Drop-Counter-Assisted Acid/Base Titrations in the Quantitive Analysis Laboratory. An In-Depth Guided-Inquiry Laboratory Exercise

TCE

The Chemical Educator

ISSN: 1430-4171 (electronic version)

Table of Contents

Abstract Volume 13 Issue 5 (2008) pp 289-294

Drop-Counter-Assisted Acid/Base Titrations in the Quantitive Analysis Laboratory. An In-Depth Guided-Inquiry Laboratory Exercise

Frazier Nyasulu^†,* Rebecca Barlag^† and John Macklin^‡

^†Department of Chemistry and Biochemistry, Ohio University, Athens, OH 45701, nyasulu@ohio.edu, and ^‡Department of Chemistry, University of Washington, Box 351700, Seattle, WA 98195
Received October 18, 2006. Accepted June 3, 2007.

Published online: 19 September 2008

Abstract. In this laboratory exercise, groups of seven students with seven drop-counter stations perform NaOH titrations of the following seven acids: hydrochloric, potassium hydrogen phthalate (aliquot from a saturated solution), potassium hydrogen phthalate (aliquot from a saturated solution in 0.50 M KCl), fumaric (aliquot from a saturated solution), maleic, phosphoric, and a mixture of phosphoric and hydrochloric. After completing the titrations, students perform assigned tasks that include the determination of the acid concentrations, all applicable pK_a values, K_an/K_a(n+1) (where applicable)_,and K_sp for potassium hydrogen phthalate. In the second laboratory session, using what they have learned and given acid dissociation constants and experimental conditions, the group is asked to sketch the titration curves of the following acids: sulfuric acid, oxalic acid, citric acid, a mixture of phosphoric acid and sodium hydrogen phosphate (phosphoric acid in excess) and a mixture of phosphoric acid and sodium hydrogen phosphate (sodium hydrogen phosphate in excess). Because students hand in one set of sketches per group, they must reach a consensus. In the third laboratory session, students check their predictions against experimental titration curves. In both laboratory sessions, the instructor only answers questions related to instrument difficulties and watches for laboratory safety. Students are encouraged to consult with other groups.

Key Words: Laboratories and Demonstrations; quantitative analysis; acid/base titrations; titration curves and derivative plots; guided inquiry

(*) Corresponding author. (E-mail: nyasulu@ohio.edu)

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Supporting Materials:

Laboratory documentation and instructor notes are available in a Zip file (4.16 MB)