Power of hydrogen, a term more commonly known as pH, is the measure of how concentrated hydrogen ions are in a solution. Neutral pH, like that of distilled water measures a 7. Anything below pH 7 is considered acidic; anything above is alkaline (or basic). For context, lemon juice is highly acidic with a pH of 2.2 while baking soda is alkaline with a pH of 9. (Two ‘natural’ products not meant for your face by the way, no matter what Pinterest tells you).