PH: definition

A logarithmic scale is useful not only for expressing hydronium ion concentrations, but also for expressing hydroxide ion concentrations and equilibrium constants. That is, the pH definition can be generalized to other quantities pOH = - log [OH ] p Tg = - log Tg p log... 

Rossmann, R., Sawers, G. and Bock, A. (1991) Mechanism of regulation of the formate-hydro-genlyase pathway by oxygen, nitrate, and pH Definition of the formate regulon. Mol. Microbiol., 5, 2807-14. 

However, because the determination of o(H+), a single ion activity, is thermodynamically impossible, an operational pH definition is given as outlined in (1) and (2) below ... 

The above conceptual and operational pH definitions for solutions in non-aqueous and mixed solvents are very similar to those for aqueous solutions [16]. At present, pH values are available for the RVS and some primary standards in the mixtures between water and eight organic solvents (see 5 in Section 6.2) [17]. If a reliable pH standard is available for the solvent under study, the pH can be determined with a pH meter and a glass electrode, just as in aqueous solutions. However, in order to apply the IUPAC method to the solutions in neat organic solvents or water-poor mixed solvents, there are still some problems to be solved. One of them is that it is difficult to get the RVS in such solvents, because (i) the solubility of KHPh is not enough and (ii) the buffer action of KHPh is too low in solutions of an aprotic nature [18].8) Another problem is that the response of the glass electrode is often very slow in non-aqueous solvents,9 although this has been considerably improved by the use of pH-ISFETs [19]. Practical pH measurements in non-aqueous solutions and their applications are discussed in Chapter 6. 

This pH definition for non-aqueous and mixed solvent systems is practically the same as that for aqueous solutions (Section 6.2.1). Thus, if a pH standard is available for the solvent or mixed solvent under study, the glass electrode is calibrated with it and then the pH of the sample solution is measured. The pHRVs values for 0.05 mol kg-1 KHPh have been assigned to aqueous mixtures of eight organic solvents (see 5 for pHRVs at 25 °C). Although they are for discrete solvent compositions, the pHRVs in between those compositions can be obtained by use of a multilinear regression equation [14b],... 

C) which he derived from the ionic product of water (Kw = 10 14 mol x dm 3). Some years later, Lewis introduced the concept of activity, and in 1923 Debye and Hiickel published their theory for strong electrolyte solutions. On the basis of this knowledge, Soerensen and Linderstroem-Lang [2] suggested a new pH definition in terms of the relative activity of hydrogen ions in solution ... 

EH s that are often encountered in sediment pore waters. Cores from Mono ake (not shown) had extremely alkaline pH s of 9.8 to 10.1. This high a pH definitely has an effect on the bimane reaction. Pore water samples adjusted to pH 8 gave much higher results which were similar to those obtained from the methylene blue and DTNB method (data not shown). Thiosulfate and sulfite were present in micromolar quantities in cores from all habitats. Thiosulfate was highest in the salt pond cores where it occasionally was more abundant than sulfide. While methane thiol, glutathione, and other organic thiols can be detected by the bimane method, they were not abundant (< 10 mM) in the core samples we chose to analyze. 

Anonymous, Measurement of pH. Definitions, Standards and Procedures (lUPAC pH Recommendations 2002), PureAppl. Chem., 2002, 74(11), 2169-2200. 

Unfortunately, since individual ionic activity coefficients cannot be evaluated without extrathermodynamic assumptions, the theoretical thermodynamic elegance and desirabflity of this pH definition cannot be rigorously related to experimental quantities. For this reason, the modem operational NBS (National Bureau of Standards) scale of acidity has been developed. 

Buck RP, Rondinini S, Covington AK, et al. (2001) The Measurement of pH - Definition, Standards, and Procedures. Report of the Working Party on pH, lUPAC Provisional Recommendations, lUPAC. 

Buck RP, Rondinini S, Covington AK, Baucke FGK, Brett CMA, Camoes ME, Milton MJT, Mussini T, Naumann R, Pratt KW, Spitzer P, Wilson GS (2002) Measurement of pH. Definition, standards, and procedures (lUPAC Recommendations 2002). Pure Appl Chem 74 2169-2200... 

Sprensen made this last suggestion because he had observed that potentiometric measurements realized with the help of a glass electrode (which responds to the hydroxonium ions) were not exactly matched with their [HsO ] concentration. The notion of a potentiometric chain enters in the full definition of pH. The pH definition in terms of activities, which cannot escape it, brings great complications since the activity of an ion cannot be measured (see Chap. 3). 

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