The pH scale is a way of measuring acidic or basic substances, and this scale range from 0 to 14, and if the pH value is below 7, the substance will be considered to be acidic, therefore will be releasing H+ in the reaction, and if the pH value is over 7, the substance will be basic, therefore will be releasing OH- in the reaction. The pH for lemon usually is between 2 and 3, which means that is a very acidic fruit, and this acidity is mostly because of the Citric Acid, present in the lemon fruit in other fruits as well.