an essential condition for enzymes to function correctly. For example, in human blood a mixture of carbonic acid (HCO) and bicarbonate (HCO) is present in the plasma fraction; this constitutes the major mechanism for maintaining the pH of blood between 7.35 and 7.45. Outside this narrow range (7.40 ± 0.05 pH unit), acidosis and alkalosis metabolic conditions rapidly develop, ultimately leading to death if the correct buffering capacity is not rapidly restored. If the pH value of a solution rises or falls too much, the effectiveness of an enzyme decreases in a process, known as denaturation, which is usually irreversible. The majority of biological samples that are used in research are kept in a buffer solution, often phosphate buffered saline (PBS) at pH 7.4. In industry, buffering agents are used in fermentation processes and in setting the correct conditions for dyes used in colouring fabrics. They are also used in chemical analysis and calibration of pH meters. Simple buffering agents {| class="wikitable" ! Buffering agent !! pKa !! Useful pH range |- | Citric acid || 3.13, 4.76, 6.40 || 2.1–7.4 |- | Acetic acid || 4.8 || 3.8–5.8 |- | KH2PO4 || 7.2 || 6.2–8.2 |- | CHES || 9.3 || 8.3–10.3 |- | Borate || 9.24 || 8.25–10.25 |} For buffers in acid regions, the pH may be adjusted to a desired value by adding a strong acid such as hydrochloric acid to the particular buffering agent. For alkaline buffers, a strong base such as sodium hydroxide may be added. Alternatively, a buffer mixture can be made from a mixture of an acid and its conjugate base. For example, an acetate buffer can be made from a mixture of acetic acid and sodium acetate. Similarly, an alkaline buffer can be made from a mixture of the base and its conjugate acid. "Universal" buffer mixtures By combining substances with pKa values differing by only two or less and adjusting the pH, a wide range of buffers can be obtained. Citric acid is a useful component of a buffer mixture because it has three pKa values, separated by less than two. The buffer range can be extended by adding other buffering agents. The following mixtures (McIlvaine's buffer solutions) have a buffer range of pH 3 to 8. {| class="wikitable" ! 0.2 M Na2HPO4 (mL) ! 0.1 M citric acid (mL) ! pH |- | 20.55 | 79.45 | style="background:#ff0000; color:white"