Buffer solutions Henderson-Hasselbalch equation

Titration of Weak Acids The Henderson-Hasselbalch Equation Buffer Solutions (Figure 2.17, Table 2.7)... 

Although this treatment of buffers was based on acid-base chemistry, the idea of a buffer is general and can be extended to equilibria involving complexation or redox reactions. For example, the Nernst equation for a solution containing Fe + and Fe + is similar in form to the Henderson-Hasselbalch equation. 

Any solution containing comparable amounts of a weak acid, HA, and its conjugate weak base, A-, is a buffer. As we learned in Chapter 6, we can calculate the pH of a buffer using the Henderson-Hasselbalch equation. 

We saw in Section 20.3 that the extent of dissociation of a carboxylic acid HA in an aqueous solution buffered to a given pH can be calculated with the Henderson-Hasselbalch equation. Furthermore, we concluded that at the physiological... 

The values of [HA] and [A ] in this expression are the equilibrium concentrations of acid and base in the solution, not the concentrations added initially. However, a weak acid HA typically loses only a tiny fraction of its protons, and so [HA] is negligibly different from the concentration of the acid used to prepare the buffer, [HA]initia. Likewise, only a tiny fraction of the weakly basic anions A- accept protons, and so [A-] is negligibly different from the initial concentration of the base used to prepare the buffer. With the approximations A ] [base]initia and [HA] [acid]initia, we obtain the Henderson-Hasselbalch equation ... 

In practice, the Henderson-Hasselbalch equation is used to make rapid estimates of the pH of a mixed solution intended to be used as a buffer, and then the pH is adjusted to the precise value required by adding more acid or base and monitoring the solution with a pH meter. 

We can use these numbers to express the range of buffer action in terms of the pH of the solution. The Henderson-Hasselbalch equation shows us that,... 

The buffer equation, which is often called the Henderson-Hasselbalch equation, is used to calculate the equilibrium pH of a buffer solution directly from initial concentrations. The approximation is valid as long as the difference between initial concentrations and equilibrium concentrations is negligibly small. As a rule of thumb, the buffer equation can be applied when initial concentrations of H j4 and A differ by less than a factor of 10. Example provides an illustration of the use of the buffer equation. 

Fig. 3.3 Solubility profiles of sparingly soluble drugs, based on data taken from Avdeef et al. [20]. The solutions consisted of robotically adjusted universal buffers, based on a mixture of Good buffers (see text), and contained 0.2 M KCl. The dashed lines were calculated by the Henderson-Hasselbalch equation and, as can be seen, did not accurately describe the solubility profiles. The solid curves were...

For formic acid, pATa = - log(1.8 x 10 4) = 3.74. The Henderson-Hasselbalch equation provides the pH of the original buffer solution ... 

The pATa s of the three acids help us choose the one to be used in the buffer. It is the acid with a pATa within 1.00 pH unit of 3.50. pATa = 3.74 for HCH02, pATa = 4.74 for HC2H302, and p/sTj =2.15 for H3P04. Thus, we choose HCH02 and NaCH02 to prepare a buffer with pH = 3.50. The Henderson-Hasselbalch equation is used to determine the relative amounts of each component present in the buffer solution. 

This is a more basic solution, which we can achieve by increasing the basic component of the buffer solution, the acetate ion. We find out the new acetate ion concentration with the Henderson-Hasselbalch equation. 

Buffers are solutions that resist a change in pH when we add an acid or base. A buffer contains both a weak acid (HA) and its conjugate base (A-). The acid part will neutralize any base added and the base part of the buffer will neutralize any acid added to the solution. We may calculate the hydronium ion concentration of a buffer by rearranging the Ka expression to yield the Henderson-Hasselbalch equation, which we can use to calculate the pH of a buffer ... 

We can now use these two values for the equilibrium portion of the problem. There are two options for this buffer solution. We can use these concentrations in a Ka calculation, or we can use the Henderson-Hasselbalch equation. Either method will give you the same answer however, the Henderson-Hasselbalch equation is faster. 

The common-ion effect is an application of Le Chatelicr s principle to equilibrium systems of slightly soluble salts. A buffer is a solution that resists a change in pH if we add an acid or base. We can calculate the pH of a buffer using the Henderson-Hasselbalch equation. We use titrations to determine the concentration of an acid or base solution. We can represent solubility equilibria by the solubility product constant expression, Ksp. We can use the concepts associated with weak acids and bases to calculate the pH at any point during a titration. 

