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Reasons alv considered tbr setting up the new pH scale standard for aqueous solutions, for which the major terms are e.~plained and the scope of the standard is outlined.It has been reported previously [ 1, 2] that the All-Russia Technical Physics and Electronics Research Institute VNIIFTRI has set up a standard tbr the pH scale that has been accepted as a state primary standard under the number GI~T 54-98. This has resolved the apparatus pan of supporting unity in pH measurements.The legal-standard aspect is no less important. The electrochemical laboratory at VNIIFTRI has reconsidered a series of outdated standards and methodological recommendations on pH measurement. During it, they drew up and introduced the international standard 8.134-98 "The pH scale for aqueous solutions" [4], which was confirmed by the records of the Intergovernmental Council on Standardization, Metrology, and Certification No. 13-98 of May 28, 1998, which replaces GOST 8.134-74 [3]. The need to upgrade the standard arises from demands-tor more accurate definition of pH in many areas of science and industry: in clinical diagnosis, microbiology, medicine production, the manufacture of lbod products, cosmetics, and so on. The previous version of the standard contained the pH values lor seven solutions that had been established with an error of +0.01 at temperatures from 0 to 95~ It is clear that one cannot provide metrological support on the basis of that standardization document for example to blood analyzers, for which the entire range of normal values is from 7.35 to 7.45 [5]. Also, during the last two decades, interlaboratory comparisons have modified the pH values for various buffer solutions that reproduce the practical scale for it [6].Formal pH Definition. The primary problem for the authors of the standard, members of the electrochemical laboratory at VNIIFTRI, was to select a pH scale. At present, there are several definitions of the pH scale recognized at the international level and recommended by organizations such as the National Institute for Standards and Technology of the USA, the British Standards Institution, the International Union on Theoretical and Applied Chemistry, the International Organization lbr Legal Metrology, and the International Clinical Chemical Federation. A detailed survey has been made on those scales in [7]. The variety is due to the pH concept being distinctive amongst physicochemical quantities, as the formal definition [6, 8] includes the activity of a particular form of iron: hydroxonium H3 O+ (usually, for brevity one uses the term hydrogen ions and the symbol H + to denote them). The term activity is usually defined as "a thermodynamic function that describes by means of the same equations the relationship between the change in chemical potential and the changes in measured quantities such as the concentration or partial pressure, which are analogous to the equations for ideal systems" (gases or solutions) [9].The formal definition means that the pH (hydrogen index) corresponds to the negative ...
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