The hydrogen ion concentration determines if a compound is an acid or a base. A compound that is an acid increases its hydrogen ion concentration when dissolved in water. A base on the other hand decreases the hydrogen ion concentration of pure water when it is dissolved in it.
The pH is a measure of the H+ ions in a liquid. It is calculated as – log (aH+), where aH+ denotes the activity of hydrogen atoms. An acid HX would dissociate into the ions H+ and X -. This increases the H+ concentration of water it is dissolved in. A base XOH would dissociate into ions of the form X+ and OH-. This would decrease the H+ concentration of water it is dissolved in.
The pH of pure water is taken to be 7. Acids have a pH lesser than 7 and a base has a pH greater than 7.