pH Calculator – Free Chemistry, Blood & Water pH
Use our free pH calculator to find pH for water, blood, chemistry solutions, and Ka-based acids quickly and accurately online. Calculate pH, pOH, [H+], [OH-], blood pH, and mix solutions with professional precision.
pH Value
7.00
Neutral
pOH Value
7.00
pH + pOH = 14 (at 25°C)
[H+] Concentration
1.0000e-7 M
Hydrogen ions
[OH-] Concentration
1.0000e-7 M
Hydroxide ions
pH is a measure of how acidic or basic (alkaline) a solution is. The pH scale ranges from 0 to 14, with 7 being neutral. Solutions with pH less than 7 are acidic, while those with pH greater than 7 are basic or alkaline. The term "pH" stands for "power of hydrogen" and represents the concentration of hydrogen ions (H+) in a solution.
The pH Scale Explained
- pH 0-3: Strongly acidic (battery acid, gastric acid)
- pH 3-6: Weakly acidic (lemon juice, vinegar, coffee)
- pH 7: Neutral (pure water at 25°C)
- pH 8-11: Weakly basic (seawater, baking soda)
- pH 11-14: Strongly basic (bleach, drain cleaner)
The pH scale is logarithmic, meaning each whole pH value represents a tenfold difference in acidity or alkalinity. For example, a solution with pH 3 is ten times more acidic than one with pH 4, and one hundred times more acidic than pH 5.
Our pH calculator uses fundamental chemistry formulas to provide accurate results for various pH-related calculations. Here's how each mode works:
pH Formulas Used
1. pH from Hydrogen Ion Concentration
pH = -log₁₀([H+])
Where [H+] is the molar concentration of hydrogen ions.
2. Hydrogen Ion from pH
[H+] = 10^(-pH)
Calculate hydrogen ion concentration from pH value.
3. pH and pOH Relationship
pH + pOH = 14 (at 25°C)
The sum of pH and pOH equals 14 in aqueous solutions at standard temperature.
4. pH from Ka (Weak Acids)
[H+] = √(Ka × C)
pH = -log₁₀([H+])
For weak acids, where Ka is the acid dissociation constant and C is the initial concentration.
5. Blood pH (Henderson-Hasselbalch)
pH = 6.1 + log₁₀([HCO₃⁻] / (0.03 × pCO₂))
Used in medical contexts to calculate blood pH from bicarbonate and CO₂ partial pressure.
Example 1: Calculate pH from [H+]
Problem:
Find the pH of a solution with [H+] = 1 × 10⁻⁴ M
Step 1: Use the pH formula
pH = -log₁₀([H+])
Step 2: Substitute the value
pH = -log₁₀(1 × 10⁻⁴)
Step 3: Calculate
pH = -(-4) = 4
Answer: pH = 4 (acidic solution)
Example 2: Calculate pH from Ka
Problem:
Find the pH of 0.1 M acetic acid solution (Ka = 1.8 × 10⁻⁵)
Step 1: Use the weak acid formula
[H+] = √(Ka × C)
Step 2: Substitute values
[H+] = √(1.8 × 10⁻⁵ × 0.1)
[H+] = √(1.8 × 10⁻⁶)
Step 3: Calculate [H+]
[H+] ≈ 1.34 × 10⁻³ M
Step 4: Calculate pH
pH = -log₁₀(1.34 × 10⁻³) ≈ 2.87
Answer: pH ≈ 2.87
Example 3: Mix Two Solutions
Problem:
Mix 100 mL of pH 3 solution with 100 mL of pH 5 solution
Step 1: Convert pH to [H+]
[H+]₁ = 10⁻³ = 0.001 M
[H+]₂ = 10⁻⁵ = 0.00001 M
Step 2: Calculate weighted average
[H+]mix = (0.001 × 100 + 0.00001 × 100) / 200
[H+]mix = 0.10001 / 200 = 0.000505 M
Step 3: Convert back to pH
pH = -log₁₀(0.000505) ≈ 3.30
Answer: Mixed pH ≈ 3.30
pH Scale
Key Values at 25°C
Neutral pH: 7.0
Pure Water: [H+] = [OH-] = 1×10⁻⁷ M
Kw (Ion Product): 1×10⁻¹⁴
pH + pOH: 14
Basic pH
Calculate pH, pOH, [H+], and [OH-] from any value
pH from Ka
Calculate weak acid pH using Ka constant
Blood pH
Educational blood pH from pCO₂ and HCO₃⁻
Solution Mixer
Mix two solutions and find final pH
pH is logarithmic - each unit represents a 10x change in [H+] concentration
Always check temperature - pH values assume 25°C unless specified
Use Ka calculator for weak acids - they don't fully dissociate
When mixing solutions, consider volumes for accurate results
🧪 Chemistry
Acid-base reactions, buffer preparation, titrations
🏥 Medicine
Blood gas analysis, drug formulation, diagnosis
🌊 Environmental
Water quality, soil testing, aquatic ecosystems
🍽️ Food Industry
Preservation, fermentation, quality control
🏊 Pool Maintenance
Water balance, chemical effectiveness, safety
Strong Acids (Complete Dissociation)
• HCl (Hydrochloric acid)
• H₂SO₄ (Sulfuric acid)
• HNO₃ (Nitric acid)
• HBr (Hydrobromic acid)
• HI (Hydroiodic acid)
Weak Acids (Partial Dissociation)
• CH₃COOH (Acetic acid)
• H₂CO₃ (Carbonic acid)
• HCOOH (Formic acid)
• C₆H₅COOH (Benzoic acid)
• HF (Hydrofluoric acid)
Ignoring Temperature
pH calculations assume 25°C - adjust for other temperatures
Mixing Strong & Weak
Use different methods for strong vs weak acid pH calculations
Forgetting Logarithm
pH is -log[H+], not just [H+] concentration
Volume in Mixing
Always account for total volume when mixing solutions
Pure water at 25°C has a pH of exactly 7.0, making it neutral. This occurs because water undergoes autoionization, where a small fraction of water molecules dissociate into hydrogen ions (H+) and hydroxide ions (OH-). At equilibrium, the concentrations are equal: [H+] = [OH-] = 1.0 × 10⁻⁷ M.
