The terms “tochnost’” and “pravil’nost’” as applied to the results of chemical analysis

In this paper, attention is given to the inadequacy of the Russian translation of the term “accuracy” as “pravil’nost’” in publications of the IUPAC recommendations for the presentation of the results of chemical analysis and vocabularies of analytical terms. The essence of the concepts tochnost’ and pravil’nost’ in Russian measurement terminology is considered in historical context, and the evolution of the concepts accuracy in the terminological standards on analytical chemistry (IUPAC Recommendations), metrology, and statistics is traced. It is demonstrated that the one-to-one correspondence between the terms “accuracy” and “tochnost’” and “trueness” and “pravil’nost’,” respectively, which occurs in the present-day standardized terminologies, should also be followed in analytical chemistry     

Do we really need to account for run bias when producing analytical results with stated uncertainty? Comment on 'Treatment of bias in estimating measurement uncertainty' by G. E. O'Donnell and D. B. Hibbert

Treatment of bias is an important issue relating to analytical quality. Recently, G. E. O'Donnell and D. B. Hibbert (Analyst, 2005, 130, 721) recommended to always correct analytical results for 'run bias' determined by a single analysis of a certified reference material (CRM) in each analytical run. In the authors' opinion, this is necessary for the results obtained to be comparable from run to run. It is argued here that such a recommendation is logically inconsistent and stems from misinterpretation of measurement uncertainty as being estimated under repeatability conditions. The fundamental principle underlying the measurement uncertainty methodology is that all relevant sources of error should be taken into account, which results in overall uncertainty assessment and thus provides a means for a global comparability of measurement and test results. The local, i.e. run-to-run, comparability is not a factor if analytical results are interpreted on the basis of their associated uncertainty.     

Evaluating uncertainty in analytical measurements: the pursuit of correctness

 Simple in principle, the evaluation of uncertainty, especially in chemical analysis, is not a routine task and needs great care to be correct. This can be seen, particularly, from an examination of the EURACHEM Guide, Quantifying Uncertainty in Analytical Measurement (1995), which is the most important document on the subject. The examination reveals, in the author's opinion, a shortage of correctness in some principal details of the uncertainty estimation process as presented in worked examples in the Guide, and the author has therefore formulated some "in pursuit of correctness" rules for estimating uncertainty. The rules and respective comments are concerned with the following items: (1) choosing an appropriate distribution function in type B evaluation of uncertainty, (2) the necessity for consideration of separate contributions to the combined uncertainty, and (3) taking account of actual influence factors in the uncertainty estimation process. Furthermore, the problem of estimation of conditional versus overall uncertainty is touched upon in connection with comparative trials where only internal consistency of results is required. Received: 29 January 1998 · Accepted: 10 February 1998     

Molecular Structures and Photophysical Properties of Dirhodium Fluorophosphine Complexes

The excited state properties of a series of singly bonded dirhodium compounds, consisting of Rh(0)(2), Rh(0)Rh(II)X(2), and Rh(II)(2)X(4) (X = Cl and Br) cores coordinated by three bis(difluorophosphino)methylamine ligands, have been investigated. The newly synthesized complexes with X = Br have been structurally characterized. The mixed-valence complex Rh(2)[&mgr;-CH(3)N(PF(2))(2)](3)Br(2)[(PF(2))CH(3)N(PF(2))] crystallizes in the orthorhombic space group P2(1)2(1)2(1) with a = 13.868(7) ?, b = 16.090(5) ?, c = 11.614(5) ?, V = 1591(3) ?(3), and Z = 4; the structure was refined to values of R = 0.052 and R(w) = 0.062. Orange crystals of Rh(2)[&mgr;-CH(3)N(PF(2))(2)](3)Br(4) are monoclinic with a C2/c space group: a = 14.62(6) ?, b = 12.20(2) ?, c = 14.33(1) ?; beta = 106.0(2) degrees; V = 2457(11) ?(3); Z = 4; and R = 0.058 and R(w) = 0.056. Crystalline solids and low-temperature glasses of each member of the chloride and bromide series exhibit long-lived red luminescence. Excitation profiles and temperature dependencies of the emission bandwidths and lifetimes for all complexes are characteristic of luminescence originating from a dsigma excited state. Efficient nonradiative decay is observed upon the thermal population of an excited state proximate to the lowest energy emissive excited state of these complexes. The nonradiative decay rate constant of the upper excited state is 10(2)-10(3) and 10(3)-10(4) greater than that of the emissive excited state for complexes with X = Cl and Br, respectively.     

