Learn the procedure for making bromocresol green 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.
Bromocresol green (C21H14Br4O5S) is a triphenylmethane-based dye. It is a sulfonphthalein, which is a type of dye. It has a pKa of 4.7 and the pH range is 3.8 to 5.4. It commonly dissolves in ethanol or water, but can also be dissolved in benzene and diethyl ether.
How to prepare bromocresol green indicator for titration:
- Weigh accurately 0.1 g of bromocresol green and pour it into a 100 ml volumetric flask containing 0.58 ml of 0.05 N NaOH solution and 05.00 ml of rectified spirit/ethanol, warm it.
- After the solution has been effected, add 10.00 ml of rectified spirit/ethanol and dilute to 100 ml with distilled water.
- The concentration of the prepared solution is about 1 g/L.
Bromocresol green ionizes in an aqueous solution to produce the green monoanionic form (yellow), which deprotonates at higher pH and produces the dianionic form (blue), which is stabilized by resonance.
In an acidic medium (less than 3.8), bromocresol green turns yellow, while in a basic condition (higher than 5.4), it turns green. It's commonly used in strong acid+weak base titration, for electrophoresis, and in microbial growth media.
References:
- Law.resource.org. 2022. [online] Available Here:
- Wikipedia contributors. (2022, January 5). Bromocresol green. In Wikipedia, The Free Encyclopedia. Available Here:
- Dr John Elfick, j., 2022. Learn how to prepare useful acid-base indicators. Uq.edu.au. Available Here:
- Sabnis, R. W. (2008). Handbook of Acid-Base Indicators. Boca Raton, FL: CRC Press.
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