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  Home > JCE Print > Journal of Chemical Education > Issues > 2008  > April  >
In the Classroom
JCE DigiDemos: Tested Demonstrations
Phenolphthalein—Pink Tornado Demonstration
Bruce R. Prall
Department of Chemistry, Marian College, Fond du Lac, WI 54935

checked by Jeannine Eddleton
Department of Chemistry, Virginia Polytechnic Institute and State University, Blacksburg, VA 24061

Cover
April 2008
Vol. 85 No. 4
p. 527

Abstract
The phenolphthalein–pink tornado demonstration utilizes the vortex generated by a spinning magnetic stirring bar in a 1 L graduated cylinder containing 0.01 M HCl to demonstrate Le Châtelier's principle as it applies to the phenolphthalein equilibrium in water

H2In + 2H2O arr 2H2O+ + In2-

where H2In is phenophthalein. The addition of 3–4 drops of phenolphthalein indicator solution followed immediately by 3–4 drops of 50% (w/w) NaOH to the vortex of the HCl solution results in a shift to the right in the equilibrium owing to the reaction of OH- + H3O+ to form water. This shift is accompanied by the vortex becoming visible by the appearance of a pinkish-red color caused by an increase in In2- concentration within the localized region of the vortex. The demonstration also provides one an excellent opportunity to discuss the topics of limiting reagent and reagent in excess. Some insight regarding the extent to which uniform mixing is achieved when using a magnetic stirrer is also provided.

Included is a note from the Feature Editor, Ed Vitz.

More Information
*
Citation
Prall, Bruce R. J. Chem. Educ. 2008, 85, 527.
*
Keywords
Acids / Bases; Demonstrations; Equilibrium; First-Year Undergraduate / General; Hands-On Learning / Manipulatives; High School / Introductory Chemistry; Inquiry-Based / Discovery Learning; Kinetics; pH
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History
Created:
Last Updated:
2/26/2008
3/4/2008
 Caution! 
Experiments, laboratory exercises, lecture demonstrations, and other descriptions of the use of chemicals, apparatus, instruments, computers, and computer interfaces are presented in the Journal of Chemical Education as illustrative of new or improved ideas or concepts in chemistry instruction and are directed at qualified teachers. Although every effort is made to assure and encourage safe practices and safe use of chemicals, the Journal of Chemical Education cannot assume responsibility for uses made of its published materials. Many chemicals are hazardous. Precautions for the safe use of hazardous chemicals and directions for their proper disposal are described in the Material Safety Data Sheets and on the labels. We strongly urge all those planning to use materials from our pages to make choices and to develop procedures for laboratory and classroom safety in accordance with local needs and situations.
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