Learn the procedure for making bromocresol purple indicator solution.
Bromocresol purple (C21H16Br2O5S) chemically described, as 5, 5’-dibromo-o-cresolsulphonphthalein, is a dye of the triphenylmethane family (triarylmethane dyes) and a pH indicator. It comes as a pinkish brown crystalline powder, generally used as a fluorescent stain in yeast cells.
Bromocresol purple has a structure that changes with pH, similar to bromocresol green. The acidity level changes cause the shifting equilibrium between two different structures with different colors shifts. The chemical has a sulfonate structure in a near-neutral or alkaline solution, which gives the solution a purple color. It turns into a sulfone (cyclic sulfonic ester) as the pH drops, causing the solution to turn yellow.
How to prepare bromocresol purple indicator for titration:
- Weigh accurately 40 mg of bromocresol purple and pour it into a 100.00 ml volumetric flask containing 01.48 ml of 0.05N NaOH solution and 10.00 ml of 95% ethanol, warm it.
- Once it is dissolved, dilute to 100.00 ml with distilled water.
- The concentration of the prepared solution is about 0.4 g/L.
Bromocresol purple has a pKa of 6.3 and the pH range is 5.2 to 6.8. It is turning yellow (below pH 5.2), in acidic solutions, while in a basic condition (above pH 6.8) it turns bluish-violet. It is mainly used in a medical laboratory, acid-base indicator, as a dye, and two-color indicator.
References:
- Wikipedia contributors. (2021, May 1). Bromocresol purple. In Wikipedia, The Free Encyclopedia. Available Here:
- Law.resource.org. 2022. [online] 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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