Preparation of bromothymol blue indicator solution

Learn the procedure for making bromothymol blue indicator solution.

The compounds that change color when exposed to acidic or basic solutions are called indicators. Color indicators are commonly used to measure pH and are added to the reaction mixture to identify the titration endpoint/equivalence point.

Most pH indicators are weak acids or bases. For titration, you must select a pH indicator that changes color only at the equivalence point.

How to prepare bromothymol blue indicator for titration:

• Weigh accurately 50 mg of bromothymol blue and pour it into a 100.00 ml volumetric flask containing 04.00 ml of 0.05N sodium hydroxide solution and 05.00 ml of 95% ethanol, warm it.
• After the solution has been effected, add 45.00 ml of 95% ethanol and dilute to 100.00 ml with distilled water.
• The concentration of the prepared solution is about 0.5 g/L.

Bromothymol blue (C27H28Br2O5S) is also known as bromothymol sulfone phthalein and BTB, which acts as a weak acid in a solution. It used as is a pH indicator and for measuring the presence of carbonic acid in a liquid. BTB is a suitable choice for any titration with a near-neutral equivalence point.

Bromothymol blue has a pKa of 7.1 and the pH range is 6.0 to 7.6. In an acidic medium (below pH 6.0) it turns yellow, while in a basic condition (above pH 7.6) it turns blue, and green in a neutral solution.

Bromothymol blue is prepared by the addition of elemental bromine with thymol blue in a glacial acetic acid solution. It comes in the form of a powder, crystals, or chunks. It might seem yellow or blue depending on whether it is protonated or deprotonated.

References:

1. Wikipedia contributors. (2022, January 25). Bromothymol blue. In Wikipedia, The Free Encyclopedia. Available Here:
2. Law.resource.org. 2022. [online] Available Here:
3. Dr John Elfick, j., 2022. Learn how to prepare useful acid-base indicators. Uq.edu.au. Available Here:
4. Sabnis, R. W. (2008). Handbook of Acid-Base Indicators. Boca Raton, FL: CRC Press.

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