Search

The first practical comparison of the characteristics of Ag|AgCl electrodes prepared at different National Metrology Institutes (NMIs) has been carried out as a pilot study organised by the Consultative Committee for Amount of Substance (CCQM). Each NMI prepared at least three Ag|AgCl electrodes representative of those employed in their primary pH calibration facilities. The electrodes were sent to a coordinating laboratory where comparative measurements of their potential difference to a de facto reference, slope as a function of chloride ion concentration and electrochemical impedance were made. Electrodes from most NMIs were highly repeatable and consistent within the typical rejection criteria applied during production. An analysis of the electrode slopes in a phosphate buffer containing different concentrations of NaCl revealed the influence of variances in the Ag|AgCl electrodes between NMIs on the certified pH value of the buffer solution. The difference between NMIs is consistent with submitted values in CCQM-K9, although biases were smaller in the analysis here, suggesting the presence of other unknown contributions to the uncertainty such as cell design. The smaller biases observed in this work may also reflect improvements in the Ag|AgCl electrodes since CCQM-K9 was performed. Electrochemical impedance spectroscopy suggests a similar microstructure for electrodes prepared by most NMIs.

This meta-analysis assesses the long-term (up to 70 years) within-laboratory variation of the NIST pH Standard Reference Material® (SRM) tetroxalate, phthalate, phosphate, borate, and carbonate buffers. Values of ΔpH(S), the difference between the certified pH value, pH(S), of each SRM issue and the mean of all pH(S) values for the given SRM at that Celsius temperature, t, are graphed as a function of the SRM issue and t. In most cases, |ΔpH(S)| < 0.004. Deviations from the nominal base:acid amount (mole) ratio of a buffer yield t-independent, constant shifts in ΔpH(S). The mean ΔpH(S) characterizes such deviations. The corresponding mole fraction of impurity in the conjugate buffer component is generally

The key comparison (KC) studies of the Consultative Committee for Amount of Substance—Metrology in Chemistry help ensure the reliability of chemical and biochemical measurements relevant to international trade and environmental-, health-, and safety-related decision making. The traditional final evaluation of each measurement result reported by a KC participant is a “degree of equivalence” (DEq) that quantitatively specifies how consistent each individual result is relative to a reference value. Recognizing the impossibility of conducting separate KCs for all important analytes in all important sample matrices at all important analyte levels, emphasis is now shifting to documenting broadly applicable critical or “core” measurement competencies elicited through a series of studies. To better accomplish the necessary synthesis of results, data analysis and display tools must be developed for objectively and quantitatively combining individual DEqs. The information detailed in the 11 KCs of primary method pH measurements publically available as of 2013 provides an excellent “best case” prototype for such analysis. We here propose tools that enable documenting the expected primary pH measurement performance of individual participants between pH 1 and pH 11 and from 15 °C to 37 °C. These tools may prove useful for other areas where the uncertainty of measurement is a predictable function of the measured quantity, such as the stable gases. That results for relatively simple measurement processes can be combined using relatively simple analysis and display methods does not ensure that similarly meaningful summaries can be devised for less well understood and controlled systems, but it provides the incentive to attempt to do so.

Measurements of total pH, pHT, in Harned cells are reported for 2-amino-2-hydroxymethyl-1,3-propanediol (Tris) buffers in artificial seawater (ASW) of salinity 35 at three molality ratios of Tris to Tris·HCl: 0.03:0.05, 0.04:0.04, and 0.05:0.03. The ASW formulation was derived from the 2008 International Association for the Physical Sciences of the Oceans (IAPSO) composition of seawater (Millero et al., 2008). The pHT value of each buffer was measured at 288.15 K, 298.15 K, and 308.15 K (15 °C, 25 °C, and 35 °C). Measurement uncertainties compliant with the ISO GUM (JCGM, 2008) are provided for each buffer. These new formulations extend the available data for Tris buffers in ASW to pHT values that bracket the pHT range of natural surface seawaters at 298.15 K. The data provide a framework for future multiple-point calibrations of spectrophotometric and potentiometric pHT measurements traceable to these new buffers.

