pH — the logarithm of the proton

pH = −log₁₀[H⁺]: acidity is literally a logarithm, so each unit is a tenfold change in proton concentration — pH 4 is ten times more acidic than pH 5, a hundred times more than pH 6. Sealed in-domain: pH 4.0 classifies as acid (https://narrowhighway.com/s/54db514fbdd51c90cad455bc773f8152da3b3eb6a53119da6ddb670c9464e23b), and −log₁₀(10⁻⁴) = 4 sealed on the logarithm card. This is chemistry borrowing the grammar of the senses: like decibels and star magnitudes, pH compresses a vast multiplicative range into a small readable number. Blood is held at 7.4 within a few hundredths — the log scale shows how fiercely that must be defended.