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This paper considers the drop-time technique in electrocapillary measurements and focuses on calibration methods required to convert the drop-time t of a mercury electrode into the value of the interfacial tension γ. The relationships between t and γ, described in the literature, are reviewed and six calibration methods identified. On the basis of high-precision drop-time electrocapillary measurements carried out in 0.1 mol l−1 KCl aqueous solution, with γ values known from the absolute measurement method, the performance of the calibration methods, expressed in terms of measurement bias and uncertainty, is evaluated and compared. Better performance is observed for the methods that involve a nonlinear model such as γ=αtβ fitted to calibration data, instead of a single-point linear calibration commonly used. The measurement uncertainty U ⩽ 0.12 mN m−1 evaluated with these advanced calibration approaches corresponds to the level of accuracy achieved with the classical capillary electrometer.

In the paper commented upon, the electrocapillary curves for mercury in ionic liquids, obtained by static (pendant drop) and dynamic (drop-time) measurement methods, were compared to support the inference about ultraslow relaxation at the interface. Yet, the static, i.e. equilibrium interfacial tensions proved to be more in magnitude (at E

The rational strategy in the evaluation of analytical measurement uncertainty is to combine the “whole method” performance data, such as precision and recovery, with the uncertainty contributions from sources not adequately covered by those data. This paper highlights some common mistakes in evaluating the uncertainty when pursuing that strategy, as revealed in current chromatographic literature. The list of the uncertainty components usually taken into account is discussed first and fallacies with the LOD- and recovery uncertainties are noted. Close attention is paid to the uncertainty arising from a linear calibration normally used. It is demonstrated that following a well-known formula for the standard deviation of an analytical result obtained from a straight line calibration leads to double counting the precision contribution to the uncertainty budget. Furthermore, the precision component itself is often estimated improperly, based on the number of replicates taken from the precision assessment experiment. As a result, the relative uncertainty from linear calibration is overestimated in the budget and may become the largest contribution to the combined uncertainty, which is clearly shown with an example calculation based on the literature data.

Although relatively small compared to other sources of uncertainty in an analytical procedure, uncertainty in volumetric operations needs to be properly evaluated. In this paper, the problem of volumetric uncertainty is addressed with the critical examination of the procedure for its evaluation recommended in the EURACHEM Guide Quantifying Uncertainty in Analytical Measurement. Some characteristic features of volumetric apparatus as a measuring device are considered in relation to accuracy specification usually expressed in the form of "capacity tolerances". On the basis of the underlying metrology, written standards to which volumetric ware is manufactured, and evidences available from experimental studies, it is shown that the tolerance already includes a random error inherent in the use of volumetric apparatus. Therefore, no additional allowance, except for temperature effects, needs be made if the uncertainty is derived from the tolerance. A detailed analysis of relevant uncertainty sources is presented, with two different procedures for evaluating the uncertainty identified; one of them relies on the prescribed tolerance while the other is based on the experimental estimation of the actual performance in the user's hand. The uncertainty budget for each of these two approaches is evaluated, analysed and illustrated with a numerical example.

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.

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.