The negative logarithm of the hydrogen ion concentration in a solution, pH = log10 1/H+. If the hydrogen ion concentration of a solution increases, as happens with increasing acidity, the pH will decrease, and vice versa. The pH is measured on a scale of 0–14; a neutral medium (such as pure water) has a pH of 7.0, numbers above 7.0 indicate relative alkalinity, numbers below 7.0 indicate relative acidity. Most pH values in natural systems lie in the range 4.0–9.0. Human blood has a pH of 7.4, ocean water 8.1–8.3, water in saline environments may have a pH around 9.0 or higher, while for water in acidic soils it may be as low as 4.0 or less.