Contents lists available at ScienceDirect Talanta journal homepage: www.elsevier.com/locate/talanta Seawater pH measurements with a combination glass electrode and high ionic strength TRIS-TRIS HCl reference bufers – An uncertainty evaluation approach Bárbara Anes ⁎ , Ricardo J.N. Bettencourt da Silva, Cristina Oliveira, M. Filomena Camões Centro de Química Estrutural, Faculdade de Ciências da Universidade de Lisboa, C8 Campo Grande, 1749-016 Lisboa, Portugal ARTICLE INFO Keywords: pH Seawater Uncertainty Least squares regression model Monte Carlo method ABSTRACT The comparison of pH measurements in seawater collected at diferent locations or occasions, is meaningful if the same measurand (i.e. the quantity intended to be measured) is determined, if adequate measurement pro- cedures are used, including the selection of calibrators, and if the measurement uncertainty is known. Depending on the purpose of this evaluation, the measurement uncertainty should be smaller than a defned target value. The measured pH should have a sound physical-chemical meaning to allow the adequate assessment of its impacts. In the present procedure TRIS-TRIS HCl solutions, of diferent molality ratios, prepared in artifcial seawater with reference values estimated by primary measurements, were used to obtain proper calibrators for the pH meter used for the analysis of seawater samples. This work presents the uncertainty evaluation of pH measurements in seawater, performed by potentiometry using a combination glass electrode, from the interpolation uncertainty evaluated by the Least Squares Regression Model and by Monte Carlo Simulations of measured potentials and reference values. The uncertainty evaluation was critically assessed. The developed algorithms were implemented in a user-friendly MS-Excel fle available as Electronic Supplementary Material. Seawater pH was measured with an expanded uncertainty of 0.019 enabling discriminating diferences of pH of two samples larger than 0.029. 1. Introduction pH, as a master variable for the CO 2 system, is of fundamental im- portance to the study of the oceans. The most common and fastest way of measuring pH in routine analysis is the classical potentiometric method that consists on mea- surements of the electromotive force, e.m.f., of a cell, often with a combination glass electrode selective to hydrogen ions. The potential diference which occurs in the cell varies linearly with pH according to the Nernst equation [1]. The relationship is achieved through linear regression which, in case of multipoint calibration, is given by the calibration function described by Eq. (1) [4]: = E E k pH a std std (1) where E std is the potential diference measured in the standard bufer solution with the assigned pH std reference value, E a is the intercept, and k’ is the practical slope determined by the linear regression. The pH value of a sample, pH (X) , is obtained from the measured potential diference, E (x) through Eq. (2): = k pH [E E ] (X) a (X) (2) To measure the pH of seawater a complex matrix with high ionic strength of approximately 0.67moldm −3 appropriate calibration bufer solutions must be adopted [16,5,8]. Since the commercially available pH bufer solutions are characterized by their low ionic strength, I (< 0.1 mol dm −3 ) [4], hence inadequate for these specifc measurements, appropriate calibrators have to be produced by the analyst. TRIS-TRIS HCl bufers have been studied for pH measurements in seawater since 1964 and in 1973 have been proposed as primary pH standards for oceanic pH measurements [10]. More recently, reference https://doi.org/10.1016/j.talanta.2018.09.075 Received 6 July 2018; Received in revised form 17 September 2018; Accepted 19 September 2018 ⁎ Corresponding author. E-mail address: bvanes@fc.ul.pt (B. Anes). Talanta 193 (2019) 118–122 Available online 29 September 2018 0039-9140/ © 2018 Elsevier B.V. All rights reserved. T