pKa and Ionization Fractions at Physiological pH

Why pKa and pH Must Be Read Together

A drug’s pKa does not, by itself, say what fraction of the drug is charged in a physiological compartment. That fraction depends on the relationship between the pKa and the compartment’s pH. A compound may therefore have different charged fractions in plasma and in another compartment with a different pH, even when its pKa is unchanged.

This tutorial considers only monoprotic compounds: compounds represented by one relevant acid or base equilibrium. The equations give equilibrium fractions for the two forms in that pair. They are useful for interpreting a property input and making a compartment-specific calculation, but do not by themselves predict permeability, tissue distribution, or pharmacokinetics.

Calculation Rules for the Two Equilibria

For an acid \(HA\), the charged form is \(A^-\). The Henderson–Hasselbalch relationship expresses the ratio of charged to neutral forms as \(10^{pH-pK_a}\). Converting that ratio to a fraction gives the following calculation rules.

For a base \(B\), the charged form is \(BH^+\), and the neutral form is \(B\). Here the Henderson–Hasselbalch ratio of neutral to charged forms is \(10^{pH-pK_a}\). Consequently, the fraction equations have the opposite placement of the numerator compared with the acid equations.

Here brackets denote concentrations of the indicated forms, while \(f\) denotes a fraction of the total amount in the two-form pair. The fractions are dimensionless; multiply by 100 to express them as percentages. A pKa has no units, and pH and pKa must be expressed on the same scale. The familiar reference point is \(pH=pK_a\): the two forms have equal concentrations, so each fraction is 0.5.

A short justification connects the ratio to the fraction. If the charged-to-neutral ratio is \(r\), then the charged amount is \(r\) times the neutral amount. The total is therefore \(1+r\) neutral-amount units, giving a charged fraction of \(r/(1+r)\) and a neutral fraction of \(1/(1+r)\). For an acid, \(r=10^{pH-pK_a}\); for a base, that ratio applies to the neutral form instead.

Worked Calculations

Assumptions, Checks, and Failure Modes

These equations describe an idealized equilibrium between two forms of one monoprotic compound. Their direct use assumes that the pKa and pH refer to the same relevant environment, the compound is represented adequately by the stated equilibrium, and the system has reached equilibrium. The calculation also treats the relevant solution as sufficiently ideal for the Henderson–Hasselbalch relationship to be used with concentrations as a practical approximation. Binding, complex formation, or other chemical processes can make the fraction of freely dissolved compound differ from this simple estimate.

Physiological pH is not one universal value for every body compartment. Use the pH specified for the compartment of interest rather than automatically applying plasma pH throughout a model. Likewise, an experimental pKa can depend on measurement conditions and may not transfer exactly to every environment. If a compound has multiple ionizable groups or more than two relevant charge states, these monoprotic equations are not a complete description; a single charged-versus-neutral fraction can then be misleading.

Several quick checks catch common errors. First, the charged and neutral fractions must each lie between 0 and 1 and must sum to 1, apart from rounding. Second, the direction must match the chemistry: acids become more deprotonated as pH rises, while bases become more protonated as pH falls. Third, compare pH with pKa before calculating: equality means a 50:50 split, and a difference of one pH unit gives a 10:1 ratio of the relevant forms.

A frequent mistake is to use the acid equation for a base because both equations contain \(10^{pH-pK_a}\). The correction is to identify the charged species first: \(A^-\) for the acid, \(BH^+\) for the base. Then check the direction against pH and pKa. For example, a base with pH below pKa should not be calculated as mostly neutral.

Another mistake is to confuse an ionization fraction with a property that summarizes partitioning between phases. The calculated fraction describes the distribution between two forms under the stated equilibrium conditions. It does not alone establish how the compound partitions between octanol and water or across a biological membrane. The earlier material on lipophilicity distinguishes those kinds of measurements; here the calculation supplies a specific equilibrium fraction at a specified pH.

Finally, more digits in the output do not create more certainty. Calculated fractions inherit uncertainty in pKa, pH, and the assumptions used. In a PBPK input assessment, retaining a sensible number of significant figures and documenting the pH and pKa basis makes the calculation easier to audit.

Scope and Use in PBPK

Charged and neutral fractions are interpretable physicochemical quantities that can help characterize an ionizable compound under a stated pH condition. They are not, on their own, a complete mechanistic description of drug movement. A fraction calculation does not account for transporters, membrane properties, binding, multiple ionizable sites, or non-equilibrium processes. It is therefore best treated as a well-defined input or check, not as a stand-alone prediction of absorption or tissue distribution.

The principal alternative to a direct fraction calculation is to report the pH and pKa relationship as a ratio, such as 794.3 charged-to-neutral for the acid in the first example. The ratio is useful for seeing how strongly one form predominates; the fraction is usually easier to compare across compounds and to interpret as a proportion of total compound. Both describe the same two-form equilibrium when the stated assumptions apply.