4: Titration of a Diprotic Acid
- Page ID
- 516589
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\(\newcommand{\avec}{\mathbf a}\) \(\newcommand{\bvec}{\mathbf b}\) \(\newcommand{\cvec}{\mathbf c}\) \(\newcommand{\dvec}{\mathbf d}\) \(\newcommand{\dtil}{\widetilde{\mathbf d}}\) \(\newcommand{\evec}{\mathbf e}\) \(\newcommand{\fvec}{\mathbf f}\) \(\newcommand{\nvec}{\mathbf n}\) \(\newcommand{\pvec}{\mathbf p}\) \(\newcommand{\qvec}{\mathbf q}\) \(\newcommand{\svec}{\mathbf s}\) \(\newcommand{\tvec}{\mathbf t}\) \(\newcommand{\uvec}{\mathbf u}\) \(\newcommand{\vvec}{\mathbf v}\) \(\newcommand{\wvec}{\mathbf w}\) \(\newcommand{\xvec}{\mathbf x}\) \(\newcommand{\yvec}{\mathbf y}\) \(\newcommand{\zvec}{\mathbf z}\) \(\newcommand{\rvec}{\mathbf r}\) \(\newcommand{\mvec}{\mathbf m}\) \(\newcommand{\zerovec}{\mathbf 0}\) \(\newcommand{\onevec}{\mathbf 1}\) \(\newcommand{\real}{\mathbb R}\) \(\newcommand{\twovec}[2]{\left[\begin{array}{r}#1 \\ #2 \end{array}\right]}\) \(\newcommand{\ctwovec}[2]{\left[\begin{array}{c}#1 \\ #2 \end{array}\right]}\) \(\newcommand{\threevec}[3]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \end{array}\right]}\) \(\newcommand{\cthreevec}[3]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \end{array}\right]}\) \(\newcommand{\fourvec}[4]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \\ #4 \end{array}\right]}\) \(\newcommand{\cfourvec}[4]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \\ #4 \end{array}\right]}\) \(\newcommand{\fivevec}[5]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \\ #4 \\ #5 \\ \end{array}\right]}\) \(\newcommand{\cfivevec}[5]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \\ #4 \\ #5 \\ \end{array}\right]}\) \(\newcommand{\mattwo}[4]{\left[\begin{array}{rr}#1 \amp #2 \\ #3 \amp #4 \\ \end{array}\right]}\) \(\newcommand{\laspan}[1]{\text{Span}\{#1\}}\) \(\newcommand{\bcal}{\cal B}\) \(\newcommand{\ccal}{\cal C}\) \(\newcommand{\scal}{\cal S}\) \(\newcommand{\wcal}{\cal W}\) \(\newcommand{\ecal}{\cal E}\) \(\newcommand{\coords}[2]{\left\{#1\right\}_{#2}}\) \(\newcommand{\gray}[1]{\color{gray}{#1}}\) \(\newcommand{\lgray}[1]{\color{lightgray}{#1}}\) \(\newcommand{\rank}{\operatorname{rank}}\) \(\newcommand{\row}{\text{Row}}\) \(\newcommand{\col}{\text{Col}}\) \(\renewcommand{\row}{\text{Row}}\) \(\newcommand{\nul}{\text{Nul}}\) \(\newcommand{\var}{\text{Var}}\) \(\newcommand{\corr}{\text{corr}}\) \(\newcommand{\len}[1]{\left|#1\right|}\) \(\newcommand{\bbar}{\overline{\bvec}}\) \(\newcommand{\bhat}{\widehat{\bvec}}\) \(\newcommand{\bperp}{\bvec^\perp}\) \(\newcommand{\xhat}{\widehat{\xvec}}\) \(\newcommand{\vhat}{\widehat{\vvec}}\) \(\newcommand{\uhat}{\widehat{\uvec}}\) \(\newcommand{\what}{\widehat{\wvec}}\) \(\newcommand{\Sighat}{\widehat{\Sigma}}\) \(\newcommand{\lt}{<}\) \(\newcommand{\gt}{>}\) \(\newcommand{\amp}{&}\) \(\definecolor{fillinmathshade}{gray}{0.9}\)PURPOSE
- 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 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 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.
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 covers safety precautions for handling caustic substances, such as unknown acids and sodium hydroxide, stressing the importance of avoiding contact with skin and clothing. It details the required equipment, including pH probes and burets, and outlines a titration procedure for a diprotic weak acid with NaOH, which involves dilution, mixing, and careful pH monitoring until reaching pH 11. Additionally, it emphasizes proper waste disposal and cleaning equipment.
- 4.3: Titration of a Diprotic Acid - Data and Report
- This page presents structured data collection and analysis from a titration experiment, featuring a table for titration data like buret readings and pH values from two trials. It includes post-lab questions on generating titration curves, estimating equivalence points, calculating half-equivalence points and their pKa and Ka values, and finding the unknown acid concentration. Finally, it emphasizes comparing pKa values to identify the potential unknown acid.