pH Calculator
Calculate pH, pOH, Henderson-Hasselbalch equation & buffer solutions
🔬 pH Calculator from Hydrogen Ion Concentration
📏 pH Scale Reference
0 (Acidic)
3
7 (Neutral)
11
14 (Basic)
📐 Key pH Formulas
pH = -log₁₀[H⁺]
pOH = -log₁₀[OH⁻]
pH + pOH = 14 (at 25°C)
[H⁺] = 10^(-pH)
[OH⁻] = 10^(-pOH)
Kw = [H⁺][OH⁻] = 1.0 × 10⁻¹⁴ (at 25°C)
Henderson-Hasselbalch: pH = pKa + log([A⁻]/[HA])
pKa = -log₁₀(Ka)
pH Calculator - Complete Guide
This pH calculator helps you solve acid-base chemistry problems including pH, pOH, buffer solutions, and the Henderson-Hasselbalch equation.
Understanding pH
pH measures the acidity or basicity of a solution on a logarithmic scale from 0-14:
- pH < 7: Acidic (more H⁺ ions)
- pH = 7: Neutral (pure water at 25°C)
- pH > 7: Basic/Alkaline (more OH⁻ ions)
The pH and pOH Relationship
At 25°C, pH + pOH = 14. This comes from the water dissociation constant Kw = 1.0 × 10⁻¹⁴. As temperature changes, Kw changes, affecting this relationship.
Henderson-Hasselbalch Equation
This equation relates pH to the pKa and the ratio of conjugate base to weak acid:
- pH = pKa + log([A⁻]/[HA])
- When [A⁻] = [HA], pH = pKa
- Useful for buffer calculations and titration curves
Buffer Solutions
Buffers resist pH changes when small amounts of acid or base are added. They work best when:
- pH is within ±1 of the pKa
- Concentrations of acid and conjugate base are similar
- Total buffer concentration is adequate
Common pKa Values
- Acetic acid: 4.76
- Carbonic acid (pKa₁): 6.35
- Phosphoric acid (pKa₂): 7.20
- Tris buffer: 8.06
- Ammonium ion: 9.25
Frequently Asked Questions
How do I calculate pH from [H⁺]?
Use the formula pH = -log₁₀[H⁺]. For example, if [H⁺] = 1.0 × 10⁻³ M, then pH = -log(10⁻³) = 3.
What is the Henderson-Hasselbalch equation used for?
It's used to calculate the pH of buffer solutions, design buffers at specific pH values, and understand titration curves. It relates pH to the pKa and the ratio of conjugate base to acid.
Why does pH + pOH = 14?
This comes from the water dissociation equilibrium: Kw = [H⁺][OH⁻] = 10⁻¹⁴ at 25°C. Taking -log of both sides: pKw = pH + pOH = 14.
How do I make a buffer at a specific pH?
Choose an acid with pKa close to your target pH (within ±1). Use Henderson-Hasselbalch to calculate the ratio of conjugate base to acid needed, then prepare the solution.
Is this pH calculator free?
Yes! This pH calculator is completely free with no limitations or sign-up required.