pH Selection: Total Charges and Ionic Forms Curves and Calculations

Description

The definition of total charge and its calculations as well as the ionic forms concentration calculations (by pKa only or pKa and LogD) may cause some confusion when using pH selector.

Solution

• The total charge is the sum of the contributing charges for each molecule at a given pH, not the sum of all charges across all structures.
• In the graph area, depending on the option selected in the Show area, you can view the following curves for each target chemical structure:
  • LogD curves
  • Dominant ionic forms (%)
  • Total charges
• When observing the total charges curve, the total charge value (y-axis) is the weighted charge at a given pH (x-axis).
  • For example, for Etifenin, at pH 14, the % of species at charge -3 is 60.78% and 39.13% at charge -2. This will represent a weighted charge of -2.6 (calculated as (-3)*(0.6078)+(-2)*(0.3913) = -2.6) at pH 14 on the graph.
• The ionic forms concentration can be calculated by using either pKa only or pKa and LogD.
  • Using pKa only—calculates distribution of ionic forms versus pH using the algorithm WITHOUT taking into account the existence of lipophilic phase, i.e., just in water solution. For chromatography, it may be used in case of very hydrophilic stationary phase.
  • Using pKa and LogD—calculates distribution of ionic forms taking into account the existence of lipophilic phase. In case of chromatography, the hydrophobic stationary phase (e.g., C18) is an analog of lipophilic phase, and this option should be used for columns with such type of stationary phases.