2027 · High School Specialised Subject · Biochemistry

2.3 Buffering against pH Changes

Buffer composition, Henderson–Hasselbalch and buffer capacity

How a weak acid and its conjugate base resist pH change, the Henderson–Hasselbalch relation, buffer capacity, and the physiological buffers that keep cells and blood near pH 7.4.

6 SECTIONS16 FIGURESEnglish edition

01

A buffer is a mixture of a weak acid and its conjugate base

Added H⁺ is absorbed by the conjugate base; added OH⁻ is neutralised by the weak acid
The buffer pair must be present in comparable amounts to work well
A buffer does not fix pH; it slows the change

Figure 2-3-1A buffer is a mixture of a weak acid and its conjugate base — reference figure

Table 2.3-4Assessment criteria for diabetic ketoacidosis

Table 2.3-5Terms used in ketoacidosis

02

Henderson–Hasselbalch relates pH, pKa and buffer concentration

pH = pKa + log([A⁻]/[HA])
When the ratio is 1, pH equals pKa; each tenfold change in ratio shifts pH by one unit
The relation assumes ideal behaviour and activities approximated by concentrations

Figure 2-3-2A buffer is a mixture of a weak acid and its conjugate base — reference figure

Table 2.3-2Ratio, pH shift and buffer usefulness

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02

Henderson–Hasselbalch relates pH, pKa and buffer concentration

pH = pKa + log([A⁻]/[HA])
When the ratio is 1, pH equals pKa; each tenfold change in ratio shifts pH by one unit
The relation assumes ideal behaviour and activities approximated by concentrations

Figure 2-3-2A buffer is a mixture of a weak acid and its conjugate base — reference figure

03

Buffer capacity depends on concentration and composition

Capacity rises with total buffer concentration
Capacity is greatest within about one pH unit of the pKa
Outside that window the buffer is a poor choice regardless of concentration

Figure 2-3-3A buffer is a mixture of a weak acid and its conjugate base — reference figure

04

Weak acids and bases buffer cells and tissues

Phosphate buffer operates inside cells near pH 7.2
The bicarbonate system with CO₂ handles the volatile acid load of respiration
Proteins contribute buffering through histidine and terminal groups

Figure 2-3-4Henderson–Hasselbalch relates pH, pKa and buffer concentration — reference figure

Table 2.3-3Chemical buffering, respiration and the kidney acting together

Table 2.3-1Buffering in both directions

05

Untreated diabetes can cause life-threatening acidosis

Ketone bodies accumulate when insulin is insufficient
Bicarbonate is consumed as the acids are neutralised
When compensation fails, blood pH falls and the condition becomes an emergency

Figure 2-3-5Henderson–Hasselbalch relates pH, pKa and buffer concentration — reference figure

06

[Separate note 2-1] Becoming your own experimental animal

Early investigators tested acid–base balance on themselves
The observations established the link between ventilation and blood pH
Modern work would require ethical approval and far safer methods

Figure 2-3-6Henderson–Hasselbalch relates pH, pKa and buffer concentration — reference figure

References

**Table 2.3-1**Buffering in both directions
Adding strong acid H⁺	Adding strong base OH⁻
A⁻ + H⁺ → HA	HA + OH⁻ → A⁻ + H₂O
A⁻ decreases and HA increases	HA decreases and A⁻ increases
A buffer low in A⁻ is vulnerable to acid	A buffer low in HA is vulnerable to base

**Table 2.3-2**Ratio, pH shift and buffer usefulness
[A⁻]/[HA]	pH − pKa	Interpretation
1:10	−1.00	Weak acid predominates; poor defence against acid load
1:1	0.00	Balanced buffering in both directions
10:1	+1.00	Conjugate base predominates; poor defence against base load
100:1	+2.00	Outside the usual effective buffering range

**Table 2.3-3**Chemical buffering, respiration and the kidney acting together
Component	Main role	Teaching point
Bicarbonate / CO₂	Acid–base control of blood and extracellular fluid	The lung removes CO₂, so the system behaves as an open system
Phosphate	Buffering inside cells and in the renal tubule	The effective pKa depends on conditions
Haemoglobin and protein	Bind and release H⁺, damping pH change	Surface groups have a range of pKa values
Respiratory system	Adjusts pCO₂	Compensatory route for metabolic disturbance
Kidney	H⁺ excretion and HCO₃⁻ regulation	Essential for sustained excretion of nonvolatile acid

**Table 2.3-4**Assessment criteria for diabetic ketoacidosis
Domain	Educational reference criterion	Caution in interpretation
Diabetes and glucose	Known diabetes or glucose ≥ 200 mg/dL	DKA can occur with normal or low glucose
Ketone elevation	β-hydroxybutyrate ≥ 3.0 mmol/L and others	Urine ketones do not quantify the main ketone bodies
Metabolic acidosis	pH < 7.3 or HCO₃⁻ < 18 mmol/L	Timing of sampling matters; other conditions must be excluded

**Table 2.3-5**Terms used in ketoacidosis
Term	Core meaning
Ketosis	Raised ketone bodies; not necessarily accompanied by acidosis
Ketoacidosis	Metabolic acidosis associated with increased ketone acids
Acidemia	Blood pH below the normal range
Compensatory hyperventilation	CO₂ excretion that partly raises pH in metabolic acidosis
Bicarbonate consumption	Fall in HCO₃⁻ as an acid load is taken up

A buffer is a mixture of a weak acid and its conjugate base — reference figure — Figure 2-3-1A buffer is a mixture of a weak acid and its conjugate base — reference figure

A buffer is a mixture of a weak acid and its conjugate base — reference figure — Figure 2-3-2A buffer is a mixture of a weak acid and its conjugate base — reference figure

A buffer is a mixture of a weak acid and its conjugate base — reference figure — Figure 2-3-2A buffer is a mixture of a weak acid and its conjugate base — reference figure

A buffer is a mixture of a weak acid and its conjugate base — reference figure — Figure 2-3-3A buffer is a mixture of a weak acid and its conjugate base — reference figure

Henderson–Hasselbalch relates pH, pKa and buffer concentration — reference figure — Figure 2-3-4Henderson–Hasselbalch relates pH, pKa and buffer concentration — reference figure

Henderson–Hasselbalch relates pH, pKa and buffer concentration — reference figure — Figure 2-3-5Henderson–Hasselbalch relates pH, pKa and buffer concentration — reference figure

Henderson–Hasselbalch relates pH, pKa and buffer concentration — reference figure — Figure 2-3-6Henderson–Hasselbalch relates pH, pKa and buffer concentration — reference figure