Polyprotic Acids and Buffers

Introduction

• Monoprotic acids: acids that contain one ionizable hydrogen atom
• Polyprotic acids: acids that contain more than one ionizable hydrogen atom
• Polyprotic acids ionize in steps. 
• The acid equilibrium constant (Ka) becomes smaller with each successive step.

Buffers

• Buffer: a solution containing appreciable amounts of both a weak acid and its conjugate base (or a weak base and its conjugate acid). 
• Buffer solutions resist changes in pH upon addition of a small amount of strong acid or base.
• Buffers are good at getting rid of strong acids and bases.
• The rule: the addition of  H+  (strong acid) to a buffer converts an equimolar amount of  A- to HA.  
  • The addition of OH- to a buffer converts an equimolar amount of Ha to A-.  
  • Remember that in a buffer,  [H+]  =   Ka (  (HA))/(A-)   ( HA and A- represent any acid and its conjugate base.)
• Buffer capacity: amount of strong acid or base that can be added to a given volume of a buffer solution before the pH changes significantly. 
  • All buffers have a limited capacity of how much H3O+ or OH– they can “soak up.”
  • Eventually, all the HA reacts with the added OH–
  • Eventually, all the A– reacts with the added H3O+
• Buffer capacity depends on the number of moles of the weak acid and its conjugate base that are in the mixture. 
• More moles of buffer components leads to a higher buffer capacity. 
• Once the buffer components near depletion, large changes in pH result. 
• The pH of a buffer system can be calculated if we know the concentration of the weak acid [HA] and its conjugate base [A–].
• Henderson-Hasselbalch equation can be used to calculate the pH of a buffer.