Secondary pH standards and their uncertainty in the context of the problem of two pH scales

Establishing a traceability route with all measurement and uncertainty relationships determined is an important aspect of traceability, and seems to be particularly striking in pH measurement. In this paper the issue of evaluation of secondary pH standards measured with reference to a primary standard in a differential cell with free diffusion type liquid junctions is considered. Relatively high uncertainty, U=0.015, has been assigned to such standards in the recent IUPAC Recommendations on pH (2001), because of a specific residual liquid-junction potential treated statistically as a contribution to the combined uncertainty. Close inspection of the problem leads to the conclusion that a correction for the residual liquid-junction potential should be applied to the measured value of a secondary pH standard. This can be considered as a correction for a known systematic effect on the traceability route. With available experimental data it is demonstrated that such a correction can reasonably be made for well-studied standard buffer systems. In this way the uncertainty associated with secondary pH standards is kept to a low level, and, what is more, the problem of two pH scales, a multi-standard scale and a single-standard scale, gains a proper solution. The need for different treatment of residual liquid-junction potentials at different levels in the measurement hierarchy is noted. Much attention is also given to rational categorization of pH standards in the hierarchy.     

Measurement uncertainty and chemical analysis

The author considers issues covered by A.B. Blank in his letter “Uncertainty in Measurements and Chemical Analysis” published in Zhurnal Analiticheskoi Khimii in 2005 (vol. 60, no. 12, p. 1316). It is noted that the negative attitude towards the introduction of the measurement uncertainty concept is to a great extent due to the incorrect translation of the term “uncertainty” as “pogreshnost’” (error) in the guides published in Russian earlier. It is shown that the estimation of uncertainty, which combines both random and so-called “systematic” error components, is consistent with the structure of the error of measurements (determinations) made by following specified procedures. Reporting of the results together with their uncertainties is necessary for the proper interpretation of the experimental data; this reflects current trends in the analytical quality assurance based on metrological principles.     

Evaluation of the residual liquid junction potential contribution to the uncertainty in pH measurement: A case study on low ionic strength natural waters

The residual liquid junction potential (RLJP) needs to be accounted for in pH uncertainty estimation. Attempts to do this and the currently available methods for evaluating the RLJP are critically discussed and their weak sides are pointed out. In this work an empirical approach to the problem is proposed. It is based on the use of the RLJP bias estimated on a variety of measurement conditions for a specific class of analytical objects essentially differing in ionic strength from the pH calibration buffers. The data from five independent studies, including interlaboratory comparisons, on pH measurement in low ionic strength waters were used to find the overall bias observed in the 10⁻⁴ mol dm⁻³ strong acid solution. The procedure includes quantifying the uncertainty of bias values from separate studies by combination of the relevant uncertainty components and testing the consistency of the data. The weighted mean bias in pH was found to be 0.043 ± 0.007 (k = 2). With this estimate, the pH measurement uncertainties calculated according to the previously suggested procedure (I. Leito, L. Strauss, E. Koort, V. Pihl, Accredit. Qual. Assur. 7 (2002) 242-249.) can be enlarged to take the uncorrected bias into account. The resulting uncertainties on the level of 0.10-0.14 (k = 2) are obtained in this way for pH measurement in water and poorly buffered aqueous solutions in the range of pH 7.5-3.5.