Читать книгу Biochemistry For Dummies - John Moore T., Richard Langley H., John T. Moore - Страница 28

To determine a buffer’s pH, you can use a K_a or K_b calculation, as we discuss in the section “Swapping hydrogens between acids and bases,” earlier in this chapter, or you can use the Henderson-Hasselbalch equation, which is a shortcut.

The Henderson-Hasselbalch equation takes two forms:

and

The terms in either form are the same as those we define earlier in the chapter. For example, suppose you want to calculate the pH of a buffer composed of 0.15 M pyruvic acid and 0.25 M sodium pyruvate. Referring back to Table 2-2, you see that the K_a of pyruvic acid is .

The pK_a would be 2.50. Therefore:

The greater the values of [CA] and [CB], the greater the buffer capacity of the solution. The buffer capacity indicates how much acid or base may be added to a buffer before the buffer ceases to function as a buffer. A buffer in which the would have a much higher buffer capacity for adding either acids or bases than a buffer in which the . If there were a buffer in which and its , the buffer would have a higher buffer capacity for additions of a base than for additions of an acid because the buffer contains more acid than base. For the buffer to be as flexible as possible, the concentrations of the conjugate acid-base pair should be as close to equal as possible and as high as possible.