The Method of Standard Additions or The Standard Addition Method, often referred to by its acronym as just SAM, is a proverbial workhorse in both inorganic and organic quantitative analytical chemistry and in related disciplines such as geochemistry. Its advantage in mitigating the effects of matrix interferences compared with the calibration curve approach is well known and is one of its major benefits. It is presented in virtually all standard textbooks on analytical chemistry in varying degrees of complexity. Yet the story of how it originated, and by whom, is not well known. It is generally believed that it originated in the early 1950s, introduced by spectroscopists. We have determined that the priority of its use and discovery apparently belongs exclusively to Hans Hohn (1906– 1978), a mining chemist, dating from the exposition of the method in his 1937 book on polarography, Chemische Analysen mit dem Polarographen. How the method became established in other disciplines quite different from polarography is not completely clear. It is likely that it was rediscovered independently in at least two separate disciplines, X-ray fluorescence (XRF) and atomic spectroscopy, almost simultaneously in 1953, 16 years after Hohn’s original description.

This paper discusses the basis and historical development of the traceability chain for pH. The quantity pH, first introduced in 1909, is among the most frequently measured analytical quantities. The practical measurement of the pH value of a sample is inexpensive, easy to perform, and yields a rapid result. However, the problems posed by the traceability of pH are not easy to solve. Most pH measurements are performed by potentiometry, using a glass electrode as the pH sensor. Such pH electrodes must be calibrated at regular intervals. Confidence in the reliability of pH measurements requires establishment of a metrological hierarchy including an uncertainty budget for calibration that links the pH measured in the sample to an internationally agreed and stated reference. For pH, this reference is the primary measurement of pH. A traceability chain can be established that links field measurements of pH to primary buffer solutions that are certified using this primary method. This allows the user in the field to estimate the measurement uncertainty of the measured pH data. As the realization of the primary measurement is sophisticated and time-consuming, primary standards are generally realized at national metrology institutes. A number of potentiometric methods are suitable for the determination of the pH of reference buffer solutions by comparison with the primary standard buffers. The choice between the methods should be made according to the uncertainty required for the application. For reference buffer solutions that have the same nominal composition as the primary standard, the differential potentiometric cell, often called the Baucke cell, is recommended.

A review of the state of the art of electrochemical methods at the highest metrology level in national metrology institutes (NMIs) is given, with emphasis on standardization work (primary methods) in the fields of pH and electrolytic conductivity, as well as use of coulometry. Attention is also given to certain technical issues in the implementation of these methods.

A concise graphical depiction, the “onion” chart, is described for presenting uncertainty budgets for any measurand, y, having a set of component variances, $$u_i^2 (y)$$ . The onion chart consists of a concentric series of rings. Each ring is constructed from the set of fractional contributions, $$u_i^2 (y)\, / \sum {u_i^2 (y)}$$ . The outer ring includes all $$u_i^2 (y)$$ and depicts the combined standard uncertainty, u c (y), where u c 2(y) ≡ ∑u i 2(y). Moving inward, each successive ring deletes the largest remaining $$u_i^2 (y)$$ . A “reduced” $$\sum {u_i^2 (y)}$$ is calculated for that ring from the remaining set of $$u_i^2 (y)$$ , with the fractional contributions calculated using the “reduced” $$\sum {u_i^2 (y)}$$ . This format facilitates rapid visual comparison of the importance of each component, with minor components becoming visible as the larger components are successively deleted. An example is presented using data from pH metrology, including the physical interpretation of the resulting diagram.

