pH Calculator - Free Online Chemistry & Science Tool

pH Calculator

Calculate pH, [H⁺], pOH, and [OH⁻] using logarithmic relationships

Calculate pH

Calculate [H⁺]

Calculate pOH & [OH⁻]

Hydrogen Ion Concentration [H⁺]

M (mol/L)

µM

pH Value

pH scale: 0 (acidic) to 14 (basic), 7 = neutral

Input Type

pOH Value

Hydroxide Ion Concentration [OH⁻]

M (mol/L)

Common Solutions (Optional)

This will fill the appropriate field based on current calculation mode.

pH Scale Reference:

0-1
Strong Acid

2-3
Acidic

4-5
Weak Acid

6
Slightly Acidic

7
Neutral

8-9
Slightly Basic

10-11
Basic

12-14
Strong Base

pH Value

7.00

Neutral (Pure Water at 25°C)

7.00

[H⁺]

1.00×10⁻⁷ M

pOH

7.00

[OH⁻]

1.00×10⁻⁷ M

Neutral

Equal concentrations of H⁺ and OH⁻ ions

pH & pOH Formulas

pH = -log₁₀[H⁺]

pOH = -log₁₀[OH⁻]

pH: Negative logarithm of hydrogen ion concentration

pOH: Negative logarithm of hydroxide ion concentration

Relationship: pH + pOH = 14 (at 25°C)

Neutral: pH = 7, [H⁺] = [OH⁻] = 1×10⁻⁷ M

Acidic: pH < 7, [H⁺] > 1×10⁻⁷ M

Basic/Alkaline: pH > 7, [H⁺] < 1×10⁻⁷ M

People Also Ask

🧪 What is pH and how is it calculated?

pH = -log₁₀[H⁺]. Measures acidity: 0-6.9 = acidic, 7 = neutral, 7.1-14 = basic. Logarithmic scale: pH 3 is 10× more acidic than pH 4.

⚗️ How to convert between pH and [H⁺] concentration?

[H⁺] = 10^(-pH). Example: pH 3 → [H⁺] = 10⁻³ = 0.001 M. pH 8 → [H⁺] = 10⁻⁸ = 0.00000001 M.

🌡️ Why is pure water pH 7 at 25°C?

Water autoionizes: H₂O ⇌ H⁺ + OH⁻. At 25°C, [H⁺]=[OH⁻]=1×10⁻⁷ M. pH=-log(1×10⁻⁷)=7. Temperature changes Kw, affecting neutral pH.

📊 What's the difference between pH and pOH?

pH measures H⁺ ions (acidity), pOH measures OH⁻ ions (basicity). pH + pOH = 14 at 25°C. Low pH = high pOH, and vice versa.

🔬 How do pH indicators work?

Indicators are weak acids/bases with different colored conjugate forms. Color changes at specific pH ranges (phenolphthalein: colorless<8.2, pink>8.2).

🌍 What are real-world pH applications?

Agriculture (soil pH), medicine (blood pH 7.35-7.45), pools (7.2-7.8), food preservation, brewing, environmental monitoring, batteries.

What is pH?

pH (potential of Hydrogen) is a logarithmic scale used to specify the acidity or basicity of an aqueous solution. It measures the concentration of hydrogen ions [H⁺] in a solution. The pH scale typically ranges from 0 to 14, with 7 being neutral, values less than 7 being acidic, and values greater than 7 being basic.

Why is pH Important?

pH affects chemical reactions, biological functions, environmental systems, and industrial processes. It determines solubility of substances, enzyme activity, corrosion rates, and is critical in many biological systems including human blood which must maintain pH 7.35-7.45.

