@Borek @Fra
Sorry to reopen the thread, but I have another little doubt.
I have found that it is possible to evaluate the solubility of an acidic drug through the following equation:
Stot = So + So*10^(pH-pKa).
Plotting Stot vs pH then a sigmoid is obtained.
Is it correct to follow the following reasoning for the construction of the sigmoid? : when pH is at least two units lower than the pKa, then [A-] in solution can be neglected, so the So*10^(pH-pKa) term of the equation can be neglected and Stot = So = [HA], so in the plot we obtain a straight line parallel to the X axis.
However, when pH increases beyond this "condition", [A-] is no longer negligible and no term in the equation can be neglected, so the equation is used entirely and the curve begins to rise upwards.
At this point, when pH is at least two units higher than the pKa, the So term of the equation can be neglected, being [HA] negligible, so, in theory, as pH increases, the curve should rise towards infinity, but in reality this does not happen, as the [A-] does not increase to infinity.
Therefore, when pH is at least two units higher than the pKa, the curve again becomes parallel to the X axis, since all the HA can now be considered converted into A- and Stot no longer increases.
Is this reasoning correct? Thank you for your patience.