You will please note that at neutrality the molarity of the hydronium ion is 10 -7 . The 7 plays a part in the pH scale by indicating neutrality. The scale reaches a maximum at 14. The pH of a solution may be determined by the use of an electronic instrument known as a pH meter, or through the use of chemical indicators. Acid-base indicators are dyes which undergo slight changes in molecular structure and colour when the pH value of the solution changes.
Specific colours correspond to specific pH values.
There is a list of acid-base indicators in the databook.