The pH level of lemon juice is 2.0. The pH scale is used to measure how acidic or alkaline/base a solution is. The pH scale runs from 0 to 14; a pH of 7 is neutral, a pH lower than 7 is acidic and higher than 7 is alkaline.
Lemon juice contains citric acid and has a low pH level, whereas pure water has a neutral pH of 7. Red cabbage juice contains a water-soluble pigment that is a natural pH indicator. The juice of a red cabbage changes color to indicate the acidity of a solution: red indicates acid and greenish-yellow indicates alkaline, while neutral solutions turn the juice purple.