pH Calculator
Calculate pH, pOH, H+ concentration, and OH- concentration. Convert between pH and hydrogen ion concentration with automatic classification of acidic, basic, or neutral solutions.
Enter the pH value. pH ranges from 0 (very acidic) to 14 (very basic), with 7 being neutral.
Enter a value above to calculate pH, pOH, and ion concentrations
Table of Contents
What is pH?
pH is a measure of the acidity or basicity of a solution. The pH scale ranges from 0 to 14, where:
- pH < 7: Acidic solution (higher H+ concentration)
- pH = 7: Neutral solution (equal H+ and OH- concentrations)
- pH > 7: Basic (alkaline) solution (lower H+ concentration, higher OH- concentration)
The pH scale is logarithmic, meaning each unit represents a 10-fold change in hydrogen ion concentration. For example, a solution with pH 3 has 10 times more H+ ions than a solution with pH 4.
pH is defined as the negative logarithm (base 10) of the hydrogen ion concentration:
pH = -log₁₀[H⁺]
Similarly, pOH measures the hydroxide ion concentration:
pOH = -log₁₀[OH⁻]
At 25°C (77°F), the relationship between pH and pOH is:
pH + pOH = 14
How it Works
Our pH calculator can calculate all pH-related values from any one input:
- From pH: Calculates H+ concentration, pOH, and OH- concentration
- From H+ concentration: Calculates pH, pOH, and OH- concentration
- From pOH: Calculates OH- concentration, pH, and H+ concentration
- From OH- concentration: Calculates pOH, pH, and H+ concentration
The calculator automatically:
- Converts between pH and H+ concentration using the logarithmic relationship
- Uses the pH + pOH = 14 relationship to calculate complementary values
- Classifies the solution as acidic, neutral, or basic
- Displays concentrations in both standard and scientific notation
All calculations assume standard conditions (25°C, 1 atm pressure) where the ion product of water (Kw) is 1.0 × 10⁻¹⁴.
Formulas
pH Calculation
pH = -log₁₀[H⁺]
Where [H⁺] is the hydrogen ion concentration in moles per liter (mol/L or M).
H+ Concentration from pH
[H⁺] = 10⁻ᵖᴴ
This is the inverse of the pH calculation.
pOH Calculation
pOH = -log₁₀[OH⁻]
Where [OH⁻] is the hydroxide ion concentration in moles per liter (mol/L or M).
OH- Concentration from pOH
[OH⁻] = 10⁻ᵖᴼᴴ
This is the inverse of the pOH calculation.
pH and pOH Relationship
pH + pOH = 14
This relationship holds true at 25°C, where the ion product of water (Kw = [H⁺][OH⁻]) equals 1.0 × 10⁻¹⁴.
Examples
Example 1: Pure Water (pH = 7)
Given: pH = 7.0
Calculation:
- [H⁺] = 10⁻⁷ = 0.0000001 M = 1.0 × 10⁻⁷ M
- pOH = 14 - 7 = 7.0
- [OH⁻] = 10⁻⁷ = 0.0000001 M = 1.0 × 10⁻⁷ M
Result: Neutral solution with equal H+ and OH- concentrations.
Example 2: Acidic Solution (pH = 3)
Given: pH = 3.0
Calculation:
- [H⁺] = 10⁻³ = 0.001 M = 1.0 × 10⁻³ M
- pOH = 14 - 3 = 11.0
- [OH⁻] = 10⁻¹¹ = 0.00000000001 M = 1.0 × 10⁻¹¹ M
Result: Acidic solution with high H+ concentration and low OH- concentration.
Example 3: Basic Solution (pH = 11)
Given: pH = 11.0
Calculation:
- [H⁺] = 10⁻¹¹ = 0.00000000001 M = 1.0 × 10⁻¹¹ M
- pOH = 14 - 11 = 3.0
- [OH⁻] = 10⁻³ = 0.001 M = 1.0 × 10⁻³ M
Result: Basic solution with low H+ concentration and high OH- concentration.
Example 4: From H+ Concentration
Given: [H⁺] = 0.0001 M = 1.0 × 10⁻⁴ M
Calculation:
- pH = -log₁₀(0.0001) = -log₁₀(10⁻⁴) = 4.0
- pOH = 14 - 4 = 10.0
- [OH⁻] = 10⁻¹⁰ = 1.0 × 10⁻¹⁰ M
Result: Acidic solution with pH = 4.0.
Common Use Cases
- Chemistry Education: Students learning about acids, bases, and pH calculations
- Laboratory Work: Preparing buffer solutions and adjusting pH in experiments
- Water Quality Testing: Analyzing pH levels in drinking water, pools, and environmental samples
- Agriculture: Testing and adjusting soil pH for optimal plant growth
- Food Science: Monitoring pH in food processing and preservation
- Aquaculture: Maintaining proper pH levels in fish tanks and aquatic systems
- Medical Applications: Understanding pH in biological systems and medical diagnostics
- Industrial Processes: Controlling pH in manufacturing and chemical processes
- Research: Analyzing pH-dependent reactions and chemical equilibria
Frequently Asked Questions
pH stands for "potential of Hydrogen" or "power of Hydrogen." It's a measure of the acidity or basicity of a solution based on the concentration of hydrogen ions (H+).
The pH scale is logarithmic because hydrogen ion concentrations in solutions vary over many orders of magnitude. A logarithmic scale makes it easier to work with these wide-ranging values. Each pH unit represents a 10-fold change in H+ concentration.
Yes, technically pH can be negative (for very concentrated acids) or greater than 14 (for very concentrated bases), but these are rare in practice. Our calculator accepts values from 0 to 14, which covers the vast majority of practical applications.
pH measures the concentration of hydrogen ions (H+), while pOH measures the concentration of hydroxide ions (OH-). They are inversely related: as pH increases, pOH decreases, and vice versa. At 25°C, pH + pOH = 14.
Yes, temperature affects the ion product of water (Kw), which changes the relationship between pH and pOH. At 25°C, pH + pOH = 14, but at other temperatures this value changes.
To convert pH to H+ concentration: [H+] = 10^(-pH). To convert H+ concentration to pH: pH = -log₁₀[H+]. Our calculator does these conversions automatically.
A neutral solution has equal concentrations of H+ and OH- ions. At 25°C, this occurs at pH = 7. Pure water is neutral, with [H+] = [OH-] = 1.0 × 10⁻⁷ M.
Yes, the calculator works for both strong and weak acids and bases. However, for weak acids and bases, you may need to account for incomplete dissociation.