Traceability of pH to the Mole

Water

Maria Filomena Camཞs¹  Bárbara Anes² 

[1] Centro de Química Estrutural, Department of Chemistry and Biochemistry, Faculty of Sciences of the University of Lisbon; C8- Campo Grande, Lisboa 1749-016, Portugal; E-Mail

关键词: free acidity;    pH;    high ionic strength buffer solutions;    measured and modelled mean activity coefficients;    hydrogen ion concentration;    calibration;    traceability;   

DOI  :  10.3390/w7084247

来源: mdpi

【 摘 要 】

Free acidity of aqueous solutions was initially defined in 1909 by Søren Peter Lauritz Sørensen as pH = −lgc_H+ (c/mol·dm⁻³ or m/mol·kg⁻¹ of the free hydrogen ions in solution, H⁺) soon (1910) was changed to pH = pa_H+ = −lga_H+, integrating the new concepts of activity, a_i and activity coefficient γ_i, for the ionic species i under concern, H⁺ in this case; it is a_i = −lg(m_iγ_i). 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, γ_H+, 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 pm_H+ = −lgm_H+ profiting from the fact that, contrary to single ion activity coefficients, mean activity coefficients, , validates the model for its intended use.

【 授权许可】

CC BY   
© 2015 by the authors; licensee MDPI, Basel, Switzerland.