pH

pH is the term used to describe the acidity or alkalinity of a solution. The pH scale is based on the number of hydrogen ions and is expressed in mmol/L. If a solution has a pH of 7 then it is considered to be a neutral solution such as water where the hydrogen + ions are present in equal concentration with the hydroxyl ions OH+.

A pH below 7 is an acid solution. This dissociates into H+ ions and OH+ ions, with more H+ ions than OH+ ions.

A pH greater than 7 is an alkaline solution. This dissociates into OH+ ions and H+ ions, with more OH+ ions than H+ ions. In the human body there needs to be a balance between intake and removal of H+ if normal body function is to be maintained.

Three systems regulate the acid–base balance:

• Buffers – metabolic
  
• Lungs – respiratory
  
• Kidney – metabolic.

In the event of abnormalities, these three systems function together in an attempt to compensate.

Buffers

The function of the buffers is to counteract changes to the pH by either increased absorption or release of H+. The carbonic acid–bicarbonate is the most important buffer system. Carbonic acid (H₂CO₃) is the weaker acid and sodium bicarbonate (NaHCO₃) is the weak base. In solution, dissociation occurs: