pH Calculator

How to Use the pH Calculator

The pH Calculator converts between pH values and hydrogen ion concentrations, helping you understand the acidity or basicity of solutions. It also calculates pOH, hydroxide ion concentration, and identifies whether a solution is acidic, neutral, or basic.

The pH Scale

pH ranges from 0 (most acidic) to 14 (most basic), with 7 being neutral. Each whole number change represents a 10-fold change in hydrogen ion concentration. A solution with pH 3 is 10 times more acidic than pH 4 and 100 times more acidic than pH 5.

Calculating pH

pH = -log₁₀[H⁺], where [H⁺] is the hydrogen ion concentration in moles per liter (M). For [H⁺] = 0.001 M: pH = -log(0.001) = -(-3) = 3. Conversely, [H⁺] = 10^(-pH). For pH 5.5: [H⁺] = 10⁻⁵·⁵ = 3.16 × 10⁻⁶ M.

pH and pOH Relationship

pH + pOH = 14 (at 25°C). If pH = 3, then pOH = 11. pOH = -log₁₀[OH⁻], where [OH⁻] is hydroxide ion concentration. Also, [H⁺] × [OH⁻] = 10⁻¹⁴ (the water autoionization constant Kw). The calculator shows all four values from any single input.

Common pH Values

Battery acid: pH 0. Lemon juice: pH 2. Vinegar: pH 2.5. Coffee: pH 5. Pure water: pH 7. Blood: pH 7.4. Baking soda: pH 9. Ammonia: pH 11. Bleach: pH 12.5. Drain cleaner: pH 14. The human body tightly regulates blood pH between 7.35 and 7.45.

Buffered Solutions

Buffers resist pH changes when small amounts of acid or base are added. They consist of a weak acid and its conjugate base. Blood is buffered by the carbonic acid/bicarbonate system. Understanding buffers is essential in biochemistry, environmental science, and industrial chemistry.

Frequently Asked Questions

Q: Can pH be negative or greater than 14?

A: Yes, in extreme cases. Concentrated hydrochloric acid can have pH below 0 (e.g., 10M HCl has pH ≈ -1). Concentrated sodium hydroxide can have pH above 14. The 0-14 range applies to dilute aqueous solutions at 25°C where water autoionization is the limiting factor.

Q: Why is pH important in everyday life?

A: pH affects taste (acidic foods are sour), cleaning power (alkaline cleaners dissolve grease), plant growth (most plants prefer pH 6-7), swimming pool safety (pH 7.2-7.8), aquarium health, brewing, and cooking. Soil pH determines which nutrients are available to plants.

Q: How does temperature affect pH?

A: Temperature changes the water autoionization constant Kw. At 25°C, neutral pH is 7. At 37°C (body temperature), neutral pH is about 6.8. At 60°C, neutral pH is about 6.5. The pH of a solution can change with temperature even without adding acids or bases. Most pH calculations assume 25°C.