Kenneth W. Pratt

Dissolution problems with botanical reference materials

SummaryAs 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.

Major applications of electrochemical techniques at national metrology institutes

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.

Origin and early history of Die Methode des Eichzusatzes or The Method of Standard Addition with primary emphasis on its origin, early design, dissemination, and usage of terms

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.

Automated, high-precision coulometric titrimetry part II. Strong and weak acids and bases

Abstract Automated constant-current coulometric acidimetry, based on Faradays 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.

Automated, high-precision coulometric titrimetry part I. Engineering and implementation

Abstract Automated constant-current coulometry, based on Faradays achieves uncertainties (relative standard deviation) of less than 1 part in 20 000 without chemical standardization. It is applicable to acid-base, redox, and precipitation titrations of high-purity compounds and solutions. Automation of the technique permits unsupervised operation and reduces operator-dependent errors. Initial setup and sample introduction are the sole manual steps. Each assay consists of a main titration at high, constant current, bracketed by the initial and final endpoint routines, each at a lower current. The coulometric assay is analogous to a conventional titration in which two different concentrations of the titrant are used to attain optimum accuracy. The initial endpoint determination corresponds to the blank determination in a classical titration includes a statistical analysis of the random and systematic uncertainties associated with the analysis. Individual steps in the procedure are performed by a hierarchical series of subroutines to reduce program complexity. Results are presented for K2Cr2O7, benzoic acid, and solutions of strong acids.

High-accuracy differential-pulse anodic stripping voltammetry with indium as an internal standard

In(III) is employed as an internal standard for the determination of Cd, Cu, and Pb at the 20 micrograms/g level using differential pulse anodic stripping voltammetry. A multi-point calibration curve is prepared for each element using these normalized peak heights. The technique is demonstrated using NBS Standard Reference Material 1643b.

Final report on CCQM-K19.1: pH of borate buffer

The subsequent key comparison CCQM-K19.1 was carried out in 2010 as a follow-up comparison to CCQM-K19. Three laboratories (NMIJ, PTB and SMU) took part in both comparisons. Their results are consistent. The assigned uncertainties are reliable and of the same order. These results provide the link to CCQM-K19. As well as the original comparison the subsequent key comparison was organized to demonstrate the capability of the interested national metrology institutes to measure the pH value of an unknown borate buffer by a primary method at 15 °C, 25 °C and 37 °C. Eight laboratories took part in CCQM-K19.1. The participants either were not able to participate in the original comparison or participate only in the Pilot Study CCQM-P82 running in parallel to CCQM-K19. In one case the laboratory expressed doubts on the integrity of the sample provided in CCQM-K19. The result in CCQM-K19.1 is the acidity function at zero chloride molality of the unknown borate buffer solution. Most participants in the key comparison CCQM-K19.1 demonstrated improved capabilities. This especially applies to the laboratories that had originally taken part in the pilot study CCQM-P82. In some cases previous results could be confirmed. The hidden reasons for that have still to be clarified. 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).

Final report on key comparison CCQM-K20: pH of tetroxalate buffer

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).

Final report on CCQM-K91: Key comparison on pH of an unknown phthalate buffer

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).

Final report on key comparison CCQM-K73: Amount content of H+ in hydrochloric acid (0.1 mol kg−1)

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 . A parallel Pilot Study, CCQM-P19.2, was performed for NMIs that did not desire to participate in the KC. 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 method of determination of νH + was left to the individual participant. All participants used either coulometry or titrimetry with potentiometric determination of the endpoint. 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 IAWGEAWG 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.