Henderson Hasselbalch Equation

The Henderson Hasselbalch calculator computes the pH based on the dissociation constant, the acid concentration and the conjugate base concentration.

INSTRUCTIONS:  Enter the following:

• (pKa) Acid Dissociation Constant
• [A-] Conjugate Base Concentration
• [HA] Weak Acid Concentration

pH:  The calculator returns the pH

The Math / Science

The Henderson Hasselbalch Equation calculates the pH using the acidity (pKa) of a solution.  The formula for the Henderson Hasselbalch Equation is:

    pH = pKa + log₁₀([A-]/[HA])

where:

• pH = pH of buffer solution or buffer system
• pKa = dissociation constant
• HA = acid concentration
• A- = conjugate base concentration

Both concentration values are usually measured in moles/liter (mols/L), but the units cancel out in this equation to result in the unit-less value of pH. pH values are to be read on a scale of 1 to 14, with 1 being acidic and 14 being basic. A pH of 7 is considered to be neutral.

Uses

The Henderson Hasselbalch Equation is often used to calculate the pH of buffers and buffer systems. It is also used to find the isoelectric point of proteins, as well as the equilibrium pH in acid-base reactions. This equation can be used to calculate the pH of everything from water, to the blood in our bodies, and is essential to biochemistry.

Describes the relationship between pH and the extent of dissociation of a weak acid. When the pH of the solution of a weak acid is equal to the pK' of the acid, then 50% of the acid is in the undissociated form, HA, and 50% is in the dissociated form, H⁺ + A^-.

References

Wikipedia: Henderson-Hasselbalch equation