Laboratory manual of biological chemistry : with supplement / by Otto Folin.

  • Folin, Otto, 1867-1934.
Date:
[1934], ©1934
Catalogue details

Licence: Public Domain Mark

Credit: Laboratory manual of biological chemistry : with supplement / by Otto Folin. Source: Wellcome Collection.

    31/388 (page 17)
    It should be observed that, because of the omission of the minus sign, the pH increases as the [H+] becomes less. Conse¬ quently figures less than 7 signify an acid reaction, while the pH of alkaline solutions is greater than 7. Ionization of Weak Acids and Bases.—The equilibrium equation for the ionization of any weak acid may be derived from the Law of Mass Action in much the same way that the equation for water was obtained. To take a typical example, acetic acid (CH3COOII^H+-rCH3CO(>), we may write to begin with, exactly as in the case of water: 1 [H+HCH3COO-] [CH3COOH] K in this case is called the “dissociation constant” or “ionization constant” of acetic acid, and its magnitude is an indication of the strength of the acid. [CH3COOHJ signifies the concentration of undissociated acetic acid. The corresponding term in the equa¬ tion for water was [H2OJ, which was considered to be a con¬ stant, since the “concentration” of water, when nothing but water is present, can hardly be altered. But here we are dealing with a substance (acetic acid) dissolved in water, and its concentration may vary within very wide limits. Since K must remain constant, any change in one of the terms in the above equation must be compensated by a change in the others. If [CH3COO_] is increased (c.g., by the addition of sodium acetate, which dissociates into Na+ and CH3COO“), there must be an increase in [CLI3COOH] and a decrease in [LI+]. What happens is that the ionization of the acetic acid is re¬ pressed by some of the H+ and CH3COO- recombining to form undissociated acetic acid (H+T CH3COO--»CH3COOH). When alkali {c.g., sodium hydroxide) is added to a solution of acetic acid (as in the process of titration), sodium acetate is immediately formed, and here also the result is a mixture of acetic acid and sodium acetate. The [H+] at any stage of this titration can be found by means of the equation: [H+WCH.3COO-] [CH3COOHJ A 4 It is only with weak acids that this sort of equation holds. ^^ tor example, is not a constant.