What does the pH scale measure?

pH measures the acidity or alkalinity of a water-based solution on a scale of 0 to 14. It is defined as the negative base-10 logarithm of the hydrogen-ion concentration, pH = -log10[H+]. Lower values are more acidic, higher values more basic.

At 25 degrees Celsius pure water self-ionises so that hydrogen and hydroxide ions are equal at 1e-7 mol/L each, giving pH 7. The neutral point shifts slightly with temperature, so very hot water can have a neutral pH below 7.

How much stronger is one pH unit?

The scale is logarithmic, so each whole unit represents a tenfold change in hydrogen-ion concentration. A solution at pH 3 has ten times more hydrogen ions than pH 4 and one hundred times more than pH 5.

How is pOH related to pH?

In water at 25 degrees Celsius pH and pOH always add up to 14, because the product of the hydrogen-ion and hydroxide-ion concentrations is fixed at 1e-14. So pOH = 14 minus pH.

Are the listed pH values exact?

No. They are typical approximate values. Real pH varies with temperature, concentration, dissolved gases and the specific sample, so treat the table as a guide rather than a precise measurement.

What is the pH Scale Reference Table?

Reference table of approximate pH values for 28 everyday acids and bases, with a built-in pH ↔ pOH ↔ hydrogen-ion concentration calculator and a substance filter. It runs free in your browser on Gera Tools, with nothing uploaded.

pH Scale Reference Table

Name: pH Scale Reference Table
Creator: Gera Tools
License: https://creativecommons.org/licenses/by/4.0/

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A quick map of the acidity scale

The pH scale runs from strongly acidic battery acid near 0 through neutral water at 7 to caustic lye near 14. This reference lists approximate pH values for common household and natural substances, sorted from most acidic to most basic, alongside a calculator that converts a pH value into its pOH and its hydrogen- and hydroxide-ion concentrations.

How it works

pH is the negative base-10 logarithm of the hydrogen-ion concentration:

pH  = -log10[H+]
[H+] = 10^(-pH)   mol/L
pOH = 14 - pH      (in water at 25 C)
[OH-] = 10^(-pOH)  mol/L

Because the relationship is logarithmic, the difference between two pH values is a power of ten in ion concentration. Moving from pH 5 to pH 2 is a 1,000-fold increase in acidity, not a threefold one. The calculator clamps input to the 0–14 range and reports both ion concentrations in scientific notation.

A worked example

Suppose you measure a sample at pH 4 — typical of black coffee or tomato juice.

[H+] = 10^(-4) = 0.0001 mol/L (1 × 10⁻⁴)
pOH = 14 − 4 = 10
[OH-] = 10^(-10) = 1 × 10⁻¹⁰ mol/L

Compare that to pH 7 (pure water): [H+] = 1 × 10⁻⁷, which is 1,000 times less concentrated. So coffee is roughly 1,000 times more acidic than pure water even though the pH numbers only differ by 3.

Substances across the scale

Understanding where everyday materials sit helps you interpret both lab readings and household chemistry.

Zone	Approximate pH	Examples
Strongly acidic	0–2	Battery acid, stomach acid
Moderately acidic	2–4	Lemon juice, vinegar, cola
Mildly acidic	4–6	Coffee, tomato juice, rain
Neutral	7	Pure water
Mildly basic	7–9	Blood, seawater, baking soda solution
Moderately basic	9–11	Milk of magnesia, antacids
Strongly basic	11–14	Bleach, ammonia, lye

Values are approximate; real samples vary with concentration, temperature and impurities.

Why the scale matters in practice

pH affects nearly every chemical and biological process. Food preservation relies on keeping pH below about 4.6 — the point below which botulinum toxin cannot form. Aquarium fish often require water within a narrow band, typically 6.5–7.5 for freshwater species. Soil pH between 6 and 7 keeps most nutrients chemically available to plant roots. And in analytical chemistry, a pH-electrode reading is meaningless without knowing the temperature, because the neutral point drifts as temperature changes.

Tips and notes

Acidic = below 7, neutral = 7, basic/alkaline = above 7.
Each whole pH unit is a 10× change in [H+].
Temperature shifts the neutral point: neutral water above 25 °C reads below 7.
Listed values are approximate and depend on concentration and sample source.
pH and pOH always sum to 14 in dilute aqueous solution at 25 °C.
The scale is often quoted as 0–14 but can extend outside those bounds for very concentrated strong acids or bases.