Key pH concepts:

Logarithmic scale: pH change of 1 = 10× change in [H⁺]
Temperature dependent: Neutral pH ≠ 7 at temperatures other than 25°C
Universal indicator: Mixture that shows different colors at different pH values
pH meter: Electronic device measuring voltage difference to determine pH
Buffers: Solutions that resist pH change when acid/base is added

How to Use This Calculator

This calculator finds all pH-related values when you know any one variable:

Three Calculation Modes:

Calculate pH: Enter [H⁺] concentration → Get pH, pOH, [OH⁻]
Calculate [H⁺]: Enter pH value → Get [H⁺], pOH, [OH⁻]
Calculate pOH/[OH⁻]: Enter pOH or [OH⁻] → Get all values

The calculator provides:

Four interconnected values: pH, [H⁺], pOH, [OH⁻]
Scientific notation: Automatically formats very small/large numbers
Acidity classification: Identifies as strong acid, weak acid, neutral, etc.
Visual pH scale: Shows where your value falls on 0-14 scale
Common solution presets: Reference values for everyday substances
Temperature note: Reminder that calculations assume 25°C

Common pH Values

Reference pH values of common substances at approximately 25°C:

Substance	pH Range	[H⁺] (mol/L)	Classification	Examples/Uses
Battery Acid	0-1	1.0-0.1	Strong Acid	Car batteries, dangerous
Stomach Acid	1.5-3.5	3×10⁻² - 3×10⁻⁴	Strong Acid	Digestion, HCl solution
Lemon Juice	2.0-2.5	1×10⁻² - 3×10⁻³	Acidic	Food, cleaning
Vinegar	2.5-3.5	3×10⁻³ - 3×10⁻⁴	Acidic	Cooking, cleaning
Orange Juice	3.3-4.2	5×10⁻⁴ - 6×10⁻⁵	Weak Acid	Beverage
Black Coffee	4.8-5.1	1.6×10⁻⁵ - 8×10⁻⁶	Weak Acid	Beverage
Pure Water (25°C)	7.0	1×10⁻⁷	Neutral	Reference point
Human Blood	7.35-7.45	4.5×10⁻⁸ - 3.5×10⁻⁸	Slightly Basic	Critical for life
Sea Water	7.8-8.3	1.6×10⁻⁸ - 5×10⁻⁹	Basic	Ocean chemistry
Baking Soda	8.3-8.6	5×10⁻⁹ - 2.5×10⁻⁹	Basic	Cooking, cleaning
Ammonia Solution	11.0-12.0	1×10⁻¹¹ - 1×10⁻¹²	Strong Base	Cleaning
Bleach	12.5-13.0	3×10⁻¹³ - 1×10⁻¹³	Strong Base	Disinfecting

pH Classification Guide:

Strong Acid (0-3): Battery acid, stomach acid
Moderate Acid (3-6): Vinegar, fruit juices, coffee
Neutral (7): Pure water
Weak Base (8-10): Sea water, baking soda, blood
Strong Base (11-14): Ammonia, bleach, lye

Common Questions & Solutions

Below are answers to frequently asked questions about pH calculations:

Calculation & Formulas

How to calculate pH from [H⁺] when concentration isn't 10⁻ⁿ?

For any [H⁺] concentration: pH = -log₁₀([H⁺])

Example Calculations:

[H⁺] = 0.0025 M = 2.5×10⁻³ M

pH = -log₁₀(0.0025) = -log₁₀(2.5×10⁻³)

pH = -[log₁₀(2.5) + log₁₀(10⁻³)]

pH = -[0.3979 + (-3)] = -[-2.6021] = 2.60

Our calculator handles all concentrations automatically.

Shortcut: For [H⁺] = a×10⁻ⁿ, pH ≈ n - log₁₀(a). Example: 3.2×10⁻⁵ → pH ≈ 5 - log(3.2) = 5 - 0.505 = 4.495 ≈ 4.50.

How to handle very small or very large pH values?

pH scale is theoretically infinite, but practically limited by water's autoionization and solvent properties:

Extreme pH Values:

Superacids: pH < 0 possible (e.g., 10 M HCl ≈ -1)

Superbases: pH > 14 possible (e.g., 10 M NaOH ≈ 15)

Practical limits: Most aqueous solutions: 0-14

Concentrated acids: 10 M HCl → [H⁺] = 10 M → pH = -1

Concentrated bases: 10 M NaOH → [OH⁻] = 10 M → pOH = -1 → pH = 15

Our calculator: Accepts any [H⁺] value and calculates corresponding pH, even negative or >14 values.

