The pH scale is a logarithmic scale that gives the acidity of a solution. The scale ranges from 1 to 14. Low pH's less than 7 indicate an acidic solution, pH = 7 is considered neutral, and pH's greater than 7 indicate a basic, or alkaline, solution. The pH scale is based on the hydrogen ion concentration and is given by: pH = -log(H), where H is the hydrogen ion concentration in moles per liter. Alternatively, H = 0.1pH = 10-pH.

The hydrogen ion concentration (H) for a solution with pH = 9 is given by: