Principle of Ph/ORP Analyzer The scale of pH is generally divided into two basic attributes: acid and base. An acid must have a free hydrogen ion. A base must have a free hydroxide ion. pH is the parameter that directly reacts hydrogen ions with hydroxide ions. If there are far more hydrogen ions than hydroxide ions, the solution is acidic. If there are far more hydroxide ions than hydrogen ions, the solution is basic. If the number of hydrogen ions is equal to the number of hydroxide ions, the solution is neutral.