Practical Applications

Why is blood pH so critical (7.35-7.45)?

Human blood must maintain pH 7.35-7.45 for enzyme function, oxygen transport, and cellular processes:

Condition	Blood pH	[H⁺] Change	Effect
Normal	7.40	4.0×10⁻⁸ M	Optimal enzyme function
Acidosis	<7.35	>4.5×10⁻⁸ M	Respiratory distress, coma risk
Alkalosis	>7.45	<3.5×10⁻⁸ M	Muscle spasms, dizziness
Severe Acidosis	<7.00	>1×10⁻⁷ M	Life-threatening
Compatible with life	6.80-7.80	1.6×10⁻⁷ - 1.6×10⁻⁸	Extreme limits

Buffer systems: Blood uses bicarbonate (HCO₃⁻/H₂CO₃), phosphate, and protein buffers to maintain pH. Lungs remove CO₂, kidneys excrete H⁺. Small pH change = large physiological impact.

How is pH used in agriculture and gardening?

Soil pH affects nutrient availability, microorganism activity, and plant health:

Soil pH Requirements:

Acid-loving plants (pH 4.5-6.0): Blueberries, azaleas, rhododendrons, potatoes
Neutral plants (pH 6.0-7.5): Most vegetables, lawns, roses
Alkaline-tolerant (pH 7.0-8.0): Lilacs, clematis, asparagus
Nutrient availability: Iron available at low pH, phosphorus at neutral pH
Adjusting pH: Lime raises pH (less acidic), sulfur lowers pH (more acidic)

Measurement: Soil pH test kits or electronic meters. For potted plants, water pH also matters (6.0-7.0 ideal). Hydroponics requires precise pH control (5.5-6.5).

Science & Chemistry

How does temperature affect pH and Kw?

Water autoionization constant Kw changes with temperature, affecting neutral pH:

Temperature Dependence:

Temperature	Kw	Neutral pH	[H⁺] = [OH⁻]
0°C	1.15×10⁻¹⁵	7.47	3.39×10⁻⁸ M
25°C	1.00×10⁻¹⁴	7.00	1.00×10⁻⁷ M
37°C (body)	2.51×10⁻¹⁴	6.80	1.58×10⁻⁷ M
50°C	5.48×10⁻¹⁴	6.63	2.34×10⁻⁷ M
100°C	5.13×10⁻¹³	6.14	7.16×10⁻⁷ M

Kw = [H⁺][OH⁻] increases with temperature (endothermic reaction).

Implication: pH measurements should specify temperature. pH meters have temperature compensation. Biological systems evolved for specific temperature ranges.

What's the difference between strong and weak acids in pH calculation?

Strong acids fully dissociate, weak acids partially dissociate:

Acid Type	Dissociation	pH Calculation	Examples
Strong Acid	100%	[H⁺] = initial concentration pH = -log(initial)	HCl, HNO₃, H₂SO₄, HClO₄
Weak Acid	<100%	[H⁺] = √(Ka × initial) Use ICE table or approximation	CH₃COOH, H₂CO₃, HCN, HF
Strong Base	100%	[OH⁻] = initial concentration pOH = -log(initial), pH = 14-pOH	NaOH, KOH, Ca(OH)₂
Weak Base	<100%	[OH⁻] = √(Kb × initial) Similar to weak acid	NH₃, CH₃NH₂, C₆H₅NH₂

Weak acid example: 0.1 M acetic acid (Ka = 1.8×10⁻⁵)
[H⁺] = √(Ka × [HA]) = √(1.8×10⁻⁵ × 0.1) = √(1.8×10⁻⁶) = 1.34×10⁻³ M
pH = -log(1.34×10⁻³) = 2.87
(Strong acid at 0.1 M would be pH 1.0)