Hydroxide Ion Concentration Calculator

This calculator determines the hydroxide ion concentration ([OH⁻]) of a solution from its pH or pOH, and it also computes the corresponding pOH, pH, and hydrogen ion concentration. You enter either the pH or pOH value (or both, if you have them), and the calculator returns the missing values using the water dissociation constant at 25 °C. The result is given in moles per liter (mol/L), the standard unit for concentration in chemistry.

Formula

The hydroxide ion concentration is calculated from the pOH using the definition of pOH:

[OH⁻] = 10^−pOH

where [OH⁻] is in moles per liter (mol/L) and pOH is a dimensionless number. If you only have the pH, first find the pOH using the water dissociation relation at 25 °C:

pOH = 14 − pH

Then substitute that pOH into the first equation. The calculator also shows the hydrogen ion concentration using [H⁺] = 10^−pH, because the two concentrations are linked by the ion product of water: [H⁺] × [OH⁻] = 1.0 × 10⁻¹⁴ at 25 °C.

Worked Example

Using the default values (pH = 7, pOH = 7):

1. Start with the given pH: 7.0.
2. Calculate pOH = 14 − 7 = 7.0.
3. Calculate [OH⁻] = 10⁻⁷ = 1.00 × 10⁻⁷ mol/L.
4. Calculate [H⁺] = 10⁻⁷ = 1.00 × 10⁻⁷ mol/L.

The calculator returns [OH⁻] = 1.00 × 10⁻⁷ mol/L, which matches the known neutral water value. For a pH of 3 (acidic), pOH would be 11, and [OH⁻] = 1.00 × 10⁻¹¹ mol/L. For a pH of 12 (basic), pOH = 2, and [OH⁻] = 1.00 × 10⁻² mol/L.

When to Use This Calculator

Use this tool whenever you need the hydroxide ion concentration for a solution whose pH or pOH is known. Common situations include:

• Preparing buffer solutions in a lab or for a classroom experiment.
• Checking the alkalinity of water in aquariums, pools, or environmental samples.
• Solving textbook chemistry problems that ask for [OH⁻] from pH.
• Verifying the results of a titration or a pH meter reading.

The calculator is designed for aqueous solutions at 25 °C (room temperature). At other temperatures, the ion product of water changes, so the relation pOH = 14 − pH is not exact. For most general purposes, however, the 25 °C value is a reliable approximation.

How to Read the Result

The hydroxide ion concentration is expressed in moles per liter (mol/L), also written as M (molar). A higher [OH⁻] means a more basic (alkaline) solution. For reference:

• Neutral water: [OH⁻] = 1.0 × 10⁻⁷ mol/L
• Mildly basic (pH 8): [OH⁻] = 1.0 × 10⁻⁶ mol/L
• Strongly basic (pH 13): [OH⁻] = 0.1 mol/L

Notice that a change of 1 pH unit changes [OH⁻] by a factor of 10. So pH 9 has ten times the hydroxide concentration of pH 8. The calculator gives three decimal places, which is usually enough for lab work; if you need more precision, you can round manually.

Common Mistakes and Misconceptions

A frequent error is forgetting that pOH and pH are related by 14 only at 25 °C. At 0 °C, the sum is about 14.94; at 100 °C, it drops to about 12.26. If you are working at a different temperature, use the appropriate Kw value rather than 14.

Another mistake is mixing up pH and pOH when entering values. If you enter a pH of 10 but intend pOH = 10, the calculator will give a different [OH⁻] (10⁻¹⁰ vs. 10⁻⁴). Always double-check which value you have.

Some people also confuse hydroxide concentration with alkalinity. Alkalinity is a measure of the solution’s capacity to neutralize acid, usually reported in mg/L as CaCO₃, and it is not the same as [OH⁻]. This calculator only gives the concentration of free hydroxide ions, not the total alkaline species.

Limits of the Formula

The formula [OH⁻] = 10^−pOH is exact only for dilute aqueous solutions where activity coefficients are close to 1. In very concentrated solutions (above about 0.1 M), the effective concentration (activity) differs from the measured molarity, and the pH meter reading may not correspond exactly to the true ion concentration. For most educational, environmental, and routine lab purposes, the simple formula is sufficient.

Also, the calculator assumes the solution is purely aqueous and that no other reactions consume or produce hydroxide ions. In a buffered solution, the pH is held relatively constant, so the calculated [OH⁻] is still valid as the equilibrium concentration.

Frequently Asked Questions

How do I find hydroxide ion concentration from pH?
First calculate pOH = 14 − pH (at 25 °C), then take the antilog: [OH⁻] = 10^−pOH. For example, if pH = 9, pOH = 5, so [OH⁻] = 1 × 10⁻⁵ mol/L.

What is the hydroxide ion concentration of pure water?
At 25 °C, pure water has [OH⁻] = 1.0 × 10⁻⁷ mol/L, equal to [H⁺]. This is the neutral point where pH = pOH = 7.

Can I use this calculator for strong bases like NaOH?
Yes, for a strong base that fully dissociates, the [OH⁻] is equal to the base concentration. For example, a 0.01 M NaOH solution has [OH⁻] = 0.01 M, giving pOH = 2 and pH = 12. The calculator can work backwards from pH to find [OH⁻] if you know the pH.

Why does the calculator show [OH⁻] as a tiny number like 1e-11?
That is scientific notation for 1 × 10⁻¹¹ mol/L. Very small concentrations are common in acidic solutions. The calculator displays three decimal places, so 1e-11 appears as 0.000000000010, which is hard to read; the scientific notation is clearer.

Does temperature affect the hydroxide concentration?
Yes. The ion product of water (Kw) increases with temperature, so at higher temperatures the neutral point shifts. For instance, at 50 °C, Kw ≈ 5.5 × 10⁻¹⁴, so neutral pH is about 6.63, and pOH = 14 − pH no longer holds exactly. This calculator uses 25 °C values.

What is the difference between [OH⁻] and pOH?
[OH⁻] is the actual concentration in moles per liter, while pOH is a logarithmic scale: pOH = −log₁₀[OH⁻]. A change of 1 in pOH corresponds to a tenfold change in concentration. The calculator gives both so you can report either.

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in pH | pH value | - | 7 | 0
in pOH | pOH value | - | 7 | 0
out oh | Hydroxide ion [OH⁻] | mol/L | 10^(-pOH) | 3
out poh | pOH | - | 14 - pH | 2
out ph | pH | - | 14 - pOH | 2
out h | Hydrogen ion [H⁺] | mol/L | 10^(-pH) | 3