Chapter 10 Acids and Bases 10.7 Buffers

Buffers Buffers resist changes in pH from the addition of acid or base in the body absorb H3O+ or OH from foods and cellular processes to maintain pH are important in the proper functioning of cells and blood in blood maintain a pH close to 7.4; a change in the pH of the blood affects the uptake of oxygen and cellular processes

Buffers (continued) When an acid or base is added to water, the pH changes drastically to a buffer solution, the pH does not change very much; pH is maintained

Components of a Buffer The components of a buffer solution are acid–base conjugate pairs can be a weak acid and a salt of its conjugate base typically have equal concentrations of the weak acid and its salt can also be a weak base and a salt of its conjugate acid

Learning Check Does each of the following combinations produce a buffer solution or not? Explain. A. HCl and KCl B. H2CO3 and NaHCO3 C. H3PO4 and NaCl D. HC2H3O2 and KC2H3O2

Solution Does each of the following combinations produce a buffer solution or not? Explain. A. HCl + KCl No; HCl is a strong acid. B. H2CO3 + NaHCO3 Yes; this is a weak acid and its salt C. H3PO4 + NaCl No; NaCl does not contain a conjugate base of H3PO4. D. HC2H3O2 and KC2H3O2 Yes; this is a weak acid and its salt

Buffer Action An acetic acid/acetate buffer contains the weak acid acetic acid (HC2H3O2) and the salt of its conjugate base sodium acetate (NaC2H3O2). Acid dissociation occurs: HC2H3O2(aq) + H2O(l) C2H3O2(aq) + H3O+(aq) The salt provides a higher concentration of the conjugate base C2H3O2 than provided by the dissociation of the weak acid by itself. NaC2H3O2(aq) Na+(aq) + C2H3O2(aq)

Function of the Weak Acid in a Buffer The function of the weak acid in a buffer is to neutralize added base. The acetate ion produced by the neutralization becomes part of the available acetate. HC2H3O2(aq) + OH−(aq) C2H3O2(aq) + H2O(l) acetic acid base acetate ion water

Function of the Conjugate Base The function of the acetate ion C2H3O2 is to neutralize added H3O+. The acetic acid produced by the neutralization contributes to the available weak acid. C2H3O2(aq) + H3O+(aq) HC2H3O2(aq) + H2O(l) acetate ion acid acetic acid water

Summary of Buffer Action Buffer action occurs because the weak acid in a buffer neutralizes base the conjugate base in the buffer neutralizes acid the pH of the solution is maintained

pH of a Buffer The [H3O+] in the Ka expression is used to determine the pH of a buffer. Weak acid + H2O H3O+ + conjugate base Ka = [H3O+][conjugate base] [weak acid] [H3O+] = Ka x [weak acid] [conjugate base] pH = log [H3O+]

Henderson-Hasselbach Equation [H+] ≈ Ka[HA]/[A–] Again assuming that x is small, you can take the log of both sides and rearrange to get the Henderson-Hasselbach equation; pH = pKa + log{ [base ]/ [acid]}

Guide to Calculating pH of a Buffer

Example of Calculating Buffer pH The weak acid H2PO4 in a blood buffer H2PO4/HPO42 has a Ka = 6.2 x 108. What is the pH of the buffer if [H2PO4] = 0.20 M and [HPO42] = 0.20 M? STEP 1 Write the Ka expression for: H2PO4(aq) + H2O(l) HPO42(aq) + H3O+(aq) Ka = [HPO42][H3O+] [H2PO4] STEP 2 Rearrange the Ka for [H3O+]: [H3O+] = Ka x [H2PO4] [HPO42]

Example of Calculating Buffer pH (continued) STEP 3 Substitute [HA] and [A]: [H3O+] = 6.2 x 108 x [0.20 M] = 6.2 x 108 [0.20 M] STEP 4 Use [H3O+] to calculate pH: pH =  log [6.2 x 10 8] = 7.21

Learning Check What is the pH of a H2CO3 buffer that is 0.20 M H2CO3 and 0.10 M HCO3? Ka(H2CO3) = 4.3 x 107 1) 6.67 2) 6.37 3) 6.07

Solution What is the pH of a H2CO3 buffer that is 0.20 M H2CO3 and 0.10 M HCO3? Ka(H2CO3) = 4.3 x 107 STEP 1 Write the Ka expression for: H2CO3(aq) + H2O(l) HCO3(aq) + H3O+(aq) Ka = [HCO3][H3O+] [H2CO3] STEP 2 Rearrange the Ka for [H3O+]: [H3O+] = Ka x [H2CO3] [HCO3]

Solution (continued) STEP 3 Substitute [HA] and [A]: [H3O+] = 4.3 x 107 x [0.20 M] = 8.6 x 107 M [0.10 M] STEP 4 Use [H3O+] to calculate pH: pH =  log [8.6 x 107] = 6.07