Evaluation and validation of detailed and simplified models of the uncertainty of unified pHabsH2O measurements in aqueous solutions

Highlights • First detailed evaluation of the uncertainty of pHabsH2O measurements. • Bottom-up uncertainty evaluations proven valid for 95% confidence. • Monte Carlo Simulation of pHabsH2O measurement ladder with least-squares minimisation. • Described simplified and detailed bottom-up uncertainty evaluations are equivalent. • Measurements from 2 to 10 pHabsH2O with a 95% expanded uncertainty of 0.26–0.51.The use of the unified pH concept, pHabsH2O, applicable to aqueous and non-aqueous solutions, which allows interpreting and comparison of the acidity of different types of solutions, requires reliable and objective determination. The pHabsH2O can be determined by a single differential potentiometry measurement referenced to an aqueous reference buffer or by a ladder of differential potentiometric measurements that allows minimisation of inconsistencies of various determinations. This work describes and assesses bottom-up evaluations of the uncertainty of these measurements, where uncertainty components are combined by the Monte Carlo Method (MCM) or Taylor Series Approximation (TSM). The MCM allows a detailed simulation of the measurements, including an iterative process involving in minimising ladder deviations. On the other hand, the TSM requires the approximate determination of minimisation uncertainty. The uncertainty evaluation was successfully applied to measuring aqueous buffers with pH of 2.00, 4.00, 7.00, and 10.00, with a standard uncertainty of 0.01. The reference and estimated values from both approaches are metrologically compatible for a 95% confidence level even when a negligible contribution of liquid junction potential uncertainty is assumed. The MCM estimated pH values with an expanded uncertainty, for the 95% confidence level, between 0.26 and 0.51, depending on the pH value and ladder inconsistencies. The minimisation uncertainty is negligible or responsible for up to 87% of the measurement uncertainty. The TSM quantified measurement uncertainties on average only 0.05 units larger than the MCM estimated ones. Additional experimental tests should be performed to test these uncertainty models for analysis performed in other laboratories and on non-aqueous solutions

Desafios Químicos e Metrológicos da Alcalinidade Oceânica

Pequenas variações na composição dos sistemas oceânicos são passiveis de discriminação dependendo dos métodos e procedimentos analíticos adoptados e da expressão da incerteza associada aos valores dos resultados experimentais. O estudo das alterações químicas nos oceanos e o seu impacto na acidificação oceânica – consequência do aumento das concentrações de dióxido de carbono na atmosfera seguida da sua dissolução na agua do mar – torna o sistema carbonato num dos sistemas mais importantes, mas também dos mais complexos nos oceanos. A sua caracterização, esta dependente do conhecimento de quatro variáveis principais: pH, alcalinidade, pressão parcial de CO2 (pCO2) e Carbono Inorgânico Dissolvido (DIC). O conhecimento de duas destas quatro variáveis possibilita a determinação das restantes duas. A acidez livre, expressa por pH, e assim um parâmetro importante para a caracterização da agua do mar, afectando praticamente todos os processos biogeoquímicos no oceano. A determinação de pH em amostras de agua do mar – uma matriz complexa com elevada forca iónica, I ≈ 0.67 mol kg-1, apresenta limitações, na medida em que as soluções tampão de calibração utilizadas para a medição são, em geral, as de uso comum que correspondem a forca iónica inferior a 0.10 mol kg-1. Deste modo, tornou-se fundamental a obtenção de soluções tampão apropriadas a esta matriz. Parte do presente trabalho incidiu no estudo de soluções de complexidade crescente aproximando-nos da matriz agua do mar, no estudo das interaccoes entre as espécies químicas presentes que contribuem para o coeficiente de actividade da espécie H+, na atribuição de valores primários de pH a soluções-tampão nesta matriz para referência de calibração dos equipamentos de medida e posterior implementação de metodologias de ensaio para analise de amostras em situações de rotina, com incerteza associada. A alcalinidade total e as suas componentes, bem como as constantes de equilíbrio de ionização do acido carbónico foram alvo de estudo para permitir o amplo conhecimento do sistema carbonato. A composição química da matriz agua do mar foi avaliada por Cromatografia Iónica, possibilitando a caracterização de amostras da Costa Portuguesa.Small changes in the composition of ocean systems are susceptible of differentiation depending on the analytical methods and procedures adopted and the uncertainty associated to the values of the experimental results. The study of chemical changes in the oceans and their impact on ocean acidification – a consequence of the increase of carbon dioxide concentrations in the atmosphere followed by its dissolution in seawater – makes the carbonate system one of the most important systems, but also the most complex in the oceans. Its characterization is dependent on the knowledge of four key parameters: pH, alkalinity, partial pressure of CO2 (pCO2) and Dissolved Inorganic Carbon (DIC). The knowledge of two of these four variables allows the determination of the remaining two. The free acidity, expressed by pH, is thus an important parameter for the characterization of seawater, affecting practically all the biogeochemical processes in the ocean. The determination of pH in seawater samples – a complex matrix with a high ionic strength, I ≈ 0.67 mol kg-1, presents limitations, since the standard buffer solutions used for the calibration of the pH meters are generally those of common use, which correspond to ionic strength less than 0.10 mol kg-1. Hence, it becomes essential to obtain buffer solutions matching this matrix. Part of the present work has its focus on the study of solutions of increasing complexity approaching the seawater matrix, on the study of interactions between the chemical species that contribute to the activity coefficient of the H + species and on the assignment of primary pH values to standard buffer solutions in this matrix, for further calibration of pH meters and subsequent implementation of tested procedures for the analysis of seawater samples in routine conditions, with respective uncertainty evaluation associated to the results. Total alkalinity and its components, as well as the ionic equilibrium constants of carbonic acid, were studied to allow a broad knowledge of the carbonate system. The chemical composition of the seawater matrix was evaluated by Ion Chromatography, allowing the characterization of samples from the Portuguese Coast

