How to Make Thymolphthalein pH Indicator


Thymolphthalein pH Indicator
Thymolphthalein is colorless below pH 9.3. Somewhere between pH 9.3 and 10.5 it turns blue. It is always blue above pH 10.5. The pH indicator is used for disappearing ink, titrations, and as a laxative.

Thymolphthalein is a pH indicator that changes color from colorless to indigo blue. It is colorless below a pH of 9.3 to 10.5 and blue at more alkaline pH values. Here are instructions to make thymolphthalein pH indicator, a look at its uses, and information on where to buy it and how much it costs.

Thymolphthalein Materials

For 100 ml of a 4% thymolphthalein solution:

  • 0.04 g thymolphthalein
  • 95% ethanol
  • distilled water

Prepare Thymolphthalein Indicator

Like other phthalein dyes, thymolphthalein isn’t very soluble in water. First dissolve it in ethanol and then dilute it with water.

  1. Dissolve 0.04 g thymolphthalein in 50 mL of 95% ethanol.
  2. Dilute this solution to 100 mL with distilled water.

Thymolphthalein Uses

Thymolphthalein is used in disappearing ink, to make color-change bubbles, and as a pH indicator. It has an inky blue color and readily changes to its colorless form from exposure to air or carbon dioxide from breath.

Blue Test Tubes
The blue color of thymolphthalein is actually more indigo than blue.

Thymolphthalein (or the related compound phenolphthalein) is used in Kastle-Meyer test, which is a presumptive test for blood. The premise of the test is that the peroxidase-like activity of hemoglobin oxidizes the dye to change it from colorless to colored. To perform the test, a swab is taken of a suspected blood spot. A drop of indicator is added to the swab, followed by a drop of hydrogen peroxide. A positive test for blood is an immediate color change to blue for thymolphthalein or red/pink for phenophthalein. After about 30 seconds, the swab will change color on its own from oxidation by air.

Another use of thymolphthalein is as a laxative.

Where to Get Thymolphthalein

Thymolphthalein is fairly inexpensive and is readily available from chemical suppliers, educational suppliers, and online at Amazon. You can get it as a powder or a prepared liquid solution. A student-grade bottle of prepared solution from Amazon costs about $15. Reagent-grade powder costs more, but you also get more. Use a reagent -grade chemical for titrations and other analytical tests. If you need a lot of indicator, it’s more economical to buy the powder and prepare your own indicator solution.

It’s possible to synthesize thymolphthalein from thymol and phthalic anhydride:

References

  • Hubacher, MH; Doernberg, S; Horner, A (1953). “Laxatives: chemical structure and potency of phthaleins and hydroxyanthraquinones”. J Am Pharm Assoc Am Pharm Assoc42 (1): 23–30. doi:10.1002/jps.3030420108
  • McCullagh, James V.; Daggett, Kelly A. (2007) “Synthesis of Triarylmethane and Xanthene Dyes Using Electrophilic Aromatic Substitution Reactions.” J. Chem. Ed. 84 (11): 1799–1802 doi:10.1021/ed084p1799.
  • Sabnis, Ram W. (2010). “Phthalein Dyes”. Kirk-Othmer Encyclopedia of Chemical Technology. ISBN 978-0471238966. doi:10.1002/0471238961.phthsabn.a01

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