The National Institute of Standards and Technology (NIST) has developed several Standard Reference Materials (SRMs) based on human serum. NIST SRM 909b, Human Serum, is a lyophilized human serum material with concentrations for seven organic and six inorganic analytes at two levels certified solely by definitive methods (DMs). This material provides the vehicle by which high precision, high accuracy measurements made with DMs at NIST can be transferred through the measurement hierarchy to other laboratories. Isotope dilution gas chromatographic-mass spectrometric (GC-IDMS) methods were applied to measure cholesterol, creatinine, glucose, urea, uric acid, triglycerides, and total glycerides. Thermal ionization isotope dilution mass spectrometry (TI-IDMS) was used for determination of lithium, magnesium, potassium, calcium, and chloride. In addition, chloride was determined by coulometry, providing a comparison between two DMs. Sodium, which lacks a stable isotope that would permit isotope dilution mass spectrometric (IDMS) measurement, was determined by gravimetry. SRM 909b includes certified values for total glycerides and triglycerides, which were not certified in the previous lot of this material (SRM 909a). Improvement in uniformity of vial fill weight in the production of SRM 909b resulted in smaller certified uncertainties over previous freeze-dried serum SRMs. Uncertainties at the 99% level of confidence for relative expanded uncertainty (%) for certification of the organic analytes on a mmol/L/g basis ranged from 0.44% for urea (level II) to 5.04% for glucose (level II). (In-house studies have shown glucose to be a relatively unstable analyte in similar lyophilized serum materials, degrading at about 1% per year.) Relative expanded uncertainties (99% C.I.) for certification of inorganic analytes on a mmol/L/g basis ranged from 0.25% for chloride (level I) to 0.49% for magnesium (level II).

Results of CCQM-K99 key comparison on unknown phosphate buffer pH ∼ 7.5 at 5 °C, 15 °C, 25 °C, 37 °C and 50 °C are reported. Good agreement is found between the majority of participants. Main text To reach the main text of this paper, click on Final Report. Note that this text is that which appears in Appendix B of the BIPM key comparison database kcdb.bipm.org/. The final report has been peer-reviewed and approved for publication by the CCQM, according to the provisions of the CIPM Mutual Recognition Arrangement (CIPM MRA).

The CCQM-K48.2014 (CCQM-K114) key comparison for determination of the purity of potassium chloride was organized jointly by the inorganic analysis and electrochemical analysis working groups of CCQM to repeat a sub-set of comparisons to support CMC claims for all institutes in the relevant ranges with timely experimental data to measure the amount content of chloride in KCl. National Institute of Metrology P.R.China (NIM) acted as the coordinating laboratory of this comparison. Ten NMIs participated in this key comparison. All participants used coulometry and all of them analyzed the bromine impurity in the sample and made corrections. In general a very good overlap of results was observed and also the results for bromide are consistent with one exception. Main text To reach the main text of this paper, click on Final Report. Note that this text is that which appears in Appendix B of the BIPM key comparison database kcdb.bipm.org/. The final report has been peer-reviewed and approved for publication by the CCQM, according to the provisions of the CIPM Mutual Recognition Arrangement (CIPM MRA).

Automated constant-current coulometric acidimetry, based on Faraday's Laws, is uncertain to less than 1 part in 20 000 (relative standard deviation) and requires no chemical standardization. It is applicable to strong and weak acids and bases, with bases back-titrated after addition of excess strong acid. Initial setup and sample introduction are the sole manual steps. Assays of HCl, benzoic acid, Na 2 CO 3 , and tris(hydroxymethyl)aminomethane are presented. In the endpoint determination procedure, a generalized titration equation yields the theoretical charge remaining to the endpoint before each charge addition. The ratio of the experimental to the theoretical charge for the preceding aliquot corrects for experimental deviations.

As part of the analytical research leading to the certification of the new Apple and Peach Leaves Standard Reference Materials (SRMs), a study was undertaken to evaluate different sample dissolution techniques for losses of analyte species. Possible loss mechanisms include absorption or adsorption of analyte elements at the walls of the sample decomposition vessels, and the formation or persistence of insoluble particulate material during sample dissolution. Results of this study indicated that significant fractions of some elements were present on particles after acid dissolution, despite visual indications that dissolution was complete. In addition, large amounts of some elements remained in the platinum crucibles used to fuse samples with lithium metaborate.

The aim of the key comparison CCQM-K92 was to demonstrate the capabilities of the participating NMIs to measure electrolytic conductivity of an unknown sample. Two samples with nominal electrolytic conductivity values of 0.05 S m−1 and 20 S m−1 have been prepared for comparison. For the first time a conductivity value larger than those given in the IUPAC document [1] was measured in a CCQM comparison. Thus no calibration standards with similar conductivity value were available. The comparison was an activity of the Electrochemical Working Group (EAWG) of the CCQM and was coordinated by SMU. In the comparison NMIs from fifteen countries took part. The higher conductivity (20 S m−1) was measured by ten participants. Good agreement of the results was observed for the majority of participants. Main text. To reach the main text of this paper, click on Final Report. Note that this text is that which appears in Appendix B of the BIPM key comparison database kcdb.bipm.org/. The final report has been peer-reviewed and approved for publication by the CCQM, according to the provisions of the CIPM Mutual Recognition Arrangement (CIPM MRA).

