A pKa of 68 Makes Phosphate a Good Buffer

The pKa for phosphate, H2PO4~ - H+ + HPO42-, is 6.8, close to the desired blood pH of 7.4, so phosphate is a good buffer. In the ECF, phosphate is present as inorganic phosphate. Its concentration, however, is low (about 1 mmol/L), so it plays a minor role in extracellular buffering.

Phosphate is an important intracellular buffer, however, for two reasons. First, cells contain large amounts of phosphate in such organic compounds as adenosine triphosphate (ATP), adenosine diphosphate (ADP), and creatine phosphate. Although these compounds primarily function in energy metabolism, they also act as pH buffers. Second, intracellular pH is generally lower than the pH of ECF and is closer to the pKa of phosphate. (The cytosol of skeletal muscle, for example, has a pH of 6.9.) Phosphate is, thus, more effective in this environment than in one with a pH of 7.4. Bone has large phosphate salt stores, which also help in buffering.

^ABLE~25^ Major Chemical pH Buffers in the Body

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  • uta
    What makes a compound a good buffer?
    8 years ago

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