Learn the procedure for making cresol purple indicator solution.
The compound that change color when exposed to acidic or basic solutions are called indicator. 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.
Meta-cresol purple (C21H18O5S), commonly known as m-cresol purple or m-cresolsulfonphthalein, is a triarylmethane dye and a pH indicator. It comes in a green or brick-red powder that is acidic and hydrophobic in nature, with slightly soluble in water and ethanol. It is primarily used as an acid-base indicator and in spectrophotometric pH measurements. It's also used to stain epithelial cells and proteins.
How to prepare cresol purple indicator for titration:
- Weigh accurately 40 mg of m-cresol purple and pour it into a 100.00 ml volumetric flask containing 02.08 ml of 0.05N NaOH solution and 20.00 ml of 95% ethanol.
- Once it is dissolved, dilute to 100.00 ml with distilled water.
- The concentration of the prepared solution is about 0.4 g/L.
m-cresol purple has a pKa of 8.3 and it has two useful pH ranges are 1.2 - 2.8 (red to yellow) and 7.4 - 9.0 (yellow to violet). It changes color in both pH ranges: below pH 1.2 it turns red, above pH 2.8 it turns yellow, and below pH 7.4 it turns yellow, above pH 9.0 it turns violet.
References:
- Wikipedia contributors. (2021, October 3). Metacresol 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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