A few words about pH

pH is a measure of acidity and alkalinity. The pH scale goes from 0 to 14. At pH 7, the half way point, there is a balance between acidity and alkalinity. Such a solution is neutral.  

If the solution has excess acid, the balance is shifted so that the scale reads a number lower than 7 - the greater the acid strength, the lower the pH value.

Similarly, a solution that is alkaline has a pH that is higher than 7, and the greater the alkali strength, the higher the pH value - up to 14.

Each unit on the pH scale actually represents a 10 fold difference in acid activity. Thus, a pH 4 is ten times as acid as a pH 5 solution. A pH 3 solution, in turn has ten times the acid activity of the pH 4 solution - and 100 times the acidity of the pH 5 solution.