The purity P of laboratory chemicals is often declared in the form P > or = xy% (e.g., P > or = 97%). With a randomly chosen set of 40 compounds we found that their purity is generally closer to 100% than to the lower limit. The distribution of the purity data as found in the laboratory depends on the analytical technique used. Whereas purities determined by chromatography do not exceed 100% (because the sum of all observed peak areas is set to 100%), the purities obtained by titration can exceed 100% (because the functionality of the compound is measured). Therefore, the data for these two groups need to be dealt with in different ways. For purities based on titration we propose to use a rectangular distribution with a range from Pmin to 101%, an expected purity value which is the mean and a standard uncertainty of the purity u(P) of 29% of the range. Purities determined by chromatography can be described with a triangular distribution (ramp function). One leg of the triangle represents the range from Pmin to 100% and the right-angle is located at 100%. The expected value is the median and the uncertainty u(P) is 24% of the range. These proposals match the experimental data well.
"A general problem for many commercially available chemicals, is the fact that their purity is often only stated as a minimum guaranteed purity (e.g., purity ≥97%) or as an indicative purity value (e.g., purity ∼99%). Usually, little or even no information is given regarding the uncertainty of the stated purity value and the methods used to determine the assigned purity value . If a calibration solution is prepared from a chemical with an inaccurate or insufficiently stated purity value, one cannot use the solution to establish metrological traceability to the SI. "
[Show abstract][Hide abstract] ABSTRACT: In this study the validity and suitability of differential scanning calorimetry (DSC) to determine the purity of selected polycyclic aromatic hydrocarbons and chloramphenicol has been investigated. The study materials were two candidate certified reference materials (CRMs), 6-methylchrysene and benzo[a]pyrene, and two different batches of commercially available highly pure chloramphenicol. The DSC results were compared with those obtained by other methods, namely gas and liquid chromatography with mass spectrometric detection, liquid chromatography with diode array detection, and quantitative nuclear magnetic resonance. The purity results obtained by these different analytical methods confirm the well-known challenges of comparing results of different method-defined measurands. In comparison with other methods, DSC has a much narrower working range. This limits the applicability of DSC as purity determination method, for instance during the assignment of the purity value of a CRM. Nevertheless, this study showed that DSC can be a powerful technique to detect impurities that are structurally very similar to the main purity component. From this point of view, and because of its good repeatability, DSC can be considered as a valuable technique to investigate the homogeneity and stability of candidate purity CRMs.
[Show abstract][Hide abstract] ABSTRACT: Lead is an important constituent for the preparation of wide variety of glasses like high refractive index optical glasses, radiation shielding glasses, ceramic glazes, enamels, high electrical resistance glasses, glass soldiers and sealants, etc. Determination of exact quantity of lead is therefore very essential to obtain the desired property of different glasses. With a view to meet the necessity, the measurement uncertainty of the results of lead determination in different lead containing glasses have been evaluated. The lead content has been determined complexometrically at pH 4.3 using di-sodium salt of EDTA. The sources of uncertainty of the results of measurement have been identified as contributions from repeatability, standardization of EDTA, volume measurement by volumetric flask, burette, pipette and end point detection. Sources of uncertainty have been identified and combined following the EURACHEM guidelines. The results show that the major sources of uncertainty arise from standardization and end point detection.
MAPAN-Journal of Metrology Society of India 09/2012; 27(3). DOI:10.1007/s12647-012-0027-8 · 0.79 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Ishikawa, or cause-and-effect diagrams, help to visualize the parameters that influence a chromatographic analysis. Therefore, they facilitate the set up of the uncertainty budget of the analysis, which can then be expressed in mathematical form. If the uncertainty is calculated as the Gaussian sum of all uncertainty parameters, it is necessary to quantitate them all, a task that is usually not practical. The other possible approach is to use the intermediate precision as a base for the uncertainty calculation. In this case, it is at least necessary to consider the uncertainty of the purity of the reference material in addition to the precision data. The Ishikawa diagram is then very simple, and so is the uncertainty calculation. This advantage is given by the loss of information about the parameters that influence the measurement uncertainty.
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