Question

What is the chemistry behind phosphate buffers in the human body? and what is the chemistry behind their use? Be sure to attach all sources used for full credit.

Answer 1

Phosphates are found in the blood in two forms which are sodium dihydrogen phosphate (Na2H2PO4−), which is a weak acid, and sodium monohydrogen phosphate (Na2HPO42-), which is a weak base. When Na2HPO42- comes into contact with the strong acid, such as HCl, the base picks up a second hydrogen ion to form the weak acid Na2H2PO4− and sodium chloride, NaCl. When Na2HPO42− (the weak acid) comes into contact with the strong base, such as sodium hydroxide (NaOH), the weak acid reverts back to the weak base and produces water. Acids and bases are still present, but they hold onto the ions.

HCl + Na2HPO4→NaH2PO4 + NaCl

(strong acid) + (weak base) → (weak acid) + (salt)

NaOH + NaH2PO4→Na2HPO4 + H2O

(strong base) + (weak acid) → (weak base) + (water)

The Phosphate buffer system operates in the internal fluids of all cells. It consists of dihydrogen phosphate ions as the hydrogen ion donor ( acid ) and hydrogen phosphate ion as the ion acceptor ( base ) . If additional hydroxide ions enter the cellular fluid then they are neutralised by the dihydrogen phosphate ion. If extra hydrogen ions enter the cellular fluid then they are neutralised by the hydrogen phosphate ion.

The phosphate buffer consists of phosphoric acid (H3PO4) in equilibrium with dihydrogen phosphate ion (H2PO4-) and H+. The pK for the phosphate buffer is 6.8, which allows this buffer to function within its optimal buffering range at physiological pH.the Phosphate buffer consists of inorganic and organic bound phosphate (i.e. esters of organic substances, e.g. AMP, ADP, and ATP). Phosphate buffer is important intracellular and urine buffer. In blood it accounts for only 5 % of buffering capacity.the phosphate buffer

is especially important in the tubular fluids of the kidneys, for two reasons: (1) phosphate usually becomes greatly concentrated in the tubules, thereby increasing the buffering power of the phosphate system, and (2) the tubular fluid usually has a considerably lower pH than the extracellular fluid does, bringing the operating range of the buffer closer to the pK (6.8) of the system.

The phosphate buffer system is also important in buffering intracellular fluid because the concentration of phosphate in this fluid is many times that in the extracellular fluid. Also, the pH of intracellular fluid is lower than that of extracellular fluid and therefore is usually closer to the pK of the phosphate buffer system compared with the extracellular fluid.

Source

Brown, Lemay, and Bursten. Chemistry: the Central Science, 7th ed. p. 631.

Vander, A. et al. Human Physiology, 6th ed. WCB McGraw-Hill, Boston, 1994, p. 463-466, 492-3, 552-6.

Zumdahl. Chem. Principles, 4th ed.p. 192-5, 208-214.

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