pH Calculator

Calculate pH, pOH, hydrogen ion concentration, and hydroxide ion concentration

Enter Known Value

Hydrogen ion concentration in mol/L (M)

Results

pH
7.00
Neutral Solution
pOH
7.00
[H+] Concentration
1.00 × 10⁻⁷ M
[OH-] Concentration
1.00 × 10⁻⁷ M

Understanding pH and How to Calculate It

pH is one of the most important concepts in chemistry, measuring the acidity or basicity of a solution. Understanding pH is crucial for countless applications, from biology and medicine to environmental science and industrial processes. The pH scale provides a convenient way to express hydrogen ion concentration and predict chemical behavior.

What is pH?

pH stands for "potential of hydrogen" and is defined as the negative logarithm (base 10) of the hydrogen ion concentration:

pH = -log₁₀[H+]

The pH scale typically ranges from 0 to 14, where:

  • pH < 7: Acidic solution (higher H+ concentration)
  • pH = 7: Neutral solution (pure water at 25°C)
  • pH > 7: Basic or alkaline solution (lower H+ concentration)

Why pH Matters

pH plays a critical role in numerous areas:

  • Biological systems: Most enzymes and cellular processes function within narrow pH ranges
  • Medicine: Blood pH must stay between 7.35-7.45 for survival
  • Environmental science: Ocean acidification and acid rain are tracked through pH
  • Agriculture: Soil pH affects nutrient availability and plant growth
  • Food industry: pH affects preservation, flavor, and safety
  • Water treatment: pH must be controlled for effective purification
  • Industrial chemistry: Many reactions require specific pH conditions

Understanding pOH

Similar to pH, pOH measures hydroxide ion concentration:

pOH = -log₁₀[OH-]

At 25°C, pH and pOH are related by the water dissociation constant:

pH + pOH = 14

Key pH Calculation Formulas

Here are the essential formulas for pH calculations:

  • pH from [H+]: pH = -log₁₀[H+]
  • [H+] from pH: [H+] = 10⁻ᵖᴴ
  • pOH from [OH-]: pOH = -log₁₀[OH-]
  • [OH-] from pOH: [OH-] = 10⁻ᵖᴼᴴ
  • Water equilibrium: [H+] × [OH-] = 1.0 × 10⁻¹⁴ at 25°C
  • pH and pOH relationship: pH + pOH = 14

Practical pH Calculation Examples

Example 1: Calculating pH from [H+]

If [H+] = 1.0 × 10⁻³ M:

  • pH = -log₁₀(1.0 × 10⁻³)
  • pH = -(-3)
  • pH = 3.0 (acidic solution)

Example 2: Calculating [H+] from pH

If pH = 11.0:

  • [H+] = 10⁻¹¹·⁰
  • [H+] = 1.0 × 10⁻¹¹ M (basic solution)

Example 3: Finding pH from [OH-]

If [OH-] = 1.0 × 10⁻⁴ M:

  • pOH = -log₁₀(1.0 × 10⁻⁴) = 4.0
  • pH = 14 - pOH = 14 - 4.0 = 10.0 (basic solution)

Example 4: Pure Water

At 25°C, pure water has:

  • [H+] = [OH-] = 1.0 × 10⁻⁷ M
  • pH = pOH = 7.0 (neutral)

The pH Scale in Everyday Life

Common substances and their approximate pH values:

  • Battery acid: pH 0-1 (extremely acidic)
  • Lemon juice: pH 2-3 (very acidic)
  • Vinegar: pH 2.5-3.5 (acidic)
  • Coffee: pH 4.5-5.5 (slightly acidic)
  • Pure water: pH 7.0 (neutral)
  • Blood: pH 7.35-7.45 (slightly basic)
  • Baking soda solution: pH 8.5-9.0 (basic)
  • Ammonia: pH 11-12 (very basic)
  • Drain cleaner: pH 13-14 (extremely basic)

Factors Affecting pH

Several factors influence the pH of solutions:

  • Temperature: Water's pH changes with temperature (pH 7 is neutral only at 25°C)
  • Dilution: Adding water to acids or bases changes their pH
  • Buffer capacity: Buffered solutions resist pH changes
  • Dissolved gases: CO₂ from air can lower the pH of water
  • Ionic strength: High salt concentrations affect pH measurements

pH Measurement Methods

pH can be measured using various techniques:

  • pH meters: Electronic instruments providing accurate digital readings (±0.01 pH units)
  • pH paper/strips: Color-changing indicators for quick estimates (±0.5 pH units)
  • Indicator solutions: Chemicals that change color at specific pH ranges
  • pH electrodes: Glass electrodes that generate voltage proportional to pH

Strong vs. Weak Acids and Bases

The pH calculation method depends on acid/base strength:

  • Strong acids: Completely dissociate; pH = -log₁₀[acid concentration]
  • Strong bases: Completely dissociate; calculate [OH-], then find pH
  • Weak acids: Partially dissociate; requires Ka (acid dissociation constant)
  • Weak bases: Partially dissociate; requires Kb (base dissociation constant)

Buffer Solutions

Buffers are solutions that resist pH changes when acids or bases are added. They consist of a weak acid and its conjugate base (or a weak base and its conjugate acid). The Henderson-Hasselbalch equation calculates buffer pH:

pH = pKa + log₁₀([A-]/[HA])

Buffers are essential in biological systems, maintaining stable pH despite metabolic activities.

Practical Applications

  • Swimming pools: Maintaining pH 7.2-7.8 for comfort and effective chlorination
  • Aquariums: Different fish species require specific pH ranges
  • Brewing: Mash pH affects enzyme activity and flavor development
  • Cosmetics: Skin-friendly products typically have pH 4.5-6.5
  • Pharmaceuticals: Drug stability and absorption depend on pH
  • Wastewater treatment: pH adjustment is crucial for treatment processes

Tips for pH Calculations

  • Remember that pH is logarithmic - each unit represents a 10-fold change in [H+]
  • Lower pH means higher acidity (more H+ ions)
  • At 25°C, [H+] × [OH-] always equals 1.0 × 10⁻¹⁴
  • pH can be negative or greater than 14 for very concentrated solutions
  • Always consider temperature when interpreting pH values
  • Use scientific notation for very small concentrations

Common pH Calculation Mistakes

  • Confusing [H+] with pH (one is concentration, the other is logarithmic scale)
  • Forgetting the negative sign in pH = -log₁₀[H+]
  • Using wrong base for logarithm (must be base 10, not natural log)
  • Assuming pH 7 is always neutral (only true at 25°C)
  • Not accounting for partial dissociation of weak acids/bases

Whether you're a student learning acid-base chemistry, a scientist conducting research, a healthcare professional monitoring patient pH, or someone maintaining a pool or aquarium, our pH calculator provides quick and accurate conversions between pH, pOH, and ion concentrations. Simply enter any known value, and the calculator instantly computes all related parameters.