pH - Potential of Hydrogen - measures the acidity or alkalinity of a substance. The lower the number, the more acidic it is. Higher numbers are more alkaline. The pH scale ranges from 0 (strong acid) to 14 (highly alkaline). 7 is neutral. It's important to know the scale jumps in multiples of 10. This means that a pH of 4.0 is ten time more acidic than 5.0, and one hundred times more acidic than 6.0.