pH Calculator
Calculate pH, pOH, hydrogen ion concentration [H⁺], and hydroxide ion concentration [OH⁻] with our free pH Calculator. Convert between pH, pOH, and ion concentrations using the fundamental relationships of aqueous chemistry. Perfect for chemistry students, lab work, and educational purposes.
What Is pH?
pH (potential of hydrogen) is a logarithmic scale used to specify the acidity or basicity of an aqueous solution. The pH scale ranges from 0 to 14, with 7 being neutral. Solutions with a pH less than 7 are acidic, while those with a pH greater than 7 are basic (alkaline). The pH is defined as the negative base-10 logarithm of the hydrogen ion concentration:
pH = -log₁₀[H⁺]
Where [H⁺] is the concentration of hydrogen ions in moles per liter (molarity). For example, a solution with [H⁺] = 1.0 × 10⁻⁷ M has a pH of 7.0 (neutral).
The pH and pOH of a solution are related by the equation pH + pOH = 14 at 25°C. This relationship allows you to calculate one from the other.
How the pH Calculator Works
This calculator uses the fundamental relationships of aqueous chemistry:
- pH from [H⁺]: pH = -log₁₀[H⁺]
- pOH from [OH⁻]: pOH = -log₁₀[OH⁻]
- pH + pOH = 14 at 25°C
- [H⁺] from pH: [H⁺] = 10⁻pH
- [OH⁻] from pOH: [OH⁻] = 10⁻pOH
- [H⁺] × [OH⁻] = 10⁻¹⁴ at 25°C
Simply select the mode (pH, pOH, [H⁺], or [OH⁻]), enter your value, and the calculator will instantly compute all the other values.
Common pH Values of Everyday Substances
| Substance | pH Range | Classification |
|---|---|---|
| Stomach Acid | 1.0 – 2.0 | Strongly Acidic |
| Lemon Juice | 2.0 – 2.5 | Acidic |
| Vinegar | 2.5 – 3.0 | Acidic |
| Orange Juice | 3.0 – 4.0 | Acidic |
| Tomato Juice | 4.0 – 4.5 | Mildly Acidic |
| Pure Water | 7.0 | Neutral |
| Human Blood | 7.35 – 7.45 | Mildly Basic |
| Seawater | 7.5 – 8.5 | Basic |
| Baking Soda | 8.5 – 9.0 | Basic |
| Milk of Magnesia | 10.0 – 11.0 | Strongly Basic |
| Ammonia | 11.0 – 12.0 | Strongly Basic |
| Bleach | 12.0 – 13.0 | Very Strongly Basic |
Applications of pH
- Chemistry & Biology: pH is essential for understanding acid-base reactions, enzyme function, and metabolic processes.
- Medicine: Blood pH is tightly regulated (7.35-7.45). Abnormal pH can indicate metabolic or respiratory disorders.
- Agriculture: Soil pH affects nutrient availability and plant growth. Most plants prefer slightly acidic to neutral soil.
- Environmental Science: pH of water bodies indicates pollution levels and aquatic ecosystem health.
- Food Science: pH controls food preservation, flavor, and safety.
❓ pH Calculator FAQ
What is pH?
pH is a measure of the acidity or basicity of an aqueous solution. It is defined as the negative base-10 logarithm of the hydrogen ion concentration: pH = -log₁₀[H⁺]. The pH scale ranges from 0 (strongly acidic) to 14 (strongly basic), with 7 being neutral.
What is the difference between pH and pOH?
pH measures the concentration of hydrogen ions [H⁺], while pOH measures the concentration of hydroxide ions [OH⁻]. They are related by the equation pH + pOH = 14 at 25°C.
How do I convert pH to [H⁺]?
[H⁺] = 10⁻pH. For example, if pH = 3.0, then [H⁺] = 1.0 × 10⁻³ M.
How do I convert [H⁺] to pH?
pH = -log₁₀[H⁺]. For example, if [H⁺] = 1.0 × 10⁻⁵ M, then pH = 5.00.
What is the relationship between pH and temperature?
The pH + pOH = 14 relationship is valid at 25°C. At other temperatures, the value of Kw (the ion product of water) changes, so pH + pOH ≠ 14. This calculator allows you to select different temperatures for more accurate calculations.
What is a neutral pH?
At 25°C, a neutral solution has a pH of 7.0, where [H⁺] = [OH⁻] = 1.0 × 10⁻⁷ M. Pure water is neutral.
What is the pH of pure water?
Pure water at 25°C has a pH of 7.0. However, pure water can have a pH slightly different from 7 if it is not at 25°C or if it has dissolved CO₂, which forms carbonic acid and lowers the pH.
What is a strong acid?
A strong acid is an acid that completely dissociates in water, releasing all of its hydrogen ions. Examples include hydrochloric acid (HCl), sulfuric acid (H₂SO₄), and nitric acid (HNO₃).
What is a weak acid?
A weak acid is an acid that only partially dissociates in water. Examples include acetic acid (CH₃COOH) and carbonic acid (H₂CO₃). The pH of a weak acid solution depends on its concentration and its acid dissociation constant (Ka).
What is the pH of blood?
Human blood plasma normally has a pH between 7.35 and 7.45, making it slightly basic. The body tightly regulates blood pH within this narrow range.
What is the pH of stomach acid?
Stomach acid typically has a pH between 1.0 and 2.0, making it strongly acidic. This acidity helps break down food and kill bacteria.
What is the Henderson-Hasselbalch equation?
The Henderson-Hasselbalch equation is used to calculate the pH of a buffer solution: pH = pKa + log([A⁻]/[HA]). It relates the pH of a solution to the pKa of the acid and the ratio of the concentrations of the conjugate base and acid.
How does this calculator handle weak acids and bases?
This calculator focuses on the fundamental conversions between pH, pOH, [H⁺], and [OH⁻]. For weak acid/base calculations involving Ka or Kb, please use a dedicated weak acid/base calculator.
What is the significance of the pH scale?
The pH scale is logarithmic, meaning each unit change represents a tenfold change in hydrogen ion concentration. For example, a solution with pH 3 has 10 times more H⁺ ions than a solution with pH 4.
How do I measure pH?
pH can be measured using a pH meter (electrochemical method) or pH indicator paper (colorimetric method). pH meters are more accurate and can measure pH to ±0.01 units.
What is the pH of acid rain?
Acid rain typically has a pH between 4.0 and 5.0. Normal rainwater has a pH of about 5.6 due to dissolved CO₂ forming carbonic acid.
What is the relationship between pH and pOH?
At 25°C, pH + pOH = 14. This relationship holds because the product of [H⁺] and [OH⁻] is always 1.0 × 10⁻¹⁴ in water at 25°C.
How do I convert pOH to [OH⁻]?
[OH⁻] = 10⁻pOH. For example, if pOH = 4.0, then [OH⁻] = 1.0 × 10⁻⁴ M.
Why is pH important in everyday life?
pH affects many aspects of daily life, including the taste of food and beverages, the effectiveness of cleaning products, the health of skin and hair, and the growth of plants. Understanding pH helps in making informed choices about products and lifestyle.