Compute buffer pH, hydrogen ion concentration, and buffer ratio from pKa or Ka. Visualize the logarithmic relationship between pH and [A⁻]/[HA].
The Henderson–Hasselbalch equation provides a direct relationship between the pH of a buffer solution, the pKa of the weak acid, and the ratio of concentrations of the conjugate base ([A⁻]) to the weak acid ([HA]):
This equation is derived from the acid dissociation equilibrium (HA ⇌ H⁺ + A⁻) and the definition of Ka. It is fundamental in biochemistry, pharmacology, and environmental chemistry for predicting buffer behavior and ionization states of molecules.
In drug development, the ionization state (determined by pH relative to pKa) affects solubility, membrane permeability, and absorption. In enzymatic reactions, maintaining optimal pH via buffer systems relies on precise control of the [base]/[acid] ratio. The Henderson–Hasselbalch equation also explains the buffer capacity region: maximum buffering occurs when pH = pKa, where the ratio equals 1.
Within ±1 pH unit from pKa (ratio between 0.1 and 10), the buffer effectively resists pH changes. Outside this range, buffer capacity drops drastically. Our interactive graph illustrates this logarithmic dependency.
Blood bicarbonate buffer (pKa₁ = 6.35 for H₂CO₃) maintains pH ≈ 7.4 using a [HCO₃⁻]/[H₂CO₃] ratio ≈ 20:1. This calculator helps simulate such physiological buffers.
Given the acid dissociation constant Ka = [H⁺][A⁻]/[HA], taking negative logarithms: -log₁₀(Ka) = -log₁₀([H⁺]) - log₁₀([A⁻]/[HA]) → pKa = pH - log₁₀([A⁻]/[HA]). Rearranging gives the celebrated equation above. Our calculator automates this transformation, also converting Ka to pKa when provided.
The tool also computes [H⁺] = 10-pH, pOH = 14 - pH (at 25°C), and [OH⁻] = 10-pOH.
The Henderson–Hasselbalch equation assumes ideal behavior, neglecting activity coefficients, and is most accurate for dilute solutions (<0.1 M). For polyprotic acids, this equation applies to each dissociation step independently when the stepwise pKa values differ by at least 3 units. Temperature influences pKa; our calculator uses standard thermodynamic data (25°C) unless specified.
The graph plots pH against log₁₀(ratio) (range -3 to +3). The straight line (slope = 1) intersects the y‑axis at pH = pKa. The red marker shows your current buffer condition, helping visualize how pH shifts with changing ratio. Move the ratio or pKa and instantly see the point move along the theoretical curve.
| Acid / Conjugate pair | pKa | Application area |
|---|---|---|
| Acetic acid / Acetate | 4.76 | Biochemistry, food preservation |
| Carbonic acid (pKa₁) | 6.35 | Blood buffer, ocean acidification |
| Bicarbonate (pKa₂) | 10.33 | Alkaline buffers |
| Ammonium ion / Ammonia | 9.25 | Protein chemistry, wastewater |
| Phosphoric acid (pKa₂) | 7.21 | Biological buffers (PBS) |
| Tris (Tris‑HCl) | 8.07 | Molecular biology buffers |
To ensure drug stability and bioavailability, formulators adjust the pH of injectable solutions close to the pKa of the active ingredient. Using our calculator, researchers can compute the precise ratio of sodium salt to free acid needed to achieve target pH, minimizing irritation and degradation.
Natural water bodies contain carbonate buffers. Given the pKa₂ of bicarbonate (10.33), the calculator helps predict how pH responds to addition of acids or bases, essential for ecological impact assessment.