Simple Expression Relates pH pKa and Buffer Concentration

The titration curves of acetic acid, H2PO-, and NH+ (Fig. 2-18) have nearly identical shapes, suggesting that these curves reflect a fundamental law or relationship. This is indeed the case. The shape of the titration curve of any weak acid is described by the Henderson-

Acetic acid HAc (CH3COOH)

Acetic acid HAc (CH3COOH)

Acetate (CH3COO-)

FIGURE 2-19 The acetic acid-acetate pair as a buffer system. The system is capable of absorbing either H+ or OH- through the reversibility of the dissociation of acetic acid. The proton donor, acetic acid (HAc), contains a reserve of bound H+, which can be released to neutralize an addition of OH- to the system, forming H2O. This happens because the product [H+][OH-] transiently exceeds Kw (1 X 10-14 m2). The equilibrium quickly adjusts so that this product equals 1 X 10-14 m2 (at 25 °C), thus transiently reducing the concentration of H + . But now the quotient [H+][Ac-] / [HAc] is less than Ka, so HAc dissociates further to restore equilibrium. Similarly, the conjugate base, Ac-, can react with H+ ions added to the system; again, the two ionization reactions simultaneously come to equilibrium. Thus a conjugate acid-base pair, such as acetic acid and acetate ion, tends to resist a change in pH when small amounts of acid or base are added. Buffering action is simply the consequence of two reversible reactions taking place simultaneously and reaching their points of equilibrium as governed by their equilibrium constants, KW and Ka.

Hasselbalch equation, which is important for understanding buffer action and acid-base balance in the blood and tissues of vertebrates. This equation is simply a useful way of restating the expression for the dissociation constant of an acid. For the dissociation of a weak acid HA into H+ and A-, the Henderson-Hasselbalch equation can be derived as follows:

Then take the negative logarithm of both sides:

Essentials of Human Physiology

Essentials of Human Physiology

This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.

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