Implementação da Norma ISO/IEC 17025:2005 no contexto de alguns métodos físico-químicos

Tese de mestrado, Química (Química Analítica), Universidade de Lisboa, Faculdade de Ciências, 2009A garantia da qualidade depende da competência demonstrada pelo laboratório para produzir dados e resultados tecnicamente válidos. Actualmente, um dos principais objectivos do laboratório da Solvay Portugal é o reconhecimento da qualidade dos resultados que emite aos seus clientes. Para tal, pretende a sua acreditação segundo a norma EN ISO/IEC 17025:2005, nos parâmetros contemplados na Licença Ambiental LA 161/2008 do efluente industrial geral da fábrica, E2. O presente trabalho foi desenvolvido no laboratório da Solvay Portugal, e teve como objectivos a validação dos métodos, segundo as normas correspondentes, e cálculo de incertezas associadas aos valores dos seguintes parâmetros: Sólidos Suspensos Totais, pH, Crómio Hexavalente, Crómio Total, Azoto Amoniacal, Carência Química de Oxigénio. No desenvolvimento do trabalho foram utilizadas as seguintes técnicas: Gravimetria, Potenciometria, Espectrometria de Absorção Molecular, Titulimetria e Espectrometria de Emissão Atómica com plasma acoplado induzido, ICP-AES.Quality assurance depends on the laboratory demonstration of competence to produce technically valid data and results. Nowadays, one of the main objectives of the laboratory of Solvay Portugal is the recognition of the quality of the results it delivers to its clients. In the frame of this objective one of the targets is to obtain accreditation according to EN ISO / IEC 17025:2005, on the parameters pertaining the Environmental Agreement LA 161/2008, on the general industrial effluent from factory, E2. This work was developed in the laboratory of Solvay Portugal, and had as objective the validation of the methods, according to the corresponding norms, and calculations of uncertainty of the results of the following parameters: Total Suspended Solids, pH, Hexavalent Chromium, Total Chromium, Ammoniacal Nitrogen, Chemical Oxygen Demand In the development of this work, the following techniques were used: Gravimetry, Potentiometry, Molecular Absorption Spectrometry, Titrimetry and Induced Coupled Plasma Atomic Emission Spectrometry, ICP-AES

