4: Titration of a Diprotic Acid
- Page ID
- 516589
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\(\newcommand{\avec}{\mathbf a}\) \(\newcommand{\bvec}{\mathbf b}\) \(\newcommand{\cvec}{\mathbf c}\) \(\newcommand{\dvec}{\mathbf d}\) \(\newcommand{\dtil}{\widetilde{\mathbf d}}\) \(\newcommand{\evec}{\mathbf e}\) \(\newcommand{\fvec}{\mathbf f}\) \(\newcommand{\nvec}{\mathbf n}\) \(\newcommand{\pvec}{\mathbf p}\) \(\newcommand{\qvec}{\mathbf q}\) \(\newcommand{\svec}{\mathbf s}\) \(\newcommand{\tvec}{\mathbf t}\) \(\newcommand{\uvec}{\mathbf u}\) \(\newcommand{\vvec}{\mathbf v}\) \(\newcommand{\wvec}{\mathbf w}\) \(\newcommand{\xvec}{\mathbf x}\) \(\newcommand{\yvec}{\mathbf y}\) \(\newcommand{\zvec}{\mathbf z}\) \(\newcommand{\rvec}{\mathbf r}\) \(\newcommand{\mvec}{\mathbf m}\) \(\newcommand{\zerovec}{\mathbf 0}\) \(\newcommand{\onevec}{\mathbf 1}\) \(\newcommand{\real}{\mathbb R}\) \(\newcommand{\twovec}[2]{\left[\begin{array}{r}#1 \\ #2 \end{array}\right]}\) \(\newcommand{\ctwovec}[2]{\left[\begin{array}{c}#1 \\ #2 \end{array}\right]}\) \(\newcommand{\threevec}[3]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \end{array}\right]}\) \(\newcommand{\cthreevec}[3]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \end{array}\right]}\) \(\newcommand{\fourvec}[4]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \\ #4 \end{array}\right]}\) \(\newcommand{\cfourvec}[4]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \\ #4 \end{array}\right]}\) \(\newcommand{\fivevec}[5]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \\ #4 \\ #5 \\ \end{array}\right]}\) \(\newcommand{\cfivevec}[5]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \\ #4 \\ #5 \\ \end{array}\right]}\) \(\newcommand{\mattwo}[4]{\left[\begin{array}{rr}#1 \amp #2 \\ #3 \amp #4 \\ \end{array}\right]}\) \(\newcommand{\laspan}[1]{\text{Span}\{#1\}}\) \(\newcommand{\bcal}{\cal B}\) \(\newcommand{\ccal}{\cal C}\) \(\newcommand{\scal}{\cal S}\) \(\newcommand{\wcal}{\cal W}\) \(\newcommand{\ecal}{\cal E}\) \(\newcommand{\coords}[2]{\left\{#1\right\}_{#2}}\) \(\newcommand{\gray}[1]{\color{gray}{#1}}\) \(\newcommand{\lgray}[1]{\color{lightgray}{#1}}\) \(\newcommand{\rank}{\operatorname{rank}}\) \(\newcommand{\row}{\text{Row}}\) \(\newcommand{\col}{\text{Col}}\) \(\renewcommand{\row}{\text{Row}}\) \(\newcommand{\nul}{\text{Nul}}\) \(\newcommand{\var}{\text{Var}}\) \(\newcommand{\corr}{\text{corr}}\) \(\newcommand{\len}[1]{\left|#1\right|}\) \(\newcommand{\bbar}{\overline{\bvec}}\) \(\newcommand{\bhat}{\widehat{\bvec}}\) \(\newcommand{\bperp}{\bvec^\perp}\) \(\newcommand{\xhat}{\widehat{\xvec}}\) \(\newcommand{\vhat}{\widehat{\vvec}}\) \(\newcommand{\uhat}{\widehat{\uvec}}\) \(\newcommand{\what}{\widehat{\wvec}}\) \(\newcommand{\Sighat}{\widehat{\Sigma}}\) \(\newcommand{\lt}{<}\) \(\newcommand{\gt}{>}\) \(\newcommand{\amp}{&}\) \(\definecolor{fillinmathshade}{gray}{0.9}\)- To titrate an unknown diprotic weak acid using a pH probe.
- To use the titration curve to determine the pKa1 and pKa2 of the acid and determine a likely identity of the acid.
INTRODUCTION
This lab experiment is designed to investigate the titration of a diprotic weak acid, referred to as \(\ce{H2A}\). Unlike monoprotic acids, a diprotic acid is characterized by the presence of two acidic protons that can dissociate in a stepwise manner, each associated with its own acid dissociation reaction and a specific acid dissociation constant, Ka1 and Ka2. The primary goal of this experiment is to explore these dissociations and determine the quantitative values that define the acid's strength and behavior in an aqueous solution.
To achieve this, a potentiometric titration will be performed while continuously monitoring the pH of the acid solution. A strong base, specifically a 0.1 M \(\ce{NaOH}\) solution, will serve as the titrant, delivered from a buret.
Upon completion of the titration, the collected titrant volume and pH data will be used to generate a titration curve, a graphical representation essential for identifying critical points during the reaction. From this curve, the first and second equivalence points will be determined, which correspond to the complete neutralization of each acidic proton. Subsequently, the pKa1 and pKa2 values for the diprotic acid will be estimated from the half-equivalence points. These values will then allow for the calculation of Ka1 and Ka2. Ultimately, the experiment aims to estimate the concentration of the unknown diprotic acid and, by comparing the determined pKa values with a table of known acid ionization constants, propose a plausible identity for the unknown acid.
Diprotic acids have two equivalence points. Because the stoichiometry is 1:1 for each proton removal, there is a built-in mathematical check:
- The volume to reach the second equivalence point (\(V_{eq2}\)) should be exactly twice the volume of the first equivalence point (\(V_{eq1}\)).
- Data Check: If your \(V_{eq2}\) is not roughly \(2 \times V_{eq1}\), you likely misidentified one of the endpoints on your graph.
Henderson-Hasselbalch Equation:
\[ \text{pH}=\text{p}K_{\text{a}}+\log \left( \frac{[\text{A}^-]}{[\text{HA}]} \right)\]
Definition of pK_a:
\[ \text{p}K_\text{a} = -\log{K_\text{a}}\ \ \ \ \ \text{or}\ \ \ \ \ K_\text{a} = 10^{-\text{p}K_\text{a}} \]
- 4.1: Titration of a Diprotic Acid - Experiment
- This page provides safety guidelines for handling caustic solutions, particularly during a titration experiment involving an unknown diprotic weak acid and sodium hydroxide. It details necessary equipment and chemicals, outlines a three-part procedure for setting up and conducting the titration, and emphasizes accurate data recording of pH changes and buret readings.
- 4.2: Titration of a Diprotic Acid - Pre-lab
- This page covers the titration of a diprotic acid (H2A) with NaOH, detailing two dissociation reactions and their corresponding constants (Ka1 and Ka2). It emphasizes the concentrations at half-equivalence points and involves calculations for NaOH volumes to reach the first and second equivalence points. Additionally, it requires a sketch of the expected titration curve with volume markers.
- 4.3: Titration of a Diprotic Acid - Data and Report
- This page details a titration lab where students determine an unknown acid's concentration by analyzing titration curves. They collect data from two titrations, create graphs, and identify features like equivalence points. Key calculations include estimating pKa values and comparing results for internal consistency through percent differences. The ultimate goal is to identify the unknown acid based on its pKa values.