Hydroxide Ion Concentration Calculator

Calculate the hydroxide ion concentration (OH⁻) in a solution based on pH or pOH values. This tool helps determine the basicity of aqueous solutions for chemistry applications.

How to Use This Calculator

Select whether you're providing a pH or pOH value
Enter the pH or pOH value of your solution
Optionally, specify the temperature (default is 25°C)
Click Calculate to see the hydroxide ion concentration

Formula Used

[OH⁻] = 10^(-pOH) = 10^(-14 + pH) (at 25°C)

Where:

[OH⁻] = Hydroxide ion concentration in mol/L
pOH = Negative logarithm of hydroxide ion concentration
pH = Negative logarithm of hydrogen ion concentration
Kw = Water ion product constant (1.0 × 10⁻¹⁴ at 25°C)

Example Calculation

Real-World Scenario:

A chemist is working with a sodium hydroxide solution and needs to determine its hydroxide ion concentration based on the measured pH value of 12.5.

Given:

pH = 12.5
Temperature = 25°C

Calculation:

pOH = 14 - pH = 14 - 12.5 = 1.5

[OH⁻] = 10^(-pOH) = 10^(-1.5)

[OH⁻] = 3.16 × 10⁻² mol/L

Result: The hydroxide ion concentration is 3.16 × 10⁻² mol/L, indicating a strongly basic solution.

Why This Calculation Matters

Practical Applications

Determining solution basicity in chemical reactions
Calculating neutralization requirements in wastewater treatment
Analyzing buffer capacity in biochemical systems

Key Benefits

Quick conversion between pH/pOH and actual ion concentrations
Essential for understanding chemical equilibrium
Helps predict chemical behavior in various environments

Common Mistakes & Tips

The water ion product constant (Kw) changes with temperature. At temperatures other than 25°C, the relationship pH + pOH = 14 is not exact. For precise calculations at different temperatures, use the appropriate Kw value for that temperature.

Remember that pH measures hydrogen ion concentration while pOH measures hydroxide ion concentration. In aqueous solutions at 25°C, pH + pOH = 14. A high pH (greater than 7) indicates a basic solution with high hydroxide ion concentration, while a low pH (less than 7) indicates an acidic solution with low hydroxide ion concentration.

Frequently Asked Questions

In aqueous solutions at 25°C, pH + pOH = 14. This relationship comes from the water ion product constant (Kw = [H⁺][OH⁻] = 1.0 × 10⁻¹⁴ at 25°C). As the temperature changes, this relationship slightly changes because Kw changes with temperature.

Temperature affects the water ion product constant (Kw). As temperature increases, Kw increases, meaning that for a given pH, the hydroxide ion concentration will be different at different temperatures. For example, at 50°C, Kw ≈ 5.5 × 10⁻¹⁴, so pH + pOH ≈ 13.26 rather than 14.

A neutral solution has equal concentrations of hydrogen ions (H⁺) and hydroxide ions (OH⁻). At 25°C, a neutral solution has [H⁺] = [OH⁻] = 1.0 × 10⁻⁷ mol/L, which corresponds to pH = 7 and pOH = 7. At different temperatures, the neutral point shifts slightly due to changes in Kw.

References & Disclaimer

Scientific Disclaimer

This calculator provides estimates based on standard chemical principles. For precise laboratory work or critical applications, always verify results with experimental measurements and consider activity coefficients in concentrated solutions.

References

ChemGuide: The pH Scale - Explanation of pH, pOH, and their relationship
LibreTexts Chemistry: Autoionization of Water - Detailed explanation of Kw and its temperature dependence
NCBI Bookshelf: Acid-Base Balance - Physiological importance of pH and hydroxide ion concentrations

Accuracy Notice

This calculator assumes ideal behavior and does not account for activity coefficients in concentrated solutions. For solutions with ionic strength greater than 0.1 M, experimental measurements are recommended for accurate determination of hydroxide ion concentrations.

About the Author

Kumaravel Madhavan

Web developer and data researcher creating accurate, easy-to-use calculators across health, finance, education, and construction and more. Works with subject-matter experts to ensure formulas meet trusted standards like WHO, NIH, and ISO.

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