Why do we use phenolphthalein as an indicator in titration

Bromophenol blue rather than phenolphthalein is used as an indicator in antacid titration as bromophenol blue changes the color at pH less than 7.

Further explanation

Titration is the quantitative analysis in which the known concentration of a compound is used to determine the unknown concentration of another compound. In this, the solution of known concentration is added slowly to the solution of unknown concentration but known volume. This addition continues until complete neutralization takes place, and that is determined by equivalence point.

The equivalence point is the point at which numbers of moles of both the solution added becomes equal, and that is signified by a change in physical state of solution such as the color. The point at which the color of solution changes is called as end point of the solution and the pH at this point is determined by an indicator.

Examples of indicator are phenolphthalein, bromophenol blue, methyl red, and bromothymol blue.

• Phenolphthalein changes color at pH greater than 7

• Bromophenol blue changes color at pH less than 7

Antacid titration is an acid-base titration in which antacid that acts as bases and is mainly composed of metal hydroxides like NaOH reacts with excess amount of acid such as HCl. The resulting solution is an acidic solution having pH less than 7. The excess amount of HCl in solution is then neutralized by adding more NaOH to completely neutralize the solution and is known as back titration.

In an antacid titration, the pH is determined by using indicators showing color change below pH 7.

For example, bromophenol blue shows a change in color in range between pH 3.0 to pH 4.6 and phenolphthalein shows a color change in range between pH 8.3 to pH 10.

Hence, bromophenol blue is used as an indicator rather than phenolphthalein in antacid titration.

Learn more:

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Answer details:

Grade: Senior School

Subject: Chemistry.

Chapter: Titration.

Keywords: acid, base, titration, equivalence point, endpoint, indicators, antacid, bromophenol blue, phenolphthalein, NaOH, HCL.