Bates-Guggenheim equation

Fig. 1.8 Dependence of the mean activity coefficient y tC of NaCl on the square root of molar concentration c at 25°C. Circles are experimental points. Curve 1 was calculated according to the Debye-Hiickel limiting law (1.3.25), curve 2 according to the approximation aB = 1 (Eq. 1.3.32) curve 3 according to the Debye-Hiickel equation (1.3.31), a = 325nm curve 4 according to the Bates-Guggenheim approximation (1.3.33) curve 5 according to the Bates-Guggenheim approximation + linear term 0.1 C curve 6 according to Eq. (1.3.38) for a = 0.4nm, C = 0.055dm5-mor ...

Measurements of the pH in aqneous-organic solvent mixtures and in nonaqueous mixtures pose similar problems, but the need for such data is extensive, for instance for mobile phases used in chromatography, for electrochemistry, and for corrosion studies. When in cell (8.3) the solvent S is not pure water but its mixture with another solvent or a non-aqueous solvent altogether. Equation 8.4 still holds, but Ihe activity coefficient of the chloride ion must be changed, using the modified Bates-Guggenheim... 

The values were calculated from the modified Debye-Hiickel equation utilizing the modifications proposed by Robinson and by Guggenheim and Bates ... 

However, almost all pH measurements now use the glass electrode and, to allow for slight variations in its pH response, the electrode is calibrated against two pH standards which, if possible, bracket the pH range of the test solutions. The measured e.m.f. of an unknown solution can then be converted directly to a pH measurement. This procedure is based on the operational definition of pH as proposed by Bates and Guggenheim (1960). The equation is... 

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