Chm130 Buffers Lab
CHEM 113L EXP# 7: Buffer Preparation and pH Measurements
Revised . AMB 7-2005 Introduction: Even in quite dilute aqueous solutions, acetic acid is very slightly ionized (it would approach 99% ionization only as the concentration approaches 0.0 M):
HC2H3O2(aq) + H2O(l) Ka = 1.8 x 10-5. H3O+ + C2H3O2-
In general, if the acid is not extremely weak, the pH of a solution of a weak acid is governed by the concentration of the acid and Ka. Under similar conditions, the pH of a solution of a weak base is determined by the concentration of the weak base and Kb. In solutions containing both a weak acid and a strong acid, both acids play a role in determining the pH of the solution; however, if the concentration of the strong acid is relatively large, it will inhibit the dissociation of the weak acid.(the common-ion effect). The pH of this solution would then be calculated as if the weak acid were not present! (Likewise, in a solution containing both a strong base and a weak base, the strong base concentration would be used to calculate pH.) Buffer solutions contain both a weak acid and its conjugate weak base in appreciable concentrations. Within limits, these solutions tend to resist changes in pH upon addition of either H3O+ or OH- (because these species are largely consumed by the acidic and basic components of the buffer mixture). In buffer systems like NaC2H3O2-HC2H3O2 mixtures, the principal source of the acetic acid molecule is from the acid; the principal source of the acetate ion is from the salt. Therefore, the [H3O+] is determined by the salt/acid (or equivalently, the base/acid) mole ratio. For a conjugate acid/base pair: pH = pKa + log10 [nconjugate base] [nacid]
On the other hand, the salt, sodium acetate (NaC2H3O2), is essentially 100% dissociated into the constituent hydrated ions in a dilute aqueous solution. Sodium acetate solutions are basic because the acetate ion (the conjugate base of acetic acid) behaves as a proton acceptor with respect to
Revised . AMB 7-2005 Introduction: Even in quite dilute aqueous solutions, acetic acid is very slightly ionized (it would approach 99% ionization only as the concentration approaches 0.0 M):
HC2H3O2(aq) + H2O(l) Ka = 1.8 x 10-5. H3O+ + C2H3O2-
In general, if the acid is not extremely weak, the pH of a solution of a weak acid is governed by the concentration of the acid and Ka. Under similar conditions, the pH of a solution of a weak base is determined by the concentration of the weak base and Kb. In solutions containing both a weak acid and a strong acid, both acids play a role in determining the pH of the solution; however, if the concentration of the strong acid is relatively large, it will inhibit the dissociation of the weak acid.(the common-ion effect). The pH of this solution would then be calculated as if the weak acid were not present! (Likewise, in a solution containing both a strong base and a weak base, the strong base concentration would be used to calculate pH.) Buffer solutions contain both a weak acid and its conjugate weak base in appreciable concentrations. Within limits, these solutions tend to resist changes in pH upon addition of either H3O+ or OH- (because these species are largely consumed by the acidic and basic components of the buffer mixture). In buffer systems like NaC2H3O2-HC2H3O2 mixtures, the principal source of the acetic acid molecule is from the acid; the principal source of the acetate ion is from the salt. Therefore, the [H3O+] is determined by the salt/acid (or equivalently, the base/acid) mole ratio. For a conjugate acid/base pair: pH = pKa + log10 [nconjugate base] [nacid]
On the other hand, the salt, sodium acetate (NaC2H3O2), is essentially 100% dissociated into the constituent hydrated ions in a dilute aqueous solution. Sodium acetate solutions are basic because the acetate ion (the conjugate base of acetic acid) behaves as a proton acceptor with respect to