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. Most real-world solutions have pH values between 0 and 14. 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. Our calculator assumes standard conditions (25°C) for simplicity.
To convert pH to H+ concentration: [H+] = 10^(-pH). To convert H+ concentration to pH: pH = -log₁₀[H+]. Our calculator does these conversions automatically when you enter either value.
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. The calculator assumes you're entering the actual H+ or OH- concentration in solution.