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This is an illustration of another acid-base reaction using a buffer solution.

  • Formalin (formaldehyde solution in water)  - Solution 1
  • NaHSO3 and Na2SO3 in water  - Solution 2
  • Phenolphthalein indicator (red)
  • Thymolphthalein indicator (blue)
  • 1M MgCl2 solution (white)
  • 3 Erlenmeyer flasks



Each of the three flasks contains Solution 1 and a small amount of either Phenolphthalein, Thymolphthalein, or 1M MgCl2 to which 200 mL of Solution 2 are added with mixing. After approximately 30 seconds each flask rapidly changes colors as a result of the acid-base reaction between bisulfite and formaldehyde, producing hydroxide ions (HO-). The sulfite/bisulfite mixture serves as a buffer and the solution remains acidic until the bisulfite is consumed. When the solutions have become basic, the indicators turn colors or magnesium ions (Mg2+) precipitate as Mg(OH)2.

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Other Demos: Chemiluminescence of Luminol, Photoresists, High Energy Emitters for Organic Light-emitting Diodes, The Spectrum and Complementary Colors, "Titrations" with Dry Ice, The Red-White-Blue Clock

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Chemical Demonstrations – A Handbook for Teachers of Chemistry by Bassam Z. Shakhashiri, The University of Wisconsin Press, Vols. 1-4 (ISBN: 0-299-08890-1).