Reference Value pH Standard

The IUPAC definition of pH39 is based upon a 0.05M solution of potassium hydrogenphthalate as the reference value pH standard (RVS). In addition, six further primary standard solutions are also defined which between them cover a range of pH values lying between 3.5 and 10.3 at room temperature, and these are further supplemented by a number of operational standard solutions which extend the pH range covered to 1.5-12.6 at room temperature. The composition of the RVS solution, of three of the primary standard solutions and of two of the operational standard solutions is detailed below, and their pH values at various temperatures are given in Table 15.4. It should be noted that the concentrations are expressed on a molal basis, i.e. moles of solute per kilogram of solution. 

Now the origin of the scale must be defined, i.e. a pH value must be selected for a standard (as close as possible to the value expected on the basis of definition 1.4.46). A solution of potassium hydrogen phthalate with a molality of 0.05 mol kg-1 has been selected as the reference value pH standard (RVS). 

Table 8.17 Standard Reference Values pH for the Measurement of Acidity in...

C 0.5012 mol 2 at 15°C). It is clearly unwise to associate a pH meter reading too closely with pH unless under very controlled conditions, and still less sensible to relate the reading to the actual hydrogen-ion concentration in solution. For further discussion of pH mea.surements, see Pure Appl. Chem. 57, 531-42 (1985) Definition of pH Scales, Standard Reference Values, Measurement of pH and Related Terminology. Also C E News, Oct. 20. 1997. p. 6. 

Table 3.7 Values of pH(RVS) for the reference value standard of 0.05 mol kg-1 potassium hydrogen phthalate at various temperatures... 

Covington, A. K., R. G. Bates and R. A. Durst, Definition of pH scales, standard reference values, measurement of pH and related terminology, Pure Appl. Chem., 57, 531 (1985). 

Figure 3, Standard reduction potentials associated with the chemistry of oxygen values in upper and lower halves of diagram refer to pH 0,0 and pH 7,0 conditions,...

For a value to be traceable it must be related to stated references. By definition and convention the stated references are taken to include SI [6] reference values (e.g., atomic mass values), reference materials (RMs), as well as primary, reference, and standard methods. It is sometimes stated that chemical measurements are traceable to the mole. This is an incomplete statement as chemical measurements are simultaneously traceable to a number of references, inter alia, the mole, kg, meter, etc. Whilst it is considered desirable to employ high level references, such as the SI, where feasible, this is not always necessary in terms of fit for purpose criteria. Neither is it possible to relate all types of analyte (fat, fiber, protein, pH, etc.) to the SI. The key issue is that the references should be stated and fit for purpose. 

Variation of Reference Electrode Potentials with Temperature pH Values of Standard Solutions Used in the Calibration of Glass Electrodes Temperature vs. pH Correlation of Standard Solutions Used for the Calibration of Electrodes Solid Membrane Electrodes Liquid Membrane Electrodes... 

Covington, A.K., Bates, R.G. and Durst, R.A., Definition of pH Scales, Standard Reference Values, Measurement of pH and Related Terminology, Pure Appl. Chem. 57 (1985) 531-542. 

P. R. Mussini,T. Mussini, and S. Rondinini, Reference value standards and primary standards for pH measurements in D2O and aqueous-organic solvent mixtures New accessions and assessments. Pure Appl. Chem. 69 (1997), 1007-1014. 

Here the pH is the value for acidity referred to the standard state defined from the limiting reference behavior for the hydrogen ions in the solvent in question, and PH is the pH value of the aqueous reference buffer corrected for liquid junction and transfer activity coefficient as follows ... 

A precision pH-meter (accuracy of 0.005 pH units) and a glass and calomel reference electrode calibrated against pH standards were used to measure pH values (activity units) which were converted into - logio [H ] using the Debye-Hiickel equation. However, the absolute accuracy that can be achieved with the applied electrode calibration is at best +0.02 pH-units, which is not sufficient for the determination of accurate results from potentiometric titration data. This problem becomes evident from the results shown in Table 1 of [1971K1C/STE]. The value of = 0.5 was reached at pH = (3.00 + 0.05) in titrations starting at initial acid concentrations below 0.01 M (/ = 0.013-0.016 M) and at pH = (3.40 + 0.05) in titrations starting at initial acid concentrations of about 0.03 M (/ = 0.037 M). 

The hydrogen electrode is the ultimate standard electrode not only for the determination of (relative) potentials, but also for the determination of pH values. Owing to the experimental difficulties associated with it, however, it is seldom used for routine measurements, but rather for the evaluation of secondary reference and pH electrodes such as the calomel reference electrode and the glass pH electrode. 

Operational standards (OS) are also defined which are traceable to the Reference Value Standard (RVS). Values are assigned by means of the operational cells I and II where the Uquid junctions are the free diffusion type reproducibly formed in 1 mm vertical capillary tubes. These operational standards are not restricted in number provided certain preparation criteria are met, and pH(OS) values for 16 solutions are given in Table 4. These OS represent an alternative procedure and are in no way to be regarded as inferior to the primary standards. As a consequence of their definition, all pH(OS) values fall on the line with slope given by the slope factor value forthe appropriate temperature in Table 1. Any ddference in liquid junction potential between the solutions of cells I and II and KCl is subsumed into the assigned value of pH(OS). 

Alternatively, a bracketting procedure may be adopted whereby two standard reference solutions are chosen whose pH values, pH(S 1), pH(S2) are on either side of pH(X). Then if the corresponding potential difference measurements are E(S 1), E(S2), E(X), then pH(X) is obtained from... 

The derivation of p a values from pH measurements is too familiar an exercise to be repeated here. Since pH values refer to a standard state for activities in methanol, the acidity constants will likewise have this reference state, i.e. they will be Ka values. When measurements are made in very dilute solutions (ca. 10" AO, not only are corrections to zero ionic strength small but ion-pairing of buffer salts becomes negligible. Some results of Juillard s measurements, based on the de Ligny standard buffers, are given in Appendix 3.5.5. 

Recently Himmel et al. have proposed a unified pH scale for all phases [534]. This general Brpnsted acidity scale is based on the absolute chemical potential of the proton in any medium. They set the standard chemical potential of the proton in the gas phase Pabs°(H gas) to 0 kJ/mol as a reference value. They report the Gibbs energies of proton solvation Asoi °G(H+) (based on a reference value -1105 8 kJ/mol for water) in most common solvents. Absolute pH values are assigned according to the relation... 

In practical measurements it is not advisable to use a single standard solution because the validity of Eq. (6) deteriorates as the distance of pH(X) fixim pH(S) increases. To avoid this problem standardization organizations have determined the pH value of a series of pH standards and these can all be used for reference. 

The increasing use of heavy water (D2O) in studies of reaction mechanism, in nuclear technology and in medical research has created a demand for accurate measurements of acidity in this medium. As the glass electrode responds satisfactorily to deuterium ion in D2 O, this need is provided by modern pH meters. The operational definition of pH also allows the creation of a scale, the pD scale, where the solvent is heavy water. Reference values have been assigned to three selected buffers in heavy water (Paabo Bates, 1969). Standard pD values for these buffers for various temperatures are given in Table 6.8. When pH meters are standardized against these buffers in heavy water solution, the meter readings are in pD units. 

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