86
averaged, reported Kg values were computed, was the same
for all titrations. It has been shown that, as the ionic
strength, I, is decreased, the ratio of bound to free
drug at which intercalative binding becomes significant
increases. The ratio also increases as the charge density
of the countercation increases. For the most dilute
solutions of rubidium, cesium, and tetramethy1ammonium
dihydrogen phosphate, intercalative effects begin to
become important when the logarithms of the ratios approach
+0.2. In solutions having high ionic strengths, or which
contain divalent cations, surface binding predominates even
at the highest ratios of bound to free drug. To allow re
liable comparisons of calculated surface binding association
constants between high and low ionic strength solutions, only
data where log ([BHP]/[BH]) was less than +0.2 were used.
The lower limit of the ratio which provides for reliable
values of K was estimated to be -0.8. Below -0.8, data
s
often deviated significantly from values obtained above
this limit. Using the above guidelines, the averaged values
of Kg listed in Tables 24-32 were computed. It is readily
acknowledged that the method of limiting the range of data
is somewhat arbitrary and may be eventually replaced with
more specific, but as yet unknown, guidelines.
The number of individual data points, n, used to
calculate Ks, for each titration, is given in the tables.
Specific points which were used are indicated by double
asterisks (**) in Tables 2 through 12 and 18 through 21.