemistry. For laboratory medicine, proficiency testing providers play an important role in the organization of External Quality Assessment Schemes. For the time being, the reference value or the assigned value of the comparison is calculated with the results obtained by the participants. This assigned value is not often traceable to SI units. One of the methods suggested by LNE is to ensure the metrological traceability to SI units of the assigned value for the more critical quantities carried on analytes by implementing the Joint committee for traceability in laboratory medicine reference methods.     

The role of certified reference materials in ensuring the quality of spectrochemical measurements – the present status in Romania

 The objective of quality assurance programme for spectrochemical measurements is to reduce the measurement errors to accepted limits. Reference materials are being widely used as measurement standards in the fields of industrial production, environmental protection and clinical chemistry, and are playing an important role in ensuring the quality of measurement results. This paper presents some aspects, practices and examples of the activity of the Reference Materials Laboratory of the National Institute of Metrology, Bucharest, in the field of spectrochemical measurements. An attempt to describe the role and use of reliable certified reference materials to ensure the quality of spectrochemical measurements is presented. A short review of the locally available certified reference materials used in spectrochemical measurements is given. The use of reference materials data in estimating the measurement uncertainty is discussed. An interlaboratory comparison, recently organized in Romania, is also presented as a useful response to the need for quality assurance of spectrochemical results. Received: 20 March 1999 / Accepted: 25 February 2000     

A constant current coulometer for the high precision analysis of uranium

 A constant current coulometry device was designed, built and evaluated. It is capable of controling the coulometric titration process and to deliver the necessary current and time supplies/measurements with extremely high accuracy. The device was applied to the coulometric titration of uranium. The chemical part of the system was adapted and improved and a reliable procedure was developed. The method is highly accurate, standard deviations lie around 0.06% relative. It is an absolute method and directly traceable to the SI units. Received: 4 March 1996/Revised: 23 December 1996/Accepted: 7 January 1997     

Purity determination and uncertainty evaluation of theophylline by mass balance method, high performance liquid chromatography and differential scanning calorimetry

Purity determination of pure organic substance is essential for the establishment of traceability to SI units. A mass balance method was employed to determine the purity of theophylline certified reference materials (CRM), compared with high performance liquid chromatography (HPLC) and differential scanning calorimetry (DSC). In the approach of the mass balance, the impurities were identified by ion trap time-of-flight mass spectrometer (IT-TOF-MS) and quantified by HPLC. The purity of theophylline CRM determined by mass balance method was 99.82% with an extended uncertainty of 0.1% (k = 2). The uncertainty evaluation of purity demonstrated that the accuracy of the mass balance method is better than that of HPLC and DSC. It indicated that the mass balance is suitable for the CRM and pharmaceutical standards.     

Creatinine determination in urine from the point of view of reference values

In this paper, the creation of a certified reference material for urinary creatinine is described. We used the Jaffe method and HPLC method for establishment of the certified value. Homogeneity tests are also described. We obtained material with sufficient homogeneity, stability, and with certified value (expanded uncertainty, k=2 for CI 95%) (7.77±0.27) mmol·L⁻¹. This material was consequently used for the interlaboratory comparison (EQA Czech Republic for clinical chemistry). Twenty-nine percent of the participants obtained measurement results within the interval of the certified value ± expanded uncertainty, while 85% of the participants obtained values inside the interval of the certified value ± target measurement uncertainty. Direct use of the certified reference materials for method evaluation in EQA programs means a significant advance for monitoring and documentation traceability of results in routine measurements.     

Calibration of double focusing Glow Discharge Mass Spectrometry instruments with pin-shaped synthetic standards

Calibration of two commercially available glow discharge double focusing mass spectrometers, the VG 9000 and Element GD, is described using synthetic pin standards pressed from solution doped copper and zinc matrices. A special pressing die was developed for this purpose and optimal results were obtained with the highest possible pressures, i.e., 95 kN·cm^? 2. This calibration approach permits the determination of trace element mass fractions down to μg·kg^? 1 with small uncertainties and additionally provides traceability of the GD-MS results in the most direct manner to the SI (International System of Units). Results were validated by concurrent measurements of a number of compact copper and zinc certified reference materials. The impact of the sample pin cross-section (circular or square) was investigated with the use of a new pin-sample holder system for the Element GD. The pin-sample holder was designed by the manufacturer for pin-samples having circular cross-section; however, samples with square pin cross-section were also shown to provide acceptable results. Relative Sensitivity Factors for some 50 analytes in copper (VG 9000, Element GD) and zinc matrices (VG 9000) are presented. The field of applicability of GD-MS may be considerably extended via analysis of pin geometry samples based on their ease of preparation, especially with respect to the accuracy and traceability of the results and the enhanced number of analytes which can be reliably calibrated using such samples.     