Thus, for a weak acid with a given Ka (or pKJ and a given ratio of conjugate base concentration to acid concentration, the pH may be calculated. Or, given the desired pH and IQ (pKa), the ratio of salt concentration to acid concentration can be calculated and the buffer subsequently prepared. Equations (5.26) to (5.30) are each a form of the Henderson-Hasselbalch equation for dealing with buffer solutions. 

Addition of an acid such as HCl to the buffer solution provides H" ", which combines with the acetate ion to give acetic acid. This has a twofold effect it reduces the amount of acetate ion present and, by so doing, also increases the amount of undissociated acetic acid. Provided the amount of acid added is small relative to the original concentration of base in the buffer, the alteration in base acid ratio in the Henderson-Hasselbalch equation is relatively small and has Mttle effect on the pH value. 

The Henderson-Hasselbalch equation may be employed in calculations relating to the properties and effects of buffer solutions (see Box 4.8). 

If 1 ml of 1 M HCl is added to this sodium acetate buffer solution, the pH change may be calculated as follows. Again, we require the Henderson-Hasselbalch equation ... 

The very best buffers and those best able to withstand the addition of both acid and base are those for which [HA] and [A ] cire approximately equal. When this occurs, the logarithmic term in the Henderson-Hasselbalch equation disappecirs, and the equation becomes pH = pA. When creating a buffered solution, chemists therefore choose an acid that has a pK close to the desired pH. 

Henderson-Hasselbalch equation An equation giving the relationship between the pH and the concentrations of base and acid in buffer solution. 

The addition of H+ to this solution favours the back reaction while the addition of base favours the forward reaction. The weak acid/salt pair thus acts to minimize ApH. An analogous situation exists for buffers consisting of a weak base and its salt. The pH of a buffer can be calculated from the concentration of its components by the Henderson-Hasselbalch equation... 

L. J. Henderson was a physician who wrote [H ] = /C0[acid]/[salt] in a physiology article in 1908, a year before the word buffer" and the concept of pH were invented by the biochemist S. R L Sorensen. Henderson s contribution was the approximation of setting [acid] equal to the concentration of HA placed in solution and [salt] equal to the concentration of A placed in solution. In 1916. K. A. Hasselbalch wrote what we call the Henderson-Hasselbalch equation in a biochemical journal.8... 

Equation 2.6 is the familiar Henderson-Hasselbalch equation, which defines the relationship between pH and the ratio of acid and conjugate base concentrations. The Henderson-Hasselbalch equation is of great value in buffer chemistry because it can be used to calculate the pH of a solution if the molar ratio of buffer ions ([A-]/[HA]) and the pKa of HA are known. Also, the molar ratio of HA to A- that is necessary to prepare a buffer solution at a specific pH can be calculated if the pKa is known. 

El 1. You need to prepare a buffer for biochemistry lab. The required solution is 0.5 M sodium phosphate, pH 7.0. Use the Henderson-Hasselbalch equation to calculate the number of moles and grams of monobasic sodium phosphate (NaH2P04) and dibasic sodium phosphate (Na2HP04) necessary to make 1 liter of the solution. 

The Henderson-Hasselbalch equation says that the pH of a buffer solution has a value close to the pKa of the weak acid, differing only by the amount log [base]/[add]. When [base]/[acid] = 1, then log [base]/[acid] = 0, and the pH equals the pKa. 

Ballpark Check a common error in using the Henderson-Hasselbalch equation is to invert the [base]/[acid] ratio. It is therefore wise to check that your answer makes chemical sense. If the concentrations of the acid and its conjugate base are equal, the pH will equal the pKa. If the acid predominates, the pH will be less than the pKa, and if the conjugate base predominates, the pH will be greater than the pKa. In part (a), [acid] = [NH4 + ] is greater than [base] = [NH3], and so the calculated pH (8.77) should be less than the pKa (9.25). In part (b), the desired pH is less than the pKa, so the buffer should contain more moles of acid than base, in agreement with the solution. 

PROBLEM 16.9 Use the Henderson-Hasselbalch equation to calculate the pH of a buffer solution prepared by mixing equal volumes of 0.20 M NaHC03 and 0.10 M Na2C03. (,Ka values are given in Appendix C.)... 

Halfway to the first equivalence point, we have an H2A+-HA buffer solution with [H2A+] = [HA]. The Henderson-Hasselbalch equation gives pH = PJCal = 2.34. 

A solution of a weak acid and its conjugate base is called a buffer solution because it resists drastic changes in pH. The ability of a buffer solution to absorb small amounts of added H30+ or OH- without a significant change in pH (buffer capacity) increases with increasing amounts of weak acid and conjugate base. The pH of a buffer solution has a value close to the pKa (— log Ka) of the weak acid and can be calculated from the Henderson-Hasselbalch equation ... 

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