Publication Type Journal Article
Title Quantification of the uncertainty of the visual detection of the end-point of a titration: Determination of total hardness in water
Authors Diogo Ferreira Miguel Barros Cristina M. Oliveira Ricardo Bettencourt da Silva
Groups Chem4Env
Journal MICROCHEMICAL JOURNAL
Year 2019
Month May
Volume 146
Number
Pages 856-863
Abstract For most titrations based on the visual detection of the end-point this operation is an important uncertainty component. Well-known models are available to estimate the uncertainty of gravimetric and volumetric steps involved in these titrations, but how the uncertainty from end-point detection should be estimated is not harmonized yet. This work presents a methodology to quantify the uncertainty from the detection of the end-point of a titration when a blank test is performed to subtract the excess of volume used to obtain the colour change that identifies the end of titration. This methodology was applied to the determination of water total hardness by EDTA titration. The repeatability of the end-point detection was estimated by the difference between repeatability obtained from replicate titrations of the same solution and the combined repeatability of the measurements of the analytical portion and titrant volumes. It was assessed if determinations are affected by relevant systematic effects from the replicate analysis of a standard solution. This methodology allowed the determination of the percentage contribution of all uncertainty components to the measurement uncertainty, including the detection of the end-point of the titration, necessary for the optimization of the uncertainty and cost of the analysis. The uncertainty components where combined using three methods to assess their applicability to this evaluation: the law of propagation of the uncertainty, the numerical Kragten method and the numerical Monte Carlo Method. Since the assumption of the first two methods is valid, the three methodologies for combining uncertainty components produced equivalent results. The end-point detection is responsible for 2\% to 22\% of the global uncertainty and the determinations of water total hardness are associated with an expanded relative uncertainty ranging from 1.5\% to 18\% that is fit for the monitoring of drinking waters taking performance requirements of the legislation.
DOI http://dx.doi.org/10.1016/j.microc.2019.01.069
ISBN
Publisher
Book Title
ISSN 0026-265X
EISSN 1095-9149
Conference Name
Bibtex ID ISI:000463132100110
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