Results of the CCQM-K91 key comparison on pH of an unknown phthalate buffer with a nominal pH value of pH ~ 4.01 at 25 ?C are reported. Measurements are performed at 15 ?C, 25 ?C and 37 ?C and optional also at 5 ?C and 50 ?C. Main text. To reach the main text of this paper, click on Final Report. Note that this text is that which appears in Appendix B of the BIPM key comparison database kcdb.bipm.org/. The final report has been peer-reviewed and approved for publication by the CCQM, according to the provisions of the CIPM Mutual Recognition Arrangement (CIPM MRA).

The key comparison CCQM-K96 was organized jointly by the inorganic and electrochemistry working groups of CCQM to test the abilities of the metrology institutes to measure the amount content of dichromate. Slovak Institute of Metrology with help of KRISS acted as the coordinating laboratories. Eight NMIs took part in the comparison. All participants used high accuracy constant current coulometry. Good agreement of the results was observed. Main text. To reach the main text of this paper, click on Final Report. Note that this text is that which appears in Appendix B of the BIPM key comparison database kcdb.bipm.org/. The final report has been peer-reviewed and approved for publication by the CCQM, according to the provisions of the CIPM Mutual Recognition Arrangement (CIPM MRA).

This key comparison (KC), CCQM-K73, was performed to demonstrate the capability of the participating National Metrology Institutes (NMIs) to measure the amount content of H+, νH+, in an HCl solution with a nominal νH+ of 0.1 mol kg−1. The comparison was a joint activity of the Electrochemical Working Group (EAWG) and Inorganic Analysis Working Group (IAWG) of the CCQM and was coordinated by NIST (USA) and CENAM (Mexico). The agreement of the results was not commensurate with the claimed uncertainties of the subset of participants that claimed small uncertainties for this determination. A workshop on technical issues relating to the CCQM-K73 measurements was conducted at the joint IAWG-EAWG meeting at the Bureau International des Poids et Mesures (BIPM), Paris (Sevres) in April 2010. Several possible sources of bias were investigated, but none could explain the observed dispersion among the participants' results. In the absence of a specific cause for the dispersion, the IAWG and EAWG decided to assign a Key Comparison Reference Value, KCRV, and standard uncertainty of the KCRV, uKCRV, based on the DerSimonian–Laird statistical estimator. The uKCRV is dominated by the between-laboratory scatter of results in CCQM-K73. The uncertainty estimates from the participants with the lowest reported uncertainties remain unsupported by this KC. Main text. To reach the main text of this paper, click on Final Report. Note that this text is that which appears in Appendix B of the BIPM key comparison database kcdb.bipm.org/. The final report has been peer-reviewed and approved for publication by the CCQM, according to the provisions of the CIPM Mutual Recognition Arrangement (CIPM MRA).

The CCQM-K48 key comparison for determination of the purity of potassium chloride was organized jointly by the inorganic analysis and electrochemical analysis working groups of CCQM to test the abilities of the metrology institutes to measure the amount content of chloride in KCl. National Institute of Metrology P.R.China (NIM) acted as the coordinating laboratory of this comparison with SMU and NIST as co-coordinating laboratories. Seven NMIs participated in this key comparison. All participants used coulometry (except NMIJ which used gravimetry) and all of them analyzed the impurity bromine in the sample and made corrections. In general very good agreement of the results was observed. Some possible problems were highlighted. Main text. To reach the main text of this paper, click on Final Report. Note that this text is that which appears in Appendix B of the BIPM key comparison database kcdb.bipm.org/. The final report has been peer-reviewed and approved for publication by the CCQM, according to the provisions of the CIPM Mutual Recognition Arrangement (MRA).

