Monoprotic Substance log DpH Curve Shape Analysis

Fig. 5 shows that the lipophiiicity curve has a predictable structure. In this section we will further examine this. Fig. 9a is that of a hypothetical weak base, with pKa 10, log Px 5 and log Pxh 0. It necessarily has p0/CaSCH 5, from the preceding discussion. The curve in Fig. 9a is broken down into zones of zero slope (1 and 1'), zones of curvature (2 and 2') and a zone of slope 1 (zone 3). Fig. 9 b and 9 c show the detail of zones 2 and 2'. If we believe that the precision of log D measurement is 0.01, for what value of slope is the curve 0.01 log D units away from the zero-slope line? This appears to be a slope of 0.022. A similar departure from the unit-limit slope is 1-0.022. These two slopes mark the two departures of the actual curve from the slope 1 and 0 asymptotes. Hence in this way we define the zone of curvature (zone 2 and 2') to be between point A and B in Fig. 9c and A' and B' in Fig. 9b. Point C with slope 0.5 corresponds to Scherrer pK.„ and point C' (which also has slope 0.5) corresponds to the pK.d. The practical upshot of these definitions is that the zone of curvature is 3.30 pH units wide and 1.66 log D units high. This information can be put to use in the next section.

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