Traceability of pH to the Mole

Free acidity of aqueous solutions was initially defined in 1909 by Søren Peter Lauritz Sørensen as pH = −lgcH+ (c/mol·dm−3 or m/mol·kg−1 of the free hydrogen ions in solution, H+) soon (1910) was changed to pH = paH+ = −lgaH+, integrating the new concepts of activity, ai and activity coefficient γi, for the ionic species i under concern, H+ in this case; it is ai = −lg(miγi). Since individual ions do not exist alone in solution, primary pH values cannot be assigned solely by experimental measurements, requiring extra thermodynamic model assumptions for the activity coefficient, γH+, which has put pH in a unique situation of not being fully traceable to the International System of Units (SI). Also the concept of activity is often not felt to be as perceptible as that of concentration which may present difficulties, namely with the interpretation of data. pH measurements on unknown samples rely on calibration of the measuring setup with adequate reference pH buffers. In this work, the assignment of pH values to buffers closely matching the samples, e.g., seawater, is revisited. An approach is presented to assess the quantity pmH+ = −lgmH+ profiting from the fact that, contrary to single ion activity coefficients, mean activity coefficients,   can be assessed based on experimentally assessed quantities alone, γExp ±, thus ensuring traceability to the mole, the SI base unit for amount of substance. Compatibility between γExp ± and mean activity coefficient calculated by means of Pitzer model equations, γPtz ±, validates the model for its intended use

Toward Unified pH of Saline Solutions

Fluctuations of pH in coastal systems are generally surveyed through potentiometric pH measurements. A new concept of a unified pH scale was introduced with the great advantage of enabling comparability of absolute values, pHabs, pertaining to any medium. Using water as an anchor solvent, yielding pHabsH2O, enables referencing the pHabs values to the conventional aqueous pH scale. The current work aims at contributing to implement pHabsH2O to saline solutions. To this purpose, differential potentiometric measurements, with a salt bridge of ionic liquid [N2225][NTf2], were carried out aiming at overcoming problems related to residual liquid junction potentials that affect the quality of such measurements. The ability to measure pHabsH2O with acceptable uncertainty was evaluated using Tris-Tris·HCl standard buffer solutions prepared in a background matrix close to the characteristics of estuarine systems (salinity of 20) as well as with NaCl solutions with ionic strength between 0.005 and 0.8 mol kg−1. The present study shows that for high ionic strength solutions, such as seawater, challenges remain when addressing the assessment and quantification of ocean acidification in relation to climate change. Improvements are envisaged from the eventual selection of a more adequate ionic liquid

The Condor seamount at Mid-Atlantic Ridge as a supplementary source of trace and rare earth elements to the sediments

