Science

pH Calculator

Calculate pH, pOH, [H⁺], and [OH⁻] from any one known value. Includes acid/base classification and a visual pH scale reference.

Quick Answer

pH = -log[H⁺]. pOH = 14 - pH. Enter any one value (pH, pOH, [H⁺], or [OH⁻]) and get all four calculated instantly.

pH Scale Reference

0
Battery Acid
1
Stomach Acid
2
Lemon Juice
3
Vinegar
4
Tomato Juice
5
Coffee
6
Milk
7
Pure Water
8
Sea Water
9
Baking Soda
10
Milk of Magnesia
11
Ammonia
12
Soapy Water
13
Bleach
14
Drain Cleaner
AcidicNeutralBasic

Calculate

Select your input type and enter the value. All other quantities are calculated automatically.

pH
7
pOH
7
[H⁺]
1.0000e-7 M
[OH⁻]
1.0000e-7 M
Neutral
Comparable to pure water or blood

About This Tool

The pH Calculator converts between all four related measurements of solution acidity and basicity: pH, pOH, hydrogen ion concentration [H⁺], and hydroxide ion concentration [OH⁻]. Enter any one of these values and the calculator instantly computes the other three, along with a classification of the solution as acidic, neutral, or basic. The visual pH scale provides instant context by highlighting where your solution falls relative to common substances.

Understanding the pH Scale

The pH scale was introduced by Danish chemist S.P.L. Sørensen in 1909 to simplify the expression of hydrogen ion concentrations, which can span many orders of magnitude. Rather than writing 0.00001 mol/L, we say pH 5. The logarithmic nature of pH means each unit represents a tenfold change. Moving from pH 7 to pH 4 represents a 1,000-fold increase in hydrogen ion concentration. This compression makes it practical to compare solutions ranging from extremely acidic (battery acid, pH ≈ 0) to extremely basic (drain cleaner, pH ≈ 14) on a single, manageable scale.

The Mathematics of pH

Four equations connect pH, pOH, [H⁺], and [OH⁻]: (1) pH = -log₁₀[H⁺], (2) pOH = -log₁₀[OH⁻], (3) pH + pOH = 14 (at 25°C), and (4) [H⁺] × [OH⁻] = 1.0 × 10⁻¹⁴ (the water autoionization constant, Kw). These relationships mean that knowing any single value determines all the others. The calculator applies these formulas and displays every result simultaneously.

Acid-Base Classification

Solutions are classified based on their pH at standard conditions (25°C, 1 atm): below 7 is acidic, exactly 7 is neutral, and above 7 is basic (alkaline). The calculator provides a more granular classification: strongly acidic (pH 0-3), moderately acidic (3-5), weakly acidic (5-6.5), neutral (6.5-7.5), weakly basic (7.5-9), moderately basic (9-11), and strongly basic (11-14). Each classification includes a comparison to familiar substances for intuitive understanding.

Biological and Environmental Significance

pH plays a critical role in biological systems. Human blood maintains a remarkably narrow pH range of 7.35 to 7.45 through the bicarbonate buffer system. Deviations of just 0.3 units can be life-threatening. Enzymes, which catalyze virtually all biochemical reactions, typically function within a narrow pH optimum. Pepsin in the stomach works best at pH 2, while trypsin in the small intestine requires pH 8. Soil pH determines nutrient availability for plants: most crops prefer pH 6.0 to 7.0. Environmental monitoring of water pH helps detect pollution and assess ecosystem health.

Temperature Dependence

The relationship pH + pOH = 14 is only exact at 25°C. At other temperatures, Kw changes: at 37°C (body temperature), Kw = 2.4 × 10⁻¹⁴, so pH + pOH = 13.6. At 60°C, Kw = 9.6 × 10⁻¹⁴. This means pure water at 37°C has a pH of about 6.8, even though it is still neutral (equal [H⁺] and [OH⁻]). This calculator uses the standard 25°C relationship, which is appropriate for most educational and laboratory purposes.

Frequently Asked Questions

What is pH?
pH is a logarithmic scale that measures the acidity or basicity of an aqueous solution. It is defined as pH = -log10[H+], where [H+] is the hydrogen ion concentration in moles per liter. The scale typically runs from 0 to 14 at 25 degrees Celsius: values below 7 are acidic, exactly 7 is neutral, and above 7 is basic (alkaline). Because it is logarithmic, each whole pH unit represents a tenfold change in hydrogen ion concentration. A solution with pH 3 is ten times more acidic than one with pH 4.
What is the relationship between pH and pOH?
At 25 degrees Celsius (standard temperature), pH + pOH = 14. This relationship comes from the water autoionization constant Kw = [H+][OH-] = 1.0 x 10^-14 at 25C. Taking the negative log of both sides gives pH + pOH = pKw = 14. So if you know the pH, you can find pOH by subtracting from 14, and vice versa. At different temperatures, Kw changes, so the sum of pH and pOH also changes (e.g., at 37C body temperature, Kw is about 2.4 x 10^-14).
How do I convert between pH and hydrogen ion concentration?
To convert pH to [H+]: use the formula [H+] = 10^(-pH). For example, pH 4 means [H+] = 10^-4 = 0.0001 mol/L. To convert [H+] to pH: use pH = -log10([H+]). For example, if [H+] = 3.5 x 10^-5 mol/L, then pH = -log10(3.5 x 10^-5) = 4.46. This calculator handles both conversions automatically — just select your input type and enter the value.
Can pH be negative or greater than 14?
Technically, yes. The pH scale of 0-14 applies to dilute aqueous solutions at 25C. Very concentrated strong acids can have negative pH values (for example, 10 M HCl has a theoretical pH of -1). Very concentrated strong bases can exceed pH 14. However, these extreme values are uncommon in practice, and the standard 0-14 scale covers the vast majority of chemical solutions encountered in labs and real-world applications. This calculator accepts values within the 0-14 range for pH and pOH.
Why is pH important in everyday life?
pH affects nearly everything in chemistry and biology. Blood must maintain a pH of 7.35-7.45 for survival (acidosis or alkalosis can be life-threatening). Soil pH determines which plants can grow (most prefer 6.0-7.0). Swimming pool water is maintained at pH 7.2-7.8 for comfort and effective chlorine disinfection. Food pH affects flavor, preservation, and safety (low-pH foods like pickles resist bacterial growth). Industrial processes like brewing, water treatment, and pharmaceutical manufacturing all require precise pH control.
What is a buffer solution and how does it relate to pH?
A buffer solution resists changes in pH when small amounts of acid or base are added. Buffers typically consist of a weak acid and its conjugate base (or a weak base and its conjugate acid) in roughly equal concentrations. The Henderson-Hasselbalch equation describes buffer pH: pH = pKa + log([A-]/[HA]). Common biological buffers include the bicarbonate system in blood (maintains pH ~7.4), phosphate buffers (pH ~7.2), and TRIS buffers used in molecular biology labs.

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