pH is a measure of the acidity or alkalinity of a solution on a logarithmic scale on which 7 is neutral, lower values are more acid, and higher values more alkaline. Pure water has a pH very close to 7 at 77°F (25°C). Sea water is generally in the pH range 7.5 to 8.4. The pH of seawater plays an important role in the ocean's carbon cycle, and there is evidence of ongoing ocean acidification caused by carbon dioxide emissions.