1.4: pH Buffers
- Page ID
- 367975
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\(\newcommand{\avec}{\mathbf a}\) \(\newcommand{\bvec}{\mathbf b}\) \(\newcommand{\cvec}{\mathbf c}\) \(\newcommand{\dvec}{\mathbf d}\) \(\newcommand{\dtil}{\widetilde{\mathbf d}}\) \(\newcommand{\evec}{\mathbf e}\) \(\newcommand{\fvec}{\mathbf f}\) \(\newcommand{\nvec}{\mathbf n}\) \(\newcommand{\pvec}{\mathbf p}\) \(\newcommand{\qvec}{\mathbf q}\) \(\newcommand{\svec}{\mathbf s}\) \(\newcommand{\tvec}{\mathbf t}\) \(\newcommand{\uvec}{\mathbf u}\) \(\newcommand{\vvec}{\mathbf v}\) \(\newcommand{\wvec}{\mathbf w}\) \(\newcommand{\xvec}{\mathbf x}\) \(\newcommand{\yvec}{\mathbf y}\) \(\newcommand{\zvec}{\mathbf z}\) \(\newcommand{\rvec}{\mathbf r}\) \(\newcommand{\mvec}{\mathbf m}\) \(\newcommand{\zerovec}{\mathbf 0}\) \(\newcommand{\onevec}{\mathbf 1}\) \(\newcommand{\real}{\mathbb R}\) \(\newcommand{\twovec}[2]{\left[\begin{array}{r}#1 \\ #2 \end{array}\right]}\) \(\newcommand{\ctwovec}[2]{\left[\begin{array}{c}#1 \\ #2 \end{array}\right]}\) \(\newcommand{\threevec}[3]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \end{array}\right]}\) \(\newcommand{\cthreevec}[3]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \end{array}\right]}\) \(\newcommand{\fourvec}[4]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \\ #4 \end{array}\right]}\) \(\newcommand{\cfourvec}[4]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \\ #4 \end{array}\right]}\) \(\newcommand{\fivevec}[5]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \\ #4 \\ #5 \\ \end{array}\right]}\) \(\newcommand{\cfivevec}[5]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \\ #4 \\ #5 \\ \end{array}\right]}\) \(\newcommand{\mattwo}[4]{\left[\begin{array}{rr}#1 \amp #2 \\ #3 \amp #4 \\ \end{array}\right]}\) \(\newcommand{\laspan}[1]{\text{Span}\{#1\}}\) \(\newcommand{\bcal}{\cal B}\) \(\newcommand{\ccal}{\cal C}\) \(\newcommand{\scal}{\cal S}\) \(\newcommand{\wcal}{\cal W}\) \(\newcommand{\ecal}{\cal E}\) \(\newcommand{\coords}[2]{\left\{#1\right\}_{#2}}\) \(\newcommand{\gray}[1]{\color{gray}{#1}}\) \(\newcommand{\lgray}[1]{\color{lightgray}{#1}}\) \(\newcommand{\rank}{\operatorname{rank}}\) \(\newcommand{\row}{\text{Row}}\) \(\newcommand{\col}{\text{Col}}\) \(\renewcommand{\row}{\text{Row}}\) \(\newcommand{\nul}{\text{Nul}}\) \(\newcommand{\var}{\text{Var}}\) \(\newcommand{\corr}{\text{corr}}\) \(\newcommand{\len}[1]{\left|#1\right|}\) \(\newcommand{\bbar}{\overline{\bvec}}\) \(\newcommand{\bhat}{\widehat{\bvec}}\) \(\newcommand{\bperp}{\bvec^\perp}\) \(\newcommand{\xhat}{\widehat{\xvec}}\) \(\newcommand{\vhat}{\widehat{\vvec}}\) \(\newcommand{\uhat}{\widehat{\uvec}}\) \(\newcommand{\what}{\widehat{\wvec}}\) \(\newcommand{\Sighat}{\widehat{\Sigma}}\) \(\newcommand{\lt}{<}\) \(\newcommand{\gt}{>}\) \(\newcommand{\amp}{&}\) \(\definecolor{fillinmathshade}{gray}{0.9}\)Controlling pH is critically important in the qualitative analysis of cations. Often, pH needs to be maintained in a narrow range in the analysis of cations.
A pH buffer is an aqueous solution consisting of a weak acid and its conjugate base or vice versa, which minimizes pH change when a small amount of a strong acid or a strong base is added to it.
For example, the addition of 0.020 mol \(\ce{HCl}\) into 1 L of water changes pH from 7 to 1.7, i.e., about 80% change in pH. Similarly, the addition of 0.020 mol \(\ce{NaOH}\) to the same water changes pH from 7 to 12.3, i.e., again, about 80% change in pH. In contrast to pure water, 1 L of buffer solution containing 0.50 mol a week acid acetic acid (\(\ce{CH3COOH}\)) and 0.50 mol of its conjugate base \(\ce{CH3COO^-}\) changes pH from 4.74 to 4.70 by the addition of the same 0.020 mol \(\ce{HCl}\) and from 4.74 to 4.77 by the addition of 0.020 mol \(\ce{NaOH}\), i.e., about 1% change in pH, as illustrated in Fig. 1.7.1.
The buffer contains a weak acid and its conjugate base in equilibrium. For example, acetic acid/sodium acetate buffer has the following equilibrium:
\[\ce{CH3COOH + H2O <<=> H3O^{+} + CH3COO^{-}}\nonumber\]
The molar concentration of hydronium ions [\(\ce{H3O^+}\)] defines the pH of the solution, i.e., \(\mathrm{pH}=-\log \left[\mathrm{H}_{3} \mathrm{O}^{+}\right]\). The conjugate base consumes any strong acid added to the mixture:
\[\ce{HA + CH3COO^{-} -> CH3COOH + A^{-}}\nonumber\]
, where \(\ce{HA}\) is any strong acid and \(\ce{A^-}\) is its conjugate base. The concentration of \(\ce{CH3COOH}\) increases and \(\ce{CH3COO^-}\) decrease, but pH decreases little because [\(\ce{H3O^+}\)] is almost not affected. Similarly, the weak acid consumes any strong base added.
\[\ce{MOH + CH3COOH -> CH3COO^{-} + M^{+} + H2O}\nonumber\]
, where \(\ce{M^+}\) is its conjugate acid. The concentration of \(\ce{CH3COOH}\) decreases and \(\ce{CH3COO^-}\) increases, but pH increases little because [\(\ce{H3O^+}\)] is almost not affected. Buffers are employed on several occasions during the qualitative analysis of cations.