pKa Calculator

pKa Calculator

When you drop an acid into water, something remarkable happens, it starts to dissociate, releasing protons and creating a delicate balance between the acid and its conjugate base. But not all acids behave the same way. Some practically fall apart in solution, while others hold onto their protons with a death grip. The pKa value is what tells you just how tight that grip is: a lower pKa means a stronger acid, one that’s more willing to give up its proton. Understanding pKa is essential for predicting acid strength, designing buffer solutions, and understanding biochemical systems where proton transfer is key.

This pKa calculator gives you two essential tools in one. The first mode converts between the acid dissociation constant (Ka) and pKa just enter one value and the other appears instantly. The second mode applies the Henderson-Hasselbalch equation, letting you solve for pH, pKa, or the ratio of conjugate base to acid when you have the other two values. Whether you’re a chemistry student grappling with acid-base equilibrium, a researcher designing buffer systems, or a biochemist analyzing enzyme kinetics, this calculator delivers accurate, instant results. All calculations run locally in your browser, keeping your data private.

How to Use the pKa Calculator

1. Choose your mode: “Ka ↔ pKa” for basic acid dissociation conversions, or “Henderson-Hasselbalch” for buffer calculations.

2. For Ka ↔ pKa mode: enter either the Ka value (in scientific notation, e.g., 1.8e-5) or the pKa value, the calculator computes the other automatically.

3. For Henderson-Hasselbalch mode: enter any two of the three variables: pKa, pH, or the [A⁻]/[HA] ratio and the calculator solves for the third.

4. Review the results with detailed interpretations of what the values mean.

5. Use the Copy button to save results or Share to send them to others.

How the pKa Calculator Formula Works

The calculator applies the fundamental relationships between acid dissociation constants and the Henderson-Hasselbalch equation. These are the cornerstones of acid-base chemistry and buffer calculations.

Formula: pKa = −log₁₀(Ka)

The pKa is the negative logarithm (base 10) of the acid dissociation constant. A lower pKa indicates a stronger acid. For example, acetic acid has Ka = 1.8×10⁻⁵, so pKa = −log(1.8×10⁻⁵) = 4.74.

Formula: Ka = 10^(−pKa)

Conversely, the Ka is 10 raised to the negative pKa. This conversion is essential for working with acid dissociation constants.

Formula: pH = pKa + log₁₀([A⁻]/[HA])

The Henderson-Hasselbalch equation relates the pH of a buffer solution to the pKa of the acid and the ratio of the concentrations of the conjugate base ([A⁻]) and the acid ([HA]). This equation is used when the ratio of base to acid is between 0.1 and 10, which is the buffer region.

The calculator solves for any missing variable when two are provided.

Worked Example: Converting Ka to pKa

Acetic acid has a Ka of 1.8×10⁻⁵. What is its pKa?

Step 1: Identify the known value

• Ka = 1.8×10⁻⁵

Step 2: Apply the formula
pKa = −log₁₀(1.8×10⁻⁵)

Step 3: Calculate
= −log₁₀(1.8) + (−log₁₀(10⁻⁵))
= −0.255 + 5
= 4.745

Step 4: Round to appropriate precision
pKa = 4.74

Interpretation: Acetic acid has a pKa of 4.74. This means it’s a weak acid, it only partially dissociates in water. In a buffer solution with equal concentrations of acetic acid and acetate, the pH would be 4.74.