The pH Scale – what it measures and where it comes from

The pH scale measures how acidic or alkaline a water-soluble substance is, on a scale from 0 (very acidic) to 14 (very alkaline), with 7 considered neutral.

Where does the concept come from?

The term pH was coined in 1909 by Danish chemist Søren Sørensen, while working at Carlsberg's laboratory in Denmark, where he needed a simple way to measure acidity in beer during the brewing process. The letters stand for "potentia hydrogenii," roughly "power of hydrogen" in Latin.

How the scale works

The scale is logarithmic, meaning that each whole step corresponds to a tenfold change in acid or base concentration. A substance with pH 4 is thus ten times more acidic than a substance with pH 5, not just "a little" more acidic.

Everyday examples

• Stomach acid: approximately pH 1.5-3.5
• Lemon juice: approximately pH 2
• Coffee: approximately pH 5
• Pure water: pH 7 (neutral)
• Seawater: approximately pH 8
• Soap: approximately pH 9-10

The body's own pH

Human blood is maintained within a very narrow range, approximately pH 7.35–7.45, regulated by the body's own buffer systems (including through the lungs and kidneys). This value varies very little in practice in a healthy person, regardless of what they eat – it is one of the body's most carefully regulated parameters. However, pH values naturally vary between different parts of the body: the stomach is very acidic (approximately pH 1.5–3.5) to break down food, while the skin typically ranges around pH 4.5–5.5.

Measurement

pH can be measured with litmus paper, which changes color depending on the acidity of the solution – our own pH strips are an example – or with an electronic pH meter for more precise values.