pH Calculator

1. Enter either the pH or the hydrogen ion concentration.
2. Leave the one you want to find blank.
3. Read the result, plus pOH and the hydroxide concentration.
4. The verdict shows whether the solution is acidic, neutral or basic.

Here is what each result means:

pH is a measure of how acidic or basic a water-based solution is. It is defined as the negative base-10 logarithm of the hydrogen ion concentration, so a solution with more hydrogen ions has a lower pH and is more acidic. The scale usually runs from 0 to 14, with 7 marking a neutral solution such as pure water at 25 degrees Celsius.

Because pH is a logarithm, each whole step on the scale represents a tenfold change in hydrogen ion concentration. A solution at pH 3 has ten times as many hydrogen ions as one at pH 4, and a hundred times as many as one at pH 5. This is why small pH differences matter so much in chemistry, biology and the environment.

Alongside pH sits pOH, the matching measure for hydroxide ions. In a dilute solution at 25 degrees Celsius, pH plus pOH always equals 14, so knowing one gives the other. Together, pH and pOH describe the balance between hydrogen and hydroxide ions that determines whether a solution behaves as an acid or a base.

It converts between pH and hydrogen ion concentration, then derives pOH and hydroxide concentration.

1. For pH, take the negative base-10 log of the hydrogen ion concentration.
2. For concentration, raise 10 to the power of minus the pH.
3. pOH is 14 minus the pH, and hydroxide concentration is 10 to the power of minus pOH.

pH uses logarithms; see the logarithm calculator and molarity calculator for concentrations.

A solution has a hydrogen ion concentration of 0.001 moles per liter.

Calculation: pH = -log10(0.001) = 3, which is acidic. Its pOH is 14 minus 3, which is 11, and its hydroxide concentration is 10 to the power of minus 11 moles per liter.

One definition, rearranged and extended for whatever you need.

All of these come from the single definition of pH plus the fact that pH and pOH add to 14 in water at 25 degrees Celsius. That is why entering just one value lets the calculator report all four quantities at once.

The pH scale spans the acids and bases we meet every day.

Most foods and drinks are mildly acidic, while cleaning products are often basic. Because the scale is logarithmic, battery acid is not just a few times stronger than coffee but many thousands of times higher in hydrogen ion concentration.

The Hydrogen Ion Concentration

More hydrogen ions means a lower pH and a more acidic solution, changing by one unit for every tenfold change.

Dilution

Adding water moves the pH toward 7, since it lowers the concentration of whichever ion is in excess.

Temperature

The neutral point is exactly 7 only at 25 degrees Celsius. At other temperatures neutral pH shifts slightly.

Chemistry Homework

Convert between pH, pOH and ion concentrations in an acid-base problem.

Laboratory Work

Check the acidity of a prepared solution or a titration.

Biology and Environment

Interpret the pH of soil, water, blood or aquariums.

1. Forgetting the Minus Sign

pH is the negative log of the concentration. Dropping the minus flips the sign of the answer.

2. Wrong Log Base

pH uses base-10 logarithms, not natural logs. Using ln gives a wrong value.

3. Concentration Units

The hydrogen ion concentration must be in moles per liter for a correct pH.

4. Assuming the Range Is Fixed

pH can go below 0 or above 14 for very concentrated solutions, though this is rare.

5. Ignoring Temperature

Neutral pH is 7 only at 25 degrees Celsius. It shifts a little at other temperatures.

The relationships are exact for the stated model; pH is shown to two decimals and concentrations use scientific notation when very small.

pH measures how acidic or basic a solution is. It is the negative base-10 logarithm of the hydrogen ion concentration. A concentration of 0.001 moles per liter gives a pH of 3, which is acidic.

Take the negative base-10 logarithm of the hydrogen ion concentration in moles per liter. For 0.001 moles per liter, that is minus log of 0.001, which is 3.

Raise 10 to the power of minus the pH. A pH of 3 gives a hydrogen ion concentration of 10 to the power of minus 3, which is 0.001 moles per liter.

pOH is the negative log of the hydroxide ion concentration. In water at 25 degrees Celsius, pH plus pOH always equals 14, so pOH is 14 minus the pH.

A pH below 7 is acidic, exactly 7 is neutral, and above 7 is basic or alkaline, for a solution at 25 degrees Celsius.

Because hydrogen ion concentrations span many orders of magnitude. A logarithm compresses that huge range into a simple scale, where each unit is a tenfold change in concentration.

Yes, for very concentrated strong acids or bases pH can fall below 0 or rise above 14. The 0 to 14 range covers most everyday solutions but is not a hard limit.

Yes. Neutral pH is exactly 7 only at 25 degrees Celsius. At higher temperatures the neutral point drops slightly, though a solution can still be neutral.

No. The calculation runs in your browser and nothing you enter is stored or sent anywhere, unless you choose Save, which keeps the result only on this device.

• pH (Wikipedia).
• Self-ionization of water (Wikipedia).
• Acids and bases (Maths Is Fun).