The phosphate buffer system consists of dihydrogen phosphate ions (H2PO4-), which donates hydrogen ions, and hydrogen phosphate ions (HPO42-), which accepts hydrogen ions. Both ions are in equilibrium with each other. The phosphate buffer system maintains the pH of internal cellular fluid of mammalian cells within the range of 6.9 to 7.4. When the cellular fluid becomes more acidic, HPO42- reacts with the extra hydrogen ions to form H2PO4- and the equilibrium shifts to the left. When the cellular fluid becomes alkaline, the hydroxide ions react with H2PO4- to form HPO42- and the equilibrium shifts to the right.
The carbonic acid buffer system maintains pH at about 7.4 in blood plasma, and operates in the same way as the phosphate buffer system. It consists of the carbonic acid (H2CO3) and hydrogen carbonate (HCO3-) ions in equilibrium, where H2CO3 reacts with hydroxide ions in alkaline blood, or alkalosis, and HCO3- reacts with hydrogen ions in acidic blood, or acidosis. H2CO3 is regulated by respiration. The enzyme carbonic anhydrase catalyses the conversion of H2CO3 to dissolved carbon dioxide, which is excreted by the lungs. Hence H2CO3 is present in blood plasma at a concentration up to twenty times less than of HCO3-. HCO3- is regulated in the kidneys and excess is passed in urine.
Formic acid-formate is an acidic buffer and is used to maintain a pH of about 3.5 in urine samples tested for catecholamines. This protects the urine from auto-oxidation and reduces hydrolysis. The buffer also lowers the risk of accidental injury from acid preservatives during urine specimen collection, transportation and storage. The formic acid-formate buffer is also used in liquid chromatography and mass spectroscopy applications to maintain a low pH, as the buffer does not interfere with the ionization of the sample being analyzed.
Citric acid-sodium citrate buffer is used as antacid therapy to combat gastric acidity. Instead of neutralizing the gastric acid, the buffer regulates the pH, giving it a longer duration of action and preventing rebound gastric secretion. The buffer solution has a pH of 4.8 and maintains the gastric pH roughly between 3.80 and 3.94 for up to seven hours.