What does pH actually measure?

The negative base-10 logarithm of the hydrogen ion concentration in moles per litre. Because it is logarithmic, each whole unit is a tenfold change — pH 4 has a hundred times the hydrogen ion concentration of pH 6.

How are pH and pOH related?

They always add to 14 at 25 °C, because the ion product of water fixes the relationship between hydrogen and hydroxide concentrations. A solution at pH 3 therefore has pOH 11. The constant shifts slightly at other temperatures.

How do I calculate the pH of a weak acid?

Take the square root of Ka times the concentration to get the hydrogen ion concentration, then take the negative logarithm. The approximation holds while dissociation stays under about 5% of the starting concentration.

What is the Henderson-Hasselbalch equation used for?

Finding the pH of a buffer. It is pH equals pKa plus the logarithm of the ratio of conjugate base to acid, so a buffer with equal amounts of both sits exactly at its pKa — which is where it resists pH change best.

Are my concentrations or constants transmitted?

No. Every logarithm and equilibrium calculation runs in your browser, and nothing you type is sent anywhere.

pH Calculator

Convert between pH, pOH and ion concentration, and find the pH of a weak acid, a weak base or a buffer.

The pH Calculator runs entirely in your browser. Concentrations and equilibrium constants you enter are used only for the calculation on your device.

Open the Molarity Calculator

About pH Calculator

pH Calculator works in three modes. The first converts freely between pH, pOH, hydrogen ion concentration and hydroxide concentration — supply any one and the other three follow, since pH is just the negative base-10 logarithm of the hydrogen ion concentration and pH plus pOH is 14 at 25 °C. The second handles weak acids and bases, where the concentration alone is not enough because dissociation is partial: supply Ka or Kb and the molar concentration and the standard square-root approximation gives the pH, with a warning when dissociation runs past 5% and the approximation stops holding. The third applies the Henderson-Hasselbalch equation to buffers.

Features

Converts freely between pH, pOH, hydrogen and hydroxide concentration
Weak acid and weak base pH from Ka or Kb and concentration
Buffer pH from the Henderson-Hasselbalch equation
Flags when the weak-acid approximation exceeds 5% dissociation
Shows pKa and pKb alongside the constant you entered
Accepts scientific notation such as 1.8e-5
Acidity band label and a visual position on the 0-14 scale
Allows pH outside 0-14 rather than clamping concentrated solutions

How to use the pH Calculator

Pick a mode: conversion, weak acid or base, or buffer
Enter the value you know — pH, a concentration, or a constant plus concentration
Read the pH along with pOH and both ion concentrations
Check the dissociation warning if you are working with a dilute weak acid

Example

Input

0.1 M acetic acid, Ka = 1.8e-5

Output

pH 2.87 · pOH 11.13 · [H+] 1.34e-3 M · 1.3% dissociated

A weak acid is far less acidic than a strong acid at the same concentration, which would give pH 1.

Common errors & troubleshooting

The pH of a weak acid seems too high. — That is the point of a weak acid — it only partially dissociates. 0.1 M acetic acid gives pH 2.87 while 0.1 M hydrochloric acid gives pH 1, almost a hundredfold difference in hydrogen ion concentration.
A warning appears about the approximation. — The square-root formula assumes dissociation is small compared with the starting concentration. Below about 5% it is accurate; above that you need the full quadratic, so treat the result as indicative.
The calculator returns a pH above 14 or below 0. — That is legitimate for concentrated solutions and is deliberately not clamped — 10 M hydrochloric acid really does have a negative pH on this definition.
Entering a concentration of zero gives nothing. — The logarithm of zero is undefined, so concentrations must be greater than zero. A truly zero concentration is pure water, which is pH 7.

Frequently asked questions

What does pH actually measure?: The negative base-10 logarithm of the hydrogen ion concentration in moles per litre. Because it is logarithmic, each whole unit is a tenfold change — pH 4 has a hundred times the hydrogen ion concentration of pH 6.
How are pH and pOH related?: They always add to 14 at 25 °C, because the ion product of water fixes the relationship between hydrogen and hydroxide concentrations. A solution at pH 3 therefore has pOH 11. The constant shifts slightly at other temperatures.
How do I calculate the pH of a weak acid?: Take the square root of Ka times the concentration to get the hydrogen ion concentration, then take the negative logarithm. The approximation holds while dissociation stays under about 5% of the starting concentration.
What is the Henderson-Hasselbalch equation used for?: Finding the pH of a buffer. It is pH equals pKa plus the logarithm of the ratio of conjugate base to acid, so a buffer with equal amounts of both sits exactly at its pKa — which is where it resists pH change best.
Are my concentrations or constants transmitted?: No. Every logarithm and equilibrium calculation runs in your browser, and nothing you type is sent anywhere.

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