pH Calculator
Calculate pH, pOH, hydrogen ion concentration [H⁺], and hydroxide ion concentration [OH⁻] instantly. Enter a known value to determine the acidity or basicity of any aqueous solution, then explore the complete educational guide below.
pH Calculator
Inputs
Enter a pH value or ion concentration to calculate acidity/basicity.
Understanding pH: The Measure of Acidity and Basicity
The term pH stands for "potential of hydrogen" and provides a quantitative measure of how acidic or basic an aqueous solution is. Developed by Danish chemist S.P.L. Sorensen in 1909, the pH scale has become one of the most important measurements in chemistry, biology, medicine, and environmental science. It is defined by the formula:
pH = -log₁₀[H⁺]This equation tells us that pH is the negative base-10 logarithm of the hydrogen ion concentration in moles per liter. Because the scale is logarithmic, each whole-number change in pH represents a tenfold change in hydrogen ion concentration. A solution with pH 3 has ten times more H⁺ ions than one with pH 4, and one hundred times more than a solution with pH 5.
The pH Scale: 0 to 14
At standard temperature (25 °C), the pH scale ranges from 0 to 14 for most practical purposes. Solutions are classified into three categories based on their pH value:
- Acidic (pH < 7): The solution contains more H⁺ ions than OH⁻ ions. Examples include gastric acid (pH 1.5), lemon juice (pH 2), and coffee (pH 5).
- Neutral (pH = 7): The concentrations of H⁺ and OH⁻ are equal, as in pure water at 25 °C.
- Basic / Alkaline (pH > 7): The solution contains fewer H⁺ ions than OH⁻ ions. Examples include baking soda solution (pH 9), ammonia (pH 11), and bleach (pH 13).
Strong vs. Weak Acids and Bases
Strong acids such as hydrochloric acid (HCl), sulfuric acid (H₂SO₄), and nitric acid (HNO₃) dissociate completely in water, meaning every molecule releases its hydrogen ion. This makes pH calculations straightforward: if you dissolve 0.01 mol of HCl in one liter of water, [H⁺] = 0.01 M and pH = 2.0. Weak acids like acetic acid (CH₃COOH) and carbonic acid (H₂CO₃) only partially dissociate. Calculating their pH requires the acid dissociation constant (Ka) and often involves solving a quadratic equation.
The same distinction applies to bases. Strong bases like sodium hydroxide (NaOH) and potassium hydroxide (KOH) fully dissociate, while weak bases like ammonia (NH₃) reach an equilibrium between their molecular and ionic forms.
pH in Everyday Life
pH plays a vital role far beyond the chemistry lab. Your blood must stay within a narrow pH range of 7.35 to 7.45 for enzymes and cellular processes to function properly. Soil pH determines which nutrients are available to plants: most crops thrive between pH 6.0 and 7.5. Swimming pools are maintained at pH 7.2 to 7.8 to prevent eye irritation and ensure chlorine works effectively. Even the taste and preservation of food depend on pH: yogurt has a pH around 4.5, which inhibits harmful bacteria, while soap operates at pH 9 to 10 to dissolve oils.
The pOH Relationship
While pH measures hydrogen ion concentration, pOH measures hydroxide ion concentration using an analogous formula: pOH = -log₁₀[OH⁻]. At 25 °C, these two values are linked by the water autoionization constant (Kw = 1.0 × 10⁻¹⁴):
pH + pOH = 14This relationship means that knowing any one of the four values (pH, pOH, [H⁺], or [OH⁻]) allows you to calculate all the others. Our pH calculator above does exactly that: enter one value and it computes the remaining three automatically.
pH of Common Substances
The following reference table lists approximate pH values of substances commonly encountered in daily life, laboratory work, and industrial applications. Values may vary slightly depending on concentration and temperature.
| Substance | Approximate pH | Category |
|---|---|---|
| Battery acid | 0 | Strong acid |
| Gastric acid (stomach) | 1.5 | Strong acid |
| Lemon juice | 2.0 | Acid |
| Vinegar | 2.4 | Acid |
| Orange juice | 3.5 | Acid |
| Tomato juice | 4.0 | Acid |
| Black coffee | 5.0 | Weak acid |
| Milk | 6.5 | Weak acid |
| Pure water | 7.0 | Neutral |
| Human blood | 7.4 | Slightly basic |
| Seawater | 8.1 | Weak base |
| Baking soda solution | 9.0 | Weak base |
| Milk of magnesia | 10.5 | Base |
| Ammonia solution | 11.0 | Base |
| Bleach (sodium hypochlorite) | 13.0 | Strong base |
| Drain cleaner (NaOH) | 14.0 | Strong base |
How to Calculate pH: 3 Worked Examples
Example 1: Finding pH from Hydrogen Ion Concentration
A chemist measures the hydrogen ion concentration of a vinegar sample and finds [H⁺] = 3.98 × 10⁻³ mol/L. What is the pH of this vinegar?
Step 1: Write the formula: pH = -log₁₀[H⁺]
Step 2: Substitute: pH = -log₁₀(3.98 × 10⁻³)
Step 3: Calculate: pH = -(-2.400) = 2.40
Result: The vinegar has a pH of 2.40, confirming it is acidic. Its pOH is 14 - 2.40 = 11.60.
Example 2: Finding [H⁺] from a Known pH
A swimming pool technician records the pool water pH as 7.4. What is the hydrogen ion concentration?
Step 1: Rearrange: [H⁺] = 10⁻ᵖᴴ
Step 2: Substitute: [H⁺] = 10⁻⁷·⁴
Step 3: Calculate: [H⁺] = 3.98 × 10⁻⁸ mol/L
Result: The pool water contains 3.98 × 10⁻⁸ moles of H⁺ per liter, which is slightly basic and within the ideal pool range.
Example 3: Calculating pOH and All Values from pH
An environmental scientist measures rainwater and finds pH = 5.6. Determine pOH, [H⁺], and [OH⁻].
Step 1: pOH = 14 - pH = 14 - 5.6 = 8.4
Step 2: [H⁺] = 10⁻⁵·⁶ = 2.51 × 10⁻⁶ mol/L
Step 3: [OH⁻] = 10⁻⁸·⁴ = 3.98 × 10⁻⁹ mol/L
Result: Normal rain is slightly acidic (pH 5.6) due to dissolved carbon dioxide forming carbonic acid. Rain with pH below 5.0 is considered acid rain.
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