2027 · High School Specialised Subject · Biochemistry

2.2 Ionization of Water, Weak Acids, and Weak Bases

Autoionization, pH, pKa and titration curves

Autoionization of water and the ion product, the pH scale, dissociation constants of weak acids and bases, and reading pKa from a titration curve.

5 SECTIONS18 FIGURESEnglish edition

01

Pure water ionizes only slightly

A small fraction of molecules transfers a proton to a neighbour, producing H₃O⁺ and OH⁻
At 25 °C the concentrations are equal, about 1.0 × 10⁻⁷ M each
The process is fast and dynamic, not a one-way reaction

Figure 2-2-1Pure water ionizes only slightly — reference figure

Table 2.2-1Acidic, neutral and basic conditions at 25 °C

02

Ionization of water is expressed by an equilibrium constant

Kw = [H⁺][OH⁻] = 1.0 × 10⁻¹⁴ at 25 °C
Kw changes with temperature, so neutrality is not always pH 7
Adding acid lowers [OH⁻] and adding base lowers [H⁺], keeping the product constant

Figure 2-2-2Pure water ionizes only slightly — reference figure

Table 2.2-4Equilibrium table for a weak acid

03

pH expresses the hydrogen and hydroxide concentrations

pH = −log[H⁺]; pOH = −log[OH⁻]; pH + pOH = 14 at 25 °C
A change of one pH unit is a tenfold change in concentration
Biological fluids are held near pH 7.4, with narrower ranges inside compartments

Figure 2-2-3Pure water ionizes only slightly — reference figure

Table 2.2-2pH, hydronium and hydroxide at 25 °C

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03

pH expresses the hydrogen and hydroxide concentrations

pH = −log[H⁺]; pOH = −log[OH⁻]; pH + pOH = 14 at 25 °C
A change of one pH unit is a tenfold change in concentration
Biological fluids are held near pH 7.4, with narrower ranges inside compartments

Figure 2-2-3Pure water ionizes only slightly — reference figure

04

Weak acids and bases have characteristic dissociation constants

HA ⇌ H⁺ + A⁻ with Ka = [H⁺][A⁻]/[HA]
pKa = −log Ka; a smaller pKa means a stronger acid
The fraction protonated follows the Henderson–Hasselbalch relation

Figure 2-2-4Pure water ionizes only slightly — reference figure

Table 2.2-5Dilution increases the degree of dissociation of acetic acid (25 °C)

05

A titration curve reveals the pKa of a weak acid

The curve is shallow near the half-equivalence point, where pH = pKa
Buffering is strongest at pH values close to pKa
The equivalence point itself is not the pKa; confusing the two is a common error

Figure 2-2-5Ionization of water is expressed by an equilibrium constant — reference figure

Table 2.2-6Titration of acetic acid with NaOH (calculated)

Table 2.2-3Representative pKa values and their biochemical relevance

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05

A titration curve reveals the pKa of a weak acid

The curve is shallow near the half-equivalence point, where pH = pKa
Buffering is strongest at pH values close to pKa
The equivalence point itself is not the pKa; confusing the two is a common error

Figure 2-2-5Ionization of water is expressed by an equilibrium constant — reference figure

References

**Table 2.2-1**Acidic, neutral and basic conditions at 25 °C
Condition	[H₃O⁺]	[OH⁻]	Classification
The two concentrations are equal	√Kw	√Kw	Neutral
H₃O⁺ predominates	[H₃O⁺] > [OH⁻]	[OH⁻] < [H₃O⁺]	Acidic
OH⁻ predominates	[H₃O⁺] < [OH⁻]	[OH⁻] > [H₃O⁺]	Basic

**Table 2.2-2**pH, hydronium and hydroxide at 25 °C
pH at 25 °C	[H₃O⁺] (mol/L)	[OH⁻] (mol/L)	Classification
2.00	1.0×10⁻²	1.0×10⁻¹²	Acidic
5.00	1.0×10⁻⁵	1.0×10⁻⁹	Acidic
7.00	1.0×10⁻⁷	1.0×10⁻⁷	Neutral
9.00	1.0×10⁻⁹	1.0×10⁻⁵	Basic
12.00	1.0×10⁻¹²	1.0×10⁻²	Basic

**Table 2.2-3**Representative pKa values and their biochemical relevance
Acid or conjugate acid	Equilibrium	Typical pKa near 25 °C	Biochemical relevance
Lactic acid	Lactate ⇌ H⁺ + lactate ion	≈3.86	Fermentation and metabolism
Acetic acid	CH₃COOH ⇌ H⁺ + CH₃COO⁻	≈4.74–4.76	Model weak acid for titration
Second dissociation of phosphate	H₂PO₄⁻ ⇌ H⁺ + HPO₄²⁻	≈7.2	Phosphate buffering
Ammonium ion	NH₄⁺ ⇌ H⁺ + NH₃	≈9.25	Nitrogen metabolism and acid–base balance

**Table 2.2-4**Equilibrium table for a weak acid
State	HA	H₃O⁺	A⁻
Initial (mol/L)	C	0 (approximation)	0
Change (mol/L)	−x	+x	+x
Equilibrium (mol/L)	C−x	x	x

**Table 2.2-5**Dilution increases the degree of dissociation of acetic acid (25 °C)
Initial concentration	Approximate equilibrium [H₃O⁺]	Approximate pH	Approximate dissociation
0.100 M	1.33×10⁻³ M	2.88	1.3%
0.0100 M	4.15×10⁻⁴ M	3.38	4.2%
0.00100 M	1.25×10⁻⁴ M	3.90	12.5%

**Table 2.2-6**Titration of acetic acid with NaOH (calculated)
Volume NaOH (mL)	Calculated pH	Region
0.0	2.88	Initial weak acid
5.0	4.14	Buffer region
10.0	4.57	Buffer region
12.5	4.75	Half-equivalence point
20.0	5.35	Buffer region
24.0	6.13	Just before equivalence
25.0	8.72	Equivalence point
26.0	11.29	Past equivalence
30.0	11.96	Excess OH⁻

Pure water ionizes only slightly — reference figure — Figure 2-2-1Pure water ionizes only slightly — reference figure

Pure water ionizes only slightly — reference figure — Figure 2-2-2Pure water ionizes only slightly — reference figure

Pure water ionizes only slightly — reference figure — Figure 2-2-3Pure water ionizes only slightly — reference figure

Pure water ionizes only slightly — reference figure — Figure 2-2-3Pure water ionizes only slightly — reference figure

Pure water ionizes only slightly — reference figure — Figure 2-2-4Pure water ionizes only slightly — reference figure

Ionization of water is expressed by an equilibrium constant — reference figure — Figure 2-2-5Ionization of water is expressed by an equilibrium constant — reference figure

Ionization of water is expressed by an equilibrium constant — reference figure — Figure 2-2-5Ionization of water is expressed by an equilibrium constant — reference figure