The key comparison CCQM-K20 was performed to demonstrate the capability of the participating national metrology institutes (NMIs) to measure the pH value of an unknown potassium tetroxalate buffer by the primary method. The buffer of nominal pH ~ 1.7 was measured at three temperatures: 15 °C, 25 °C, and 37 °C. The comparison was an activity of the Electrochemical Working Group (EAWG) of the CCQM and was coordinated by NIST (USA). All participants applied the primary method for pH. The result for the unknown buffer solution is the acidity function at zero chloride molality. Values for the key comparison reference value (KCRV) and its uncertainty, and the degrees of equivalence (Di) are reported. Good agreement of the results was observed for the majority of participants. Main text. To reach the main text of this paper, click on Final Report. Note that this text is that which appears in Appendix B of the BIPM key comparison database kcdb.bipm.org/. The final report has been peer-reviewed and approved for publication by the CCQM, according to the provisions of the CIPM Mutual Recognition Arrangement (MRA).

The key comparison CCQM-K18 was started in order to evaluate the equivalence of metrology institutes as a follow-up of the previous study CCQM-P52. As a sample a carbonate buffer solution was used with a composition slightly different to the usual one. There were 13 institutes participating in the comparison using a primary method for pH measurement. The fair agreement between the results reflects increased difficulty in measurement, where the buffer composition may slightly change during the measurement and extrapolation to the start time of measurement is usually necessary. Main text. To reach the main text of this paper, click on Final Report. Note that this text is that which appears in Appendix B of the BIPM key comparison database kcdb.bipm.org/. The final report has been peer-reviewed and approved for publication by the CCQM, according to the provisions of the CIPM Mutual Recognition Arrangement (MRA).

The key comparison CCQM-K34 was organized jointly by the inorganic and electrochemistry working groups of the CCQM as a follow-up to the pilot study CCQM-P36 to test the abilities of the metrology institutes to measure the amount content of acid in solid weak acids. The Slovak Institute of Metrology acted as the coordinating laboratory, and seven NMIs expressed interest in participation. All participants used constant current coulometry at different levels of sophistication. In general very good agreement of the results was observed; some possible problems were highlighted. Main text. To reach the main text of this paper, click on Final Report. Note that this text is that which appears in Appendix B of the BIPM key comparison database kcdb.bipm.org/. The final report has been peer-reviewed and approved for publication by the CCQM.

The second key comparison for the quantity pH, CCQM-K17, was carried out to assess the degree of equivalence of the national primary measurement procedures used to determine the pH of primary standard buffer solutions. The CCQM-K17 comparison allows estimations of the capability of ten national metrology institutes (NMIs) to determine the pH of a phthalate buffer of unknown concentration at different temperatures. The key comparison was co-ordinated by the CCQM Working Group on Electrochemical Analysis and piloted by the Physikalisch-Technische Bundesanstalt (PTB) with assistance from the National Institute of Standards and Technology (NIST) and the Slovak Institute of Metrology (SMU). The measurement results of the NMIs agreed not at the same level as for the first key comparison on pH CCQM-K9. The results obtained by seven of the eleven participants agree very well within the uncertainty stated by the participants. Four participants obtained results somewhat lower. The two groups of results are most obviously evident at a measurement temperature of 37 °C. Main text. To reach the main text of this paper, click on Final Report. Note that this text is that which appears in Appendix B of the BIPM key comparison database kcdb.bipm.org/. The final report has been peer-reviewed and approved for publication by the CCQM, according to the provisions of the Mutual Recognition Arrangement (MRA).

A gravimetric method for the determination of sulfate in a sulfate solution standard by the precipitation of barium sulfate is coupled with the instrumental determination of trace sulfate and precipitate contaminants to improve the accuracy and precision of the analysis. Sulfate in a solution of potassium sulfate is separated by a reverse precipitation with barium chloride in very dilute hydrochloric acid. Coulometry, ICP-MS, and flame atomic emission spectrometry (FAES) are used to quantify the level of the contamination in the barium sulfate precipitate and the solubility loss of sulfate in the filtrate, from which correction factors are calculated. Coprecipitating contaminants contribute about 0.3% to the total precipitate mass, while the analyte lost to the filtrate contributes 0.4%. Despite the poorer precision and accuracy of instrumental methods, the over-all precision and accuracy of the sulfate determination is actually improved, since the instrumental methods are used to determine only a very small part of the analyte. The expanded uncertainty (k= 2) of the method is below 0.2% relative to the precipitate mass.

