Buffer and the Effect of Ionization

5.4.2.1 Buffer

In older publications phosphate buffers have been used. However, these have the disadvantage of forming ion-pairs to positively charged basic compounds. A good solution to this problem is zwitterionic buffers, such as morpholinopropane sulfonic acid (MOPS).

5.4.2.2 Ionization Correction

For monoprotic acids and bases the following equations can be used to correct for ionization:

acids: log K = log &wapp + log (1 + 10pH"p*a) (5)

where log &wapp is the log obtained at a particular pH. Similar equations can be used for the isocratic capacity factors. Equations for bifunctional acids and bases can be found (see [43] and Chapter 7).

The relationship between the capacity factor in the neutral form (kn), fully ionized form (k,), and in a partially ionized form (k) for a simple acid is given by the Horvath equation [44, 45]:

Further modifications led to [45]:

k = 0.5 (kn - ki) tanh (pKa - pH) + 0.5 (k„ + k{) (8)

We have developed an empirical equation also based on the tanh function, namely for monoprotic solutes [27]:

acids: log kn = log k + (1 - tanh (pKa - pH + 1)) (9)

bases: log kn = log k + (1 - tanh (pH - pK,d + 1)) (10)

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