The pH scale is the measurement of the hydrogen ion (H ) concentration in an aqueous solution. It is used to measure the acidity or alkalinity of a solution. The pH scale ranges from 0 to 14. A neutral solution has a pH of 7; a solution with a pH greater than 7 is basic (or alkaline); and a solution with a pH less than 7 is acidic. The lower the pH is, the more acidic the solution is. As the pH scale is logarithmic, each whole number drop on the scale represents a tenfold increase in acidity (the concentration of H increases tenfold).