Acid dissociation constant

Acid dissociation constant is a quantitative measure of the strength of an acid in solution. It is commonly denoted as K_a and represents the equilibrium constant for the dissociation of an acid in an acid-base reaction.

For a generic weak acid, the dissociation reaction is written as:

HA ⇌ H⁺ + A⁻

where HA represents the undissociated acid, H⁺ represents the released hydrogen ion, and A⁻ represents the conjugate base of the acid.

A larger value of K_a indicates that a greater proportion of the acid dissociates in solution. Therefore, acids with higher K_a values are generally stronger acids, while acids with lower K_a values are weaker acids.

Definition

The acid dissociation constant is defined by the equilibrium expression:

K_a = ([H⁺][A⁻]) / [HA]

where:

• [H⁺] is the molar concentration of hydrogen ions
• [A⁻] is the molar concentration of the conjugate base
• [HA] is the molar concentration of the undissociated acid

In aqueous solution, the hydrogen ion is often represented more accurately as hydronium ion, H₃O⁺. The reaction may therefore also be written as:

HA + H₂O ⇌ H₃O⁺ + A⁻

and the expression may be written as:

K_a = ([H₃O⁺][A⁻]) / [HA]

pK_a

The pK_a is the negative base-10 logarithm of the acid dissociation constant:

pK_a = −log₁₀(K_a)

A lower pK_a value indicates a stronger acid, while a higher pK_a value indicates a weaker acid.

For example:

• A strong acid has a very low or negative pK_a
• A weak acid has a higher pK_a
• Acetic acid has a pK_a of approximately 4.76 in water at room temperature

Relationship to acid strength

The value of K_a reflects the position of the chemical equilibrium:

• If K_a is large, the equilibrium favors dissociation, producing more H⁺ and A⁻.
• If K_a is small, the equilibrium favors the undissociated acid, HA.
• Strong acids dissociate nearly completely in water.
• Weak acids dissociate only partially in water.

The relationship between K_a and pK_a is inverse:

• Higher K_a = lower pK_a = stronger acid
• Lower K_a = higher pK_a = weaker acid

Factors affecting acid strength

Several chemical factors influence acid strength and therefore affect the value of K_a.

Electronegativity

An acid is generally stronger when its conjugate base is stabilized by an electronegative atom. More electronegative atoms can better stabilize negative charge.

Atomic size

For acids in which the acidic hydrogen is bonded to atoms in the same group of the periodic table, acidity often increases as atomic size increases. Larger atoms can distribute negative charge over a larger volume.

Resonance

Resonance stabilizes the conjugate base by spreading negative charge over multiple atoms. Acids whose conjugate bases are resonance-stabilized are usually stronger.

Inductive effect

Electron-withdrawing groups can stabilize the conjugate base through the inductive effect. This increases acidity. For example, chloroacetic acid is stronger than acetic acid because the chlorine atom withdraws electron density.

Hybridization

The hybridization of the atom bearing the negative charge can affect acidity. Greater s-character generally stabilizes negative charge more effectively. For example, an sp-hybridized carbon is more acidic than an sp² or sp³ carbon.

Solvent effects

The solvent can influence acid dissociation by stabilizing ions. Water, a polar solvent, stabilizes charged species and supports acid-base dissociation.

Examples

Acetic acid

Acetic acid dissociation

Acetic acid is a weak carboxylic acid with the formula CH₃COOH. It partially dissociates in water to form acetate ions and hydrogen ions:

CH₃COOH ⇌ H⁺ + CH₃COO⁻

Acetic acid has a pK_a of approximately 4.76.

Phosphoric acid

File:H3PO4 speciation.png

Phosphoric acid speciation

Phosphoric acid is a triprotic acid, meaning it can donate three protons. It has three acid dissociation constants:

H₃PO₄ ⇌ H⁺ + H₂PO₄⁻

H₂PO₄⁻ ⇌ H⁺ + HPO₄²⁻

HPO₄²⁻ ⇌ H⁺ + PO₄³⁻

Each step has a different K_a and pK_a value.

Citric acid

File:Citric acid speciation.png

Citric acid speciation

Citric acid is a weak organic acid with the formula C₆H₈O₇. It contains three carboxyl groups and is therefore a triprotic acid. Each acidic proton has a separate dissociation constant.

Carbonic acid

Carbonic acid forms when carbon dioxide dissolves in water. It plays an important role in blood pH, buffer systems, and respiration.

H₂CO₃ ⇌ H⁺ + HCO₃⁻

The bicarbonate buffer system is important in physiology and acid-base homeostasis.

Acid dissociation and buffers

The acid dissociation constant is important in understanding buffer solutions. A buffer resists changes in pH when small amounts of acid or base are added.

The buffering capacity is greatest when the pH of the solution is close to the pK_a of the weak acid. This relationship is described by the Henderson-Hasselbalch equation:

pH = pK_a + log₁₀([A⁻] / [HA])

This equation is widely used in chemistry, biochemistry, pharmacology, and medicine.

Biological and medical importance

Acid dissociation constants are important in many areas of medicine, pharmacology, and biochemistry.

They help explain:

• The ionization of drugs
• Drug absorption
• Drug distribution
• Blood pH regulation
• Enzyme function
• Protein charge
• Metabolic acidosis
• Metabolic alkalosis
• Respiratory acidosis
• Respiratory alkalosis

Many medications are weak acids or weak bases. Their degree of ionization depends on their pK_a and the pH of the environment. This affects how well they cross cell membranes and how they are distributed in the body.

Common approximate pK_a values

Approximate pK_a values of selected acids

Acid	Formula	Approximate pKa	Notes
Hydrochloric acid	HCl	−6	Strong acid
Sulfuric acid	H2SO4	−3	Strong acid for first dissociation
Lactic acid	C3H6O3	3.86	Important in metabolism
Acetic acid	CH3COOH	4.76	Weak carboxylic acid
Carbonic acid	H2CO3	6.35	Important in blood buffering
Ammonium	NH4+	9.25	Conjugate acid of ammonia
Water	H2O	15.7	Very weak acid

Related concepts

• Acid
• Base
• Conjugate acid
• Conjugate base
• Base dissociation constant
• Chemical equilibrium
• pH
• pOH
• Buffer solution
• Titration
• Henderson-Hasselbalch equation
• Le Chatelier's principle

Gallery

• 
  pK_a of acetic acid
• Weak acid speciation
  Weak acid speciation
• 
  Carboxylic acid dimers
• Acetic acid pKa in dioxane-water
  Acetic acid pK_a in dioxane-water
• Chloroacetic acid pKa
  Chloroacetic acid pK_a
• Fumaric acid structure
  Fumaric acid structure
• 
  Maleic acid structure
• 
  Proton sponge
• Oxalic acid titration
  Oxalic acid titration

See also

• Acid-base reaction
• Acid-base homeostasis
• Dissociation constant
• Equilibrium constant
• Ionization
• Water ionization
• Weak acid
• Strong acid
• Weak base
• Strong base