Biochemistry For Dummies. John T. Moore

ion is the conjugate acid (CA), and the species with one less hydrogen ion is the conjugate base (CB).

      

You can express the equilibrium from the acetate example, like all equilibria, by using a mass-action expression — as long as a balance among the species is present. This expression is also known as a reaction quotient or an equilibrium constant. For acetic acid, this expression is as follows:

      The a subscript means that this expression represents an acid. The square brackets refer to the molar equilibrium concentrations of the species present. You can express the K_a as a pK_a. The calculation of pK_a is similar to the calculation of pH:

      In terms of conjugate acids and bases, every K_a expression appears as

      

No variations are allowed in this equation other than the actual formulas of the conjugate acid and base.

Like an acid, a base has a K_b value (the subscript b meaning base). A weak base, like ammonia, is part of the following equilibrium:

      The equilibrium constant expression for this equilibrium is

      The generic form of a K_b expression is

      As with a K_a expression, a K_b expression has no variations other than the actual formulas of the conjugate acid and base.

      Every acid has a K_a, and its corresponding conjugate base has a K_b. The K_a and the K_b of a conjugate acid-base pair are related by the K_w — the ionization constant for water. For a conjugate acid-base pair, . In addition, you can use the following shortcut: . (Are you enjoying this little bit of math? Biochemistry has a little more math than organic chemistry, but not nearly as much as general chemistry, so hang in there!)

      The K_b for the acetate ion, the conjugate base of acetic acid, would be associated with the following equilibrium expression:

      The K_a for the ammonium ion, the conjugate acid of ammonia, would be associated with the following equilibrium expression:

      An acid may be capable of donating more than one hydrogen ion. A biologically important example of this type of acid is phosphoric acid (H₃PO₄), which is a triprotic acid (meaning it can donate three hydrogen ions, one at a time). The equilibria for this acid are

The subscripts are modified to indicate the loss of hydrogen 1, hydrogen 2, or hydrogen 3. The associated K_a expressions are all of the form

      Here’s the breakdown for each K_a:

       

       

       

The value for each successive equilibrium constant often is significantly lower than the preceding value. Table 2-2 illustrates some biologically important acids. You can refer to this table when working buffer problems or determining which acid is stronger.

TABLE 2-2 The K_a Values for Biologically Important Acids

Acid	Ka1	Ka2	Ka3
Acetic acid (CH3COOH)
Pyruvic acid (CH3COCOOH)
Lactic acid (CH3CHOHCOOH)
Succinic acid (HOOCCH2CH2COOH)
Carbonic acid (H2CO3)
Phosphoric acid (H3PO4)