Buffer solutions Henderson-Hasselbach equation

Henderson-Hasselbach equation A simplified version of the relationships used in calculations on buffer solutions. 

The Henderson-Hasselbach equation, Equation (6.50), relates the pH of a buffer solution to the amounts of conjugate acid and conjugate base it contains ... 

The presence of H3 0+ or HO may alter drastically the observed reaction rate either because they catalyse the reaction (acid or base catalysis, see Section 3.2.3 for the Aldol reaction, and Chapter 11) or because of ionic strength effects. Proper pH control in an aqueous solution will require a buffer system which is described by the appropriate version of the Henderson-Hasselbach equation, according to whether the acid or base is the charged species ... 

Let s try another one. What is the pH of a buffer containing 0.25 M ammonia and 0.75 M ammonium chloride Well, the weak acid in this case is the ammonium ion. The chloride ion is a spectator to be ignored. Ammonia is a weak base, and the conjugate base of the ammonium ion. So, since this solution contains a weak conjugate acid-base pair, it is a buffer, and we can calculate the pH using the Henderson-Hasselbach equation. The Henderson-Hasselbach equation calls for the pKa of the acid so in this case, we need the pfor the ammonium ion. The pKh for ammonia is 4.74, so the pKa for the ammonium ion is 9.26. If we substitute this value and the values for the concentrations into the buffer equation, we find the pH of this solution is 8.78. 

Henderson-Hasselbach equation kinetic rate expression that defines the relationship between pH, pKa, and the concentrations of the acid and base components of a buffer solution... 

We emphasize the fact that the Henderson-Hasselbach equation is an approached one. Its use is valid if the concentrations [OH ] and [H3O+] are negligible in the charge balance equation. This is true only if the pKa value of the couple is not too close to 0 or to 14. In this case, the concentrations [H3O+] and [OH ] would no longer be negligible. In order for the Henderson-Hasselbach to be valid, the concentrations Cha and Ca (or Cbh and Cb) must not be too weak. This is the reason why it is often stipulated that buffer solutions are rather concentrated solutions in the weak acid and in its conjugate base. When the buffer solution is too diluted, the Henderson-Hasselbach equation is no longer valid and the pH then depends on the total concentration. It increases with the buffer dilution (Fig. 6.1). 

Equation (6.1) or (6.2) is called Henderson-Hasselbach s equation. From its examination, it appears that the pH value of a buffer solution only depends on the ratio of the concentrations of conjugate acid and basic forms and also on the nature of the acid through its pKa value. It does not depend on the total concentration Cha + Ca or Cbh -I- Cb. In other words, the pH value of a buffer solution does not change with its dilution. 

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