14 páginas, 8 figuras, 3 tablasThe Condor Seamount rises from seabed to 180 m water depth, being located 10 nautical miles southwest of the island of Faial, Azores Archipelago at the Mid-Atlantic Ridge (MAR). The vertical distribution of major, minor, trace and rare earth elements (REE) and Pb isotopes was studied in four sediment cores: one from the top of the Condor Seamount (200 m, MC9), two from the seamount base (1400 m, MC2 and MC4), and one from a deep area (1900 m, MC8). Sediments from the top of the Condor were composed by coarser particles being the fine fraction lower than 1%. Conversely the other sediments were constituted by 51–92% of fine particles (2 mm) were found at several depths of the cores sampled at the base of the seamount. The core collected in the top of the Condor showed higher carbonate content (76–86%) compared with the other cores (41–64%). The chemical compositions of MC2 and MC4 point to an enhancement of V, Cr, Co, Ni and Fe concentrations. Lower concentrations in MC8 hypothesis that Condor seamount constitutes a supplementary source of trace elements. The most plausible explanation for the enhancement found in sediments of the seamount base is the weathering of slopes with volcanic activities, which supply particles with higher element concentrations than pelagic sediments. This hypothesis is corroborated by REE data, showing increased chondrite normalized ratios in MC2 and MC4. Moreover, the REE pattern found in those cores was comparable to that existing in volcanic material with Light REE enrichment in comparison to Heavy REE. These results indicate a substantial contribution of particles derived from volcanic activities to sediments settled in the vicinity of the Condor Seamount. It is argued the potential use of REE in sediments from this region as tracers of volcanic activities. Depth profiles of 206Pb/207Pb and 206Pb/208Pb showed lower ratios in the first 8 cm sediment layers, reflecting atmospheric input of anthropogenic Pb in the last century. On the basis of Pb profiles it is proposed a baseline Pb concentration of 3.6±0.2 μg g−1 for pelagic sediments of the region with an isotopic signature of 206Pb/207Pb=1.227±0.003 and 206Pb/208Pb=0.492±0.001 signature. The isotope plots of 206Pb/207Pb versus 208Pb/206Pb showed a linear trend indicating the mixing between more radiogenic pre-industrial end-members and less radiogenic anthropogenic lead. The Pb isotope composition of sediments from the Condor area falls closer to North Atlantic Sediment Line. Sediments showed a 206Pb/204Pb signature closer to the basalts of the Capelo volcanic complex than from Mid-Ocean Ridge Basalts (MORB), which suggests the contribution of similar geological formations to sedimentary material.The current work was supported by a grant from Iceland, Liechtenstein and Norway through the EEA Financial Mechanism — Project CONDOR (PT0040).Peer reviewe

Symmetric Potentiometric Cells for the Measurement of Unified pH Values

A unified pH scale of absolute values (pHabs scale) enables the comparison of acidities in different solvents. To date, very few different experimental setups have been used for the measurement of values on this scale. The article describes the design and performance of the different symmetric cells used for unified pH measurement by several institutions. Well-established and reliable standard aqueous buffer solutions are the first step of method validation necessary to achieve a robust metrological level for more complex media. The pH of aqueous standard buffers was measured by differential potentiometry, where the potential between two glass electrodes is measured directly. All the tested electrochemical cells prove to be suitable for unified pH measurements. This validation highlights that the method is, to a large extent, independent of the used equipment, including the cell geometry. The inherent symmetry of the cell design helps to reduce the experimental workload and improve the accuracy of obtained results

Footprint of roman and modern mining activities in a sediment core from the southwestern Iberian Atlantic shelf

A 5-m long sediment core (VC2B), retrieved in the Southwestem Iberian Atlantic shelf, at 96 m water depth, was used to assess major changes in climate and human activities during the last 9.7 kyrs. Analytical measurements included sedimentological (mean grain size, and the contents of sand, silt and day), geochemical (major, minor, trace and rare earth elements; REEs) and chronological (Pb-210 and C-14) parameters. Two episodes of increment of fine-grained particles, occurring at 3050 BCE and 1350 CE, suggest the retreat of the coast line to the present level and the beginning of a wetter phase associated with the "Little Ice Age". The North American Shale Composite (NASC)-normalized REE-pattern detected in the shelf is similar to that found in the Guadiana estuarine sediments. The possibility of this estuary as a contributor to the sediment load deposited in the adjacent coastal zone was indicated. Trace elements were significantly correlated with Al until 1850 CE, pointing that grain-size rules its distribution in sediments. The depth variation of As, Cu and Pb enrichment factors relative to background values shows two periods of intense human activity that can be mainly linked to mining: (i) across the Roman Period, marked by low enrichments; and (ii) starting on the second half of the 19th century until nowadays with significantly increased enrichments, especially of Pb and Cu. In addition to As, Cu and Pb, this period is also marked by high enrichments of Hg and Zn. Despite the decrease/closure of sulphide massive deposits mining exploitation (e.g., Sao Domingos, Las Herrerias) during the second half of the 20th century, results showed ongoing input of Pb,Cu, As, Hg and Zn to coastal sediments. Thus, the legacy of contamination by these elements, mainly from leaching of slags and tailings, and remobilization/reworking of contaminated estuarine sediments, is still recorded in marine sediments. (C) 2016 Elsevier B.V. All rights reserved