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The Chemical Educator
ISSN: 1430-4171 (electronic version)
Abstract Volume 21
(2016) pp 88-92
Acid-Base Titration Curves in the General Chemistry Laboratory: an In-Depth Study
Frazier Nyasulu*, Brandon Allman, Daniel Nething, Rebecca Barlag, Lauren McMills and Phyllis Arthasery
Department of Chemistry and Biochemistry, Ohio University, Athens, OH 45701, nyasulu@ohio.edu
Received October 28, 2015. Accepted December 18, 2015.
Published: 2 May 2016
Abstract. Performing a number of carefully selected acid-base titrations provides general chemistry students with an in-depth study of titration curves and an opportunity to expand their knowledge of acid-base chemistry. Titrations are automated and performed in an identical fashion; ~1 mmol of analyte is titrated with 0.200 M titrant. Pairs of students are assigned one titration and then go from station to station to study the titration and derivative curves obtained by others. In the first of three laboratory sessions, water (blank), HCl, K2HPO4, HC2H3O2 and NH4Clare titrated with NaOH, and NaC2H3O2 and K3PO4 are titrated with HCl (this titration is terminated at 150% of the first equivalence point volume). By comparing the acid/NaOH titration curves and determining the acid ionization constant (Ka) of each acid, students attribute the differences in the titration curves to their different Ka values and observe the minimum Ka value required for an acid to show a discernable equivalence point. The lowest acid Ka that shows a discernable equivalence point under the experimental conditions is ~10-10. For NaC2H3O2 and K3PO4 titrations with HCl, the minimum Kb value of a base that shows a discernable equivalence point is ~10-10. Students observe that the smaller the change in pH in the equivalence point region in the acid/NaOH titration curve, the larger the change in pH in the equivalence point region in the conjugate base/HCl titration. In the second laboratory session, diprotic acids (maleic, fumaric and oxalic) are titrated with NaOH. Having determined Ka1 and Ka2 for maleic acid, students apply their knowledge to predict the expected titration curve for disodium maleate/HCl prior to performing this titration. Students construct knowledge about the effects of Ka1 and Ka2 on the titration curves and for maleic and fumaric acids, consider the role of intramolecular forces in determining whether the titration curves show one or two equivalence points. In the third laboratory session, students apply their knowledge from the first two laboratory sessions to sketch the expected titration curves for the triprotic systems: H3PO4/NaOH, citric acid/NaOH and K3PO4/HCl, prior to performing these titrations.
Key Words: Laboratories and Demonstrations; general chemistry; collaborative/cooperative learning; hands-on learning/manipulatives; inquiry-based/discovery learning; acids/bases; aqueous solution chemistry; constructivism; hydrogen bonding; pH; titration/titration analysis
(*) Corresponding author. (E-mail: nyasulu@ohio.edu)
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Supporting Materials:
A PDF file is available that contains instructors notes and a laboratory handout for students. (531 KB)
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