Homogeneity testing of reference materials

 Homogeneity testing is of the highest importance for the certification of reference materials, as it should demonstrate the validity of the certified values and their uncertainties in the analysis of individual units or portions thereof. However, the conclusions drawn from the results of these studies may often be questioned. It is proposed to improve this situation by quantifying the effect of homogeneity studies in terms of their impact on the uncertainty of certified values. Here it should be noted that the between-units variability directly affects the certified uncertainty, whereas the within-unit inhomogeneity only defines the minimum representative test portion. Received: 18 September 1997 · Accepted: 21 November 1997     

A framework for the application and materials in analytical chemistry

 Working group 5 of EuraChem Nederland has developed a framework for the implementation of reference materials in analytical chemistry. In this discussed paper, the framework is proposed as a tool for the development of standard operation procedures (SOPs) in laboratories. The implementation of (certified) reference materials in these SOPs is of major importance in establishing comparability and traceability in measurement results, which in turn play a crucial role in measurement in support of trade, environmental issues, and characterisation of materials. Recent developments in the field of uncertainty analysis require the application of reference materials. It is recognised that the calculation of the combined measurement uncertainty becomes almost impossible without the use of certified reference materials with a stated uncertainty. Received: 1 December 1995 Accepted: 20 December 1995     

Capabilities of femtosecond laser ablation ICP-MS for the major, minor, and trace element analysis of high alloyed steels and super alloys

Femtosecond laser ablation inductively coupled plasma mass spectrometry was used for the quantification of 23 metallurgical relevant elements in unalloyed, alloyed and highly alloyed steels, and super alloys. It was shown that by using scanning mode ablation with large ablation spot diameters (250 μm), stable and representative sampling can be achieved for the majority of elements, except for bismuth and lead. For Bi and Pb up to 46%, temporal relative standard deviation (TRSD) was encountered, whereas for most other elements, the TRSDs were below 10%. Calibration with matrix-matched and non-matrix-matched standards provided similar agreement within the uncertainty of the certified values. However, the non-matrix-matched standard-based quantification was more influenced by interferences rather than ablation- or excitation-related matrix effects. The method was validated using 34 certified reference materials. ⁵²Cr, ⁵¹V, or ⁵⁵Mn were used as internal standards due to the fact that the Fe concentration was not certified for the majority of reference materials. The determined concentrations for major and minor elements indicate that the total matrix internal standardization (100 wt.%) is applicable, which requires no knowledge about the steel samples prior to analysis.     

Contribution to the certification of B, Cd, Mg, Pb, Rb, Sr, and U in a natural water sample for the International Measurement Evaluation Programme Round 9 (IMEP-9) using ID-ICP-MS

 The present paper describes the contribution of the Institute for Reference Materials and Measurements to the certification of B, Cd, Mg, Pb, Rb, Sr, and U amount contents in a natural water sample, in round 9 of the International Measurement Evaluation Programme (IMEP-9). The analytical procedure to establish the reference values for B, Cd, Mg, Pb, Rb, Sr, and U amount contents was based on isotope dilution inductively coupled plasma-mass spectrometry used as a primary method of measurement. Applying this procedure reference values, traceable to the SI, were obtained for the natural water sample of IMEP-9. For each of the certified amount contents presented here a total uncertainty budget was calculated using the method of propagation of uncertainties according to ISO and EURACHEM guidelines. The measurement procedures, as well as the uncertainty calculations are described for all seven elements mentioned above. In order to keep the whole certification process transparent and so traceable, the preparations of various reagents and materials as well as the sample treatment and blending, the measurements themselves, and finally the data treatment are described in detail. Explanations focus on Pb as a representative example. The total uncertainties (relative) obtained were less than 2% for all investigated elements at amount contents in the pmol/kg up to the high μmol/kg range, corresponding to low μg/kg and mg/kg levels. Received: 21 October 1999 / Accepted: 29 January 2000