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Good butters. Norman Good and his colleagues developed a series of compounds that are useful buffers near neutral pH. Three of these buffers are abbreviated MES (pK2 = 6.15), MOPS |(pK= 7.15), and HEPPS (8.00). (a) Which of these buffers would be most effective near pH 7.0? (b) Near 6.0?

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dsdrajlin

Answer:

(a) MOPS

(b) MES

Explanation:

Buffers are solutions whose function is to resist drastic changes in pH when an acid or a base is added. They are formed by 2 components:

  • A weak acid and its conjugate base (e.g., HF/F⁻).               (acid buffer)
  • A weak base and its conjugate acid (e.g., NH₃/NH₄⁺).      (basic buffer)

pH and be pOH can be calculated, in each case, according to Henderson-Hasselbalch equations:

pH = pKa + log ([conjugate base]/[weak acid])           (acid buffer)

pOH = pKb + log ([conjugate acid]/[weak base])        (basic buffer)

It is possible to see that when the concentration of acid is equal to that of its base, pH = pKa or pOH = pKb. This is the point where the buffer has its optimum performance. Nevertheless, it has its most effective range of work (buffer range) when the ratio acid:base is between 0.1 and 10. If we take these values into Henderson-Hasselbalch equations, we can see that the optimum range of work is:

  • For acid buffer, pH = pKa ± 1
  • For basic buffer, pOH = pKb ± 1

In (a) (pH = 7.0), the solution is closet to the pK of the MOPS buffer, so this would be the best choice. Nevertheless, both MES and HEPPS would be suitable options, because their buffer ranges comprise pH = 7.0.

In (b) (pH = 6.0), the solution is closest to the pK of the MES buffer, so this would be the best option. Other buffers are not suitable because pH = 6.0 is out of their buffer range.

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