The most common types of buffers are mixtures of weak acid and salts of their conjugate bases, for example, acetic acid/sodium acetate. In this system the dissociation of acetic acid can be written CH3COOH→CH3COO+H where the acid dissociation constant is defined as Ka=HI [CH_ COO[ COOH Rearranging and taking the negative logarithm gives the more familiar form of the Henderson-Hasselbalch equation DH=pk+log ICHsCOo- ICH3 COOH Inspection of this equation provides several insights as to the functioning of a buffer When the concentrations of acid and conjugate base are equa log(1)=0 and the pH of the resulting solution will be equal to the pKa of the acid. The ratio of the concentrations of acid and con jugate base can differ by a factor of 10 in either direction, and the resulting pH will only change by l unit. This is how a buffer main- tains ph stability in the solution To a first approximation, the ph of a buffer solution is inde- pendent of the absolute concentration of the buffer; the pH depends only on the ratio of the acid and conjugate base present However, concentration of the buffer is important to buffer capac ity, and is considered later in this chapter When Is a Buffer not a Buffer? Simply having a weak acid and the salt of its conjugate base present in a solution doesnt ensure that the buffer will act as a buffer Buffers are most effective within t 1 pH unit of their pK Outside of that range the concentration of either the acid or its salt is so low as to provide little or no capacity for pH control Common mistakes are to select buffers without regard to the pKa of the buffer. Examples of this would be to try to use K,HPO/KH2PO4(pKa=6.7)to buffer a solution at pH 4, or to use acetic acid(pKa=4.7)to buffer near neutral pH. What Are the Criteria to Consider When Selecting a Buffer? Target pH Of primary concern is the target pH of the solution. This narrows the possible choices to those buffers with pka values within 1 pH unit of the target pH The Preparation of Buffers and Other SolutionsThe most common types of buffers are mixtures of weak acids and salts of their conjugate bases, for example, acetic acid/sodium acetate. In this system the dissociation of acetic acid can be written as CH3COOH Æ CH3COO- + H+ where the acid dissociation constant is defined as Ka = [H+ ] [CH3COO- ]/[H3COOH]. Rearranging and taking the negative logarithm gives the more familiar form of the Henderson-Hasselbalch equation: Inspection of this equation provides several insights as to the functioning of a buffer. When the concentrations of acid and conjugate base are equal, log(1) = 0 and the pH of the resulting solution will be equal to the pKa of the acid. The ratio of the concentrations of acid and con￾jugate base can differ by a factor of 10 in either direction, and the resulting pH will only change by 1 unit. This is how a buffer main￾tains pH stability in the solution. To a first approximation, the pH of a buffer solution is inde￾pendent of the absolute concentration of the buffer; the pH depends only on the ratio of the acid and conjugate base present. However, concentration of the buffer is important to buffer capac￾ity, and is considered later in this chapter. When Is a Buffer Not a Buffer? Simply having a weak acid and the salt of its conjugate base present in a solution doesn’t ensure that the buffer will act as a buffer. Buffers are most effective within ± 1 pH unit of their pKa. Outside of that range the concentration of either the acid or its salt is so low as to provide little or no capacity for pH control. Common mistakes are to select buffers without regard to the pKa of the buffer. Examples of this would be to try to use K2HPO4/KH2PO4 (pKa = 6.7) to buffer a solution at pH 4, or to use acetic acid (pKa = 4.7) to buffer near neutral pH. What Are the Criteria to Consider When Selecting a Buffer? Target pH Of primary concern is the target pH of the solution. This narrows the possible choices to those buffers with pKa values within 1 pH unit of the target pH. pH pK CH COO CH COOH = + [ ] [ ] - log 3 3 The Preparation of Buffers and Other Solutions 33