Buffers Henderson-Hasselbach equation

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

Acid-base buffers comprise both a weak acid or base and its respective salt. Calculations with buffers employing the Henderson-Hasselbach equation are introduced and evaluated, thereby allowing the calculation of the pH of a buffer. Next, titrations and pH indicators are discussed, and their modes of action placed into context. 

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 Henderson-Hasselbach equation allows the ratio of ionized un-ionized compound to be found if the pH and pKa are known. Consider carbonic acid (H2CO3) bicarbonate (HC03 ) buffer system... 

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 ... 

This is the Henderson-Hasselbach equation, or simply the buffer equation. We can use the buffer equation to calculate the pH of a buffer. We can also use it to determine the ratio of weak acid to conjugate base at a given pH. 

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. 

Given that en will be used to buffer your system, use the Henderson-Hasselbach equation calculate the expected final pH for the reaction and place your value in Table 5.5. 

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... 

At the beginning of the titration, [H" ] is equal to Vk HQA (cell E4 formula). The pH is calculated from this (cell H4 formula). Once the titration is begun and up to the equivalence point, we have a buffer of HOAc and OAc , and so we must calculate the concentrations of each. The former is the number of millimoles of remaining HOAc divided by the total volume (cell C5 formula). The [OAc ] is the number of millimoles NaOH added divided by the total volume (cell D5 formula). The pH is calculated frotn the Henderson-Hasselbach equation (cell H5 formula). At the equivalence point, [OH ] is determined by the concentration of OAc (cell F19 formula), and pOH (cell G19) and pH (cell H19) are calculated from this. The pH for the remainder of the titration is calculated from the excess millimoles NaOH and total volume (cell F20 formula), as was done for the titration of HCl. 

Mathematically and geometrically, buffer capacity, p, is deduced from the Henderson-Hasselbach equation [equation (1.2), (Section 1.4.2)]. Buffer capacity is defined by equation (1.3). 

Buffering The ability of a mixture of an acid and its conjugate base at a pH near their to minimize pH changes caused by an influx of acid or base. The Henderson-Hasselbach equation is useful relating pH, pi Tand [salt]/[acid]. 

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). 

At pH = pKa + 1 the buffer capacity falls to 33% of the maximum value. This is the approximate range within which buffering by a weak acid is effective. Note at pH = pK - 1, The Henderson-Hasselbach equation shows that the ratio [HA] [A ] is... 

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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