All analytical techniques depend on the use of calibration chemicals to relate analyte concentration to an instrumental parameter. A fundamental component in the preparation of calibration solutions is the weighing of a pure chemical or metal before preparing a solution standard. The analyst must be assured that the purity, stoichiometry, and assay of the chemical are known. These terms have different meanings, and each has an important influence. This report is intended to assist the analyst in the selection and use of chemical standards for instrumental calibration. Purity, stoichiometry, and preparation of solutions for different purposes are discussed, and a critical evaluation of the best materials available for each element is presented for use in preparing solutions or calibration standards. Information on the chemical form, source, purity, drying, and appropriate precautions is given. In some cases, multiple sources or chemical forms are available. Certain radioactive elements, the transuranic elements, and the noble gases are not considered.

The application of vibrating wire electrodes for amperometric detection in flow-injection systems is reported. Equations of response for these electrodes in a flow-through cell are derived and verified experimentally. The applicability of the equation for other convective detectors in flow-through cells is discussed. A detection limit of 5 × 10−10 M iodide and a linear dynamic range of 6.3 decades were obtained for a platinum vibrating wire electrode.

Les epaisseurs des couches de diffusion sont determinees sans connaitre l'aire de l'electrode par comparaison des courants obtenus par chronoamperometrie et voltammetrie hydrodynamique. Donnees des resultats relatifs aux electrodes a disque tournant et a fil-vibrant

Indium(III) is used as an internal standard for the determination of cadmium, copper and lead at the 20 ng g−1 level by using differential-pulse anodic stripping voltammetry; the supporting electrolyte is 1.0 mol l−1 ammonium bromide/0.25 mol l−1 nitric acid. For each solution, each stripping peak of interest is normalized to the corresponding peak height obtained in the same voltammogram for a known, added concentration of indium(III). A calibration curve is prepared for each element by using these normalized peak heights. The technique is demonstrated for NBS SRM 1643b (Trace Elements in Water). The relative standard deviations for six independent determinations of Cd, Cu, and Pb at the 20 ng g−1 level are 1.9%, 5.4%, and 1.2%, respectively. The imprecision for copper is limited by the sloping baseline at its stripping potential. The detection limit for each element is less than 1 ng g−1.

A triangular waveform generator is described which can be used for cyclic voltamraetry at moderate scan rates (

A brief review is given of the literature of vibrating electrodes. These electrodes are characterized by high sensitivity; however, the boundary-value problem describing mass transport has not been solved in closed form. The design and construction of a wire microelectrode is described which vibrates in a reciprocating fashion perpendicular to the axis of the wire. The relationship of the transport-limited electrode current to the vibrational Reynolds number was investigated for frequencies up to 510 Hz and amplitudes up to 3 mm. The exponential dependence of I 1 on Re v was determined to be approx. 0.62 for one electrode with significant deviations noted for various electrodes.

An improved procedure for the determination of iodide using ion chromatography with electrochemical detection has been developed. The advantages of the good selectivity of ion chromatography and the highly sensitive response of iodide at a Pt electrode have been combined to establish a new method with enhanced detection limits and improved linearity. The use of an eluent consisting of nitric acid and sodium nitrate has eliminated the severe tailing of iodide. The detection limit has been estimated to be 10 ng/mL. The effect of other anions in solution has been determined. The method has been applied to the determination of iodide in cadmium sulfide, potassium fluoride, and iodized sodium chloride.

Residual organic species in nitric acid digest of freeze-dried bovine liver (NBS SRM 1577a) have been identified by use of voltammetry, liquid chromatography, spectrophotometry, and classical chemical tests. Data from these techniques show that major products of microwave-assisted dissolution by nitric acid include o-, m-, and p-nitrobenzoic acids (NBA). In addition to these compounds, other organic species present in these digests irreversibly complex copper, but not zinc, and result in low values for copper by polarography. The NBAs and these other organic species are all eliminated by refluxing the nitric acid digest in perchloric acid at atmospheric pressure. Polarographic results obtained for copper following treatment with perchloric acid agree with the certified value. The use of voltammetry in the evaluation of wet ashing procedures is discussed.