Buffer pH Calculator
Calculate the pH of any buffer solution using the Henderson-Hasselbalch equation. Enter pKa, weak acid concentration, and conjugate base concentration for instant results with breakdowns and charts.
About This Calculator
The Buffer pH Calculator helps chemistry students, researchers, and laboratory professionals calculate the pH of buffer solutions using the Henderson-Hasselbalch equation. A buffer solution resists changes in pH when small amounts of acid or base are added, making buffers essential in biological systems, chemical research, and industrial processes.
The calculator uses the equation pH = pKa + log([A⁻]/[HA]), where pKa is the negative logarithm of the acid dissociation constant, [A⁻] is the concentration of the conjugate base (salt), and [HA] is the concentration of the weak acid. The ratio [A⁻]/[HA] determines the pH — when the concentrations are equal, pH equals pKa. The tool also computes pOH (pOH = 14 − pH at 25°C) and the hydrogen ion concentration [H⁺] = 10−pH.
For effective buffering, choose a weak acid whose pKa is within ±1 of your target pH. Common buffer systems include acetate (pKa 4.76), phosphate (pKa 7.21), citrate (pKa 3.13–6.40), carbonate (pKa 6.35 and 10.33), and Tris (pKa 8.07). In biological systems, buffers maintain the pH of blood (7.35–7.45), cell culture media, and enzyme reaction mixtures.
Regional Notes
Global: The Henderson-Hasselbalch equation is universal in chemistry and biochemistry. The pKa values are temperature-dependent (typically at 25°C) and should be adjusted for the working temperature using standard reference tables.
Frequently Asked Questions
What is the Henderson-Hasselbalch equation?
The Henderson-Hasselbalch equation is pH = pKa + log([A⁻]/[HA]), where pKa is the acid dissociation constant, [A⁻] is the concentration of the conjugate base, and [HA] is the concentration of the weak acid. It is used to estimate the pH of buffer solutions.
What is a buffer solution?
A buffer solution is a mixture of a weak acid and its conjugate base (or a weak base and its conjugate acid) that resists changes in pH when small amounts of acid or base are added. Common examples include acetate buffer, phosphate buffer, and bicarbonate buffer in human blood.
How do I calculate buffer pH from pKa and concentrations?
Use the Henderson-Hasselbalch equation: pH = pKa + log([conjugate base]/[weak acid]). Enter the pKa value of your weak acid, the molar concentration of the weak acid [HA], and the molar concentration of the conjugate base [A⁻]. The calculator computes pH, pOH, [H⁺], and the [A⁻]/[HA] ratio.
What is a good buffer pH range?
A buffer works best when the desired pH is within ±1 of the buffer's pKa value. For example, acetate buffer has a pKa of 4.76, making it effective in the pH range of 3.76 to 5.76. For maximum buffering capacity, the concentrations of the acid and conjugate base should be roughly equal.
What are common buffer pKa values?
Common buffer pKa values at 25°C include: acetate 4.76, phosphate 2.12/7.21/12.67, citrate 3.13/4.76/6.40, carbonate 6.35/10.33, Tris 8.07, HEPES 7.48, and ammonia 9.25. Choose a buffer with a pKa close to your target pH for optimal performance.
What buffers are present in human blood?
Human blood contains four main buffer systems: bicarbonate buffer (carbonic acid/bicarbonate, pKa 6.35), hemoglobin buffer, phosphate buffer (dihydrogen phosphate/hydrogen phosphate, pKa 7.21), and protein buffers. These work together to maintain blood pH between 7.35 and 7.45.
How does temperature affect buffer pH?
Temperature changes can affect buffer pH because pKa values are temperature-dependent. For example, Tris buffer has a large temperature coefficient (−0.028 pH/°C), meaning its pH changes significantly with temperature. Always prepare buffers at the temperature they will be used.
What is buffer capacity?
Buffer capacity is the ability of a buffer solution to resist pH changes upon addition of acid or base. It depends on the absolute concentrations of the weak acid and conjugate base pair and their ratio. Maximum buffer capacity occurs when [HA] = [A⁻], i.e., when pH = pKa.