Acid-Base Chemistry: pKa, pKb, Ka, and Related Concepts

Vocabulary flashcards covering the key terms and concepts from the acid-base lecture notes, including Ka, pKa, pKb, Henderson-Hasselbalch, protonation states, and physiological relevance.

Ka

The acid dissociation constant for HA ⇌ H+ + A−; larger Ka means a stronger acid; weak acids have Ka < 1.

pKa

The negative logarithm of Ka (pKa = -log10(Ka)); smaller pKa indicates a stronger acid and pKa is a constant for a given acid.

Kb

The base dissociation constant for B + H2O ⇌ BH+ + OH−; larger Kb means a stronger base.

pKb

The negative logarithm of Kb (pKb = -log10(Kb)); smaller pKb means a stronger base; for conjugate pairs, pKa + pKb ≈ 14.

Henderson-Hasselbalch equation

Relates pH to pKa and the ratio of conjugate base to acid: pH = pKa + log([A−]/[HA]). It can be rearranged for other forms.

Conjugate acid-base pair

Species related by proton transfer (e.g., HA/A− or BH+/B). The protonated form and the unprotonated form are conjugates.

Protonated form (acid)

The acid form with the extra proton; for HA, the protonated form is HA (neutral). For the conjugate acid BH+, the protonated form is BH+ (charged).

Unprotonated form (acid)

The acid lacking the extra proton; for HA, the unprotonated form is A− (ionized). For bases, it's B (neutral).

Protonated form (base)

The base after gaining a proton; BH+ is the protonated form and is charged.

Unprotonated form (base)

The base without the extra proton; B is the unprotonated, neutral form.

Ratio undissociated/ionized (acid)

At a given pH, the undissociated (unionized) to dissociated (ionized) ratio is 10^(pKa − pH).

Equal protonated and unprotonated forms

At pH = pKa, there are equal amounts of protonated and unprotonated forms.

pH

Negative log of hydrogen ion activity; measures acidity.

pOH

Negative log of hydroxide ion activity; pH + pOH ≈ 14 at 25°C.

14 = pKa + pKb

Relation for conjugate acid-base pairs: the sum of pKa (of the acid) and pKb (of the base) is about 14.

Ionized vs. Unionized

Ionized = charged; Unionized = uncharged; ionization state affects membrane permeability and excretion.

Ion trapping

Manipulating pH to keep a drug in its ionized form to promote excretion (urine) or, conversely, reabsorption depending on context.

Kidney excretion vs. reabsorption

In the kidney, ionized (charged) forms are generally excreted; unionized (uncharged) forms are more readily reabsorbed.

Blood-brain barrier permeability

Charged molecules cross poorly; uncharged (neutral) forms cross more readily.

Weak acid/base

Species that only partially dissociate in water; Ka < 1 for weak acids and Kb < 1 for weak bases.