The pH value, commonly used for water measurements, is a measure of acidity and alkalinity, or the alkalis and bases present in a given solution. 

The pH value is generally given on a numerical scale of 0-14. Here, the value 7 represents neutrality. The numbers on the scale increase as alkalinity increases, while the numbers on the scale decrease as acidity increases. Each unit of change corresponds to a tenfold change in acidity or alkalinity. The pH is also equal to the negative logarithm of the hydrogen ion concentration or hydrogen ion activity. The pH values of some common solutions are listed in the table on the right.