Ionic strength zwitterion contribution

The required attributes listed above effectively limit the range of primary buffers available to between pH 3 and 10 (at 25 °C). Calcium hydroxide and potassium tetraoxalate tire excluded because the contribution of hydroxide or hydrogen ions to the ionic strength is significant. Also excluded are the nitrogen bases of the type BH+ (such as tris(hydroxymethyl)aminomethane and piperazine phosphate) and the zwitterionic buffers (e.g. HEPES and MOPS (10)). These do not comply because either the Bates-Gu enheim convention is not applicable, or the liquid junction potentials are high. This means the choice of primary standards is restricted to buffers derived from oxy-carbon, -phosphorus, -boron and mono, di- and tri-protic carboxylic acids. The uncertainties (11) associated with Harned cell measurements are calculated (1) to be 0.004 in pH at NMIs, with typical variation between batches of primary standard buffers of 0.003. 

While polyelectrolyte behavior is generally not within the remit of this work, Stellwagen, et a/. s use of capillary electrophoresis to examine a fundamental physico-chemical issue is worthy of note(49). Their interest was a vexing question, namely whether or not zwitterions contribute to the ionic strength / of a solution. Many would assert that the theoretical studies of Kirkwood are substantially definitive, but some doubts remain(50,51). Stellwagen, etal. noted that the Debye-Hueckel-Onsager electrophoretic theory requires pi [Pg.57]

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