pH is a logarithmic scale of acidity and alkalinity, ranging from 0 for the most acidic substances to 14 to the most alkaline, or basic, substances. Substances that are neither acidic nor alkaline at 25 degrees centigrade, that is, neutral solutions such as pure water, usually have a pH of 7.
The pH value of any substance depends on its temperature, declining as the temperature rises. Thus, for example, the pH of pure water rises to about 7.47 at zero degrees and decreases to 6.14 at 100 degrees.
Examples of approximate pH values for common acidic substances include milk at 6.6, coffee at 5.0, orange juice at 3.0, lemon juice at 2.4, stomach acid at 1.0 and battery acid at zero. Examples of common basic substances include blood at 7.35 to 7.45, ocean water at 8.1, hand soap at 9,0 to 10, household ammonia at 11.5, bleach at 12.3, household lye at 13.5 and drain cleaner at 14.