Factors Affecting Water pH
- Temperature: Higher temperatures slightly lower pH (more acidic) while water remains neutral
- Dissolved CO₂: Forms carbonic acid, lowering pH to around 5.5-6.0 in rainwater
- Minerals: Dissolved minerals can make water slightly alkaline (pH 7.5-8.5)
- Impurities: Acids or bases dissolved in water change pH significantly
Drinking water typically has pH between 6.5 and 8.5. Distilled water exposed to air has pH around 5.8 due to dissolved CO₂. Use our pH calculator to determine exact pH values for different water conditions.
When mixing two solutions with different pH values, the final pH depends on both the pH values and the volumes of each solution. Our two-solution mixer calculates this accurately by converting pH values to hydrogen ion concentrations, computing the weighted average, and converting back to pH.
Mixing Solution Principles
- Convert each pH to [H+] using: [H+] = 10^(-pH)
- Calculate total moles of H+ from each solution: moles = [H+] × volume
- Find total volume: V_total = V₁ + V₂
- Calculate new [H+]: [H+]_final = (n₁ + n₂) / V_total
- Convert back to pH: pH_final = -log([H+]_final)
Important Considerations
- This method works best for solutions without significant buffering capacity
- Mixing acids with bases may involve neutralization reactions
- Temperature should be the same for both solutions
- The calculation assumes ideal mixing with no volume change
Use our solution mixer tool to quickly calculate the pH when diluting acids, mixing laboratory reagents, or understanding pH changes in chemical processes.
⚠️ Medical Disclaimer
This blood pH calculator is designed for educational purposes only. It should never be used for medical diagnosis, treatment decisions, or replacing professional medical advice. Always consult qualified healthcare professionals for medical concerns.
Blood pH is tightly regulated between 7.35 and 7.45 through multiple buffering systems. The primary buffer system involves the equilibrium between carbon dioxide (CO₂), carbonic acid (H₂CO₃), and bicarbonate ions (HCO₃⁻). Our calculator uses the Henderson-Hasselbalch equation to demonstrate this relationship.
Henderson-Hasselbalch Equation
pH = pKa + log([HCO₃⁻] / [H₂CO₃])
pH = 6.1 + log([HCO₃⁻] / (0.03 × pCO₂))
Blood pH Disorders
- Acidosis (pH < 7.35): Can be respiratory (high pCO₂) or metabolic (low HCO₃⁻)
- Alkalosis (pH > 7.45): Can be respiratory (low pCO₂) or metabolic (high HCO₃⁻)
- Normal Range: Maintained by lungs (CO₂ removal) and kidneys (HCO₃⁻ regulation)
Understanding blood pH is crucial for medical students, nursing students, and anyone studying physiology or clinical chemistry. Use this calculator to learn how pCO₂ and bicarbonate affect blood pH in educational scenarios.
Ka (acid dissociation constant) measures how completely an acid dissociates in water. Our Ka calculator determines the pH of weak acid solutions using the acid's Ka value and initial concentration. This is essential for chemistry students studying equilibrium and acid-base chemistry.
Understanding Ka
For a weak acid HA that dissociates according to: HA ⇌ H+ + A⁻
The equilibrium expression is: Ka = [H+][A⁻] / [HA]
Simplified Calculation for Weak Acids
When Ka is small and concentration is not too dilute:
[H+] ≈ √(Ka × C)
pH = -log₁₀([H+])
Common Ka Values at 25°C
| Weak Acid | Formula | Ka Value |
|---|---|---|
| Hydrofluoric acid | HF | 6.8 × 10⁻⁴ |
| Formic acid | HCOOH | 1.8 × 10⁻⁴ |
| Acetic acid | CH₃COOH | 1.8 × 10⁻⁵ |
| Carbonic acid | H₂CO₃ | 4.3 × 10⁻⁷ |
| Benzoic acid | C₆H₅COOH | 6.3 × 10⁻⁵ |
Smaller Ka values indicate weaker acids. Strong acids have very large Ka values (essentially infinite) because they completely dissociate. Use our Ka calculator to solve homework problems and understand weak acid equilibria in chemistry courses.