Convert pH, pOH, and hydrogen ion concentration with standard aqueous acid-base relationships at 25 degrees Celsius.
Calculate an acid-base value
Choose a conversion, then enter the known pH, pOH, or concentration.
About acid-base calculations
pH is a logarithmic measure related to hydrogen ion activity in a solution. In many introductory calculations, concentration is used as an approximation for activity. The relationship is pH equals the negative base-10 logarithm of hydrogen ion concentration in moles per liter. Reversing the relationship gives hydrogen ion concentration equal to ten raised to the negative pH. Because the scale is logarithmic, a change of one pH unit represents a tenfold change in concentration.
For dilute aqueous solutions at 25 degrees Celsius, pH and pOH add to 14. The pOH scale describes hydroxide ion concentration in the same logarithmic way. A neutral solution has pH 7 and pOH 7 under these standard conditions. A pH below 7 is acidic and a pH above 7 is basic, although neutrality changes with temperature because the ion product of water changes. This calculator uses 14 as the sum, so it should not be treated as temperature-corrected outside ordinary classroom conditions.
Suppose a solution has pH 3. Its estimated hydrogen ion concentration is 0.001 mole per liter, often written as 1 times 10 to the negative third power. A solution with hydrogen ion concentration 0.0001 mole per liter has pH 4. If the pH is 8.2, the corresponding pOH at 25 degrees Celsius is 5.8. These conversions are useful for checking laboratory exercises, solution preparation, environmental measurements, and general chemistry work.
Real solutions can depart from ideal concentration-based calculations. Ionic strength, solvent composition, temperature, calibration, and electrode condition can affect measured pH. Strong acids and bases may permit direct stoichiometric calculations, while weak acids, weak bases, buffers, and polyprotic systems require equilibrium constants and mass-balance equations. The Henderson-Hasselbalch equation is useful for some buffer problems but is not needed for the direct conversions provided here.
Use enough significant figures for the quality of the source measurement. Digits after the decimal point in pH correspond approximately to significant figures in concentration. Never use a calculated value as a substitute for proper measurement when safety, manufacturing quality, patient care, or regulatory compliance is involved. In clinical contexts, acid-base status also depends on carbon dioxide, bicarbonate, electrolytes, and physiology; this general chemistry calculator does not interpret blood gases or diagnose an acid-base disorder.
Acid-base calculation examples
These examples use ideal aqueous relationships at 25 degrees Celsius.
Known value
Calculated value
Method
pH 3
Hydrogen ions 0.001 mol/L
Ten raised to negative three.
Hydrogen ions 0.0001 mol/L
pH 4
Negative base-10 logarithm.
pH 8.2
pOH 5.8
Subtract pH from 14.
How to use the acid base calculator
Choose the conversion that matches the value you already know.
Enter the pH, pOH, or positive hydrogen ion concentration.
Select Calculate Acid-Base Result.
Report the answer with precision appropriate to the original measurement.
Acid base calculator FAQ
How do I convert pH to hydrogen ion concentration?
Raise 10 to the negative pH power. A pH of 3 therefore corresponds to approximately 0.001 mole per liter.
How do I calculate pH from concentration?
Take the negative base-10 logarithm of the hydrogen ion concentration. The concentration must be positive for the logarithm to be defined.
Why do pH and pOH add to 14?
At 25 degrees Celsius, the ion product of water leads to the familiar sum of 14. The exact sum changes with temperature.
Can pH be outside 0 to 14?
Concentrated or non-ideal solutions can have values outside the familiar classroom range. This calculator limits direct pH and pOH entries to the common aqueous range.
Can this interpret an arterial blood gas?
No, clinical acid-base interpretation requires pH, carbon dioxide, bicarbonate, and clinical context. Use an appropriate blood-gas tool and professional guidance.