Which indicator is used in the alkalinity test?

• A. Phenolphthalein
• B. Methyl Orange
• C. Bromothymol Blue
• D. Litmus

Answer: (B)

In alkalinity titration, you’re measuring how much acid is needed to lower the solution to a pH where all carbonate species have been neutralized. The endpoint that reflects total alkalinity occurs around pH 4.5, and methyl orange is the indicator that changes color right in that acidic range, giving a sharp, reliable endpoint for determining total alkalinity.

If you used phenolphthalein, you’d hit a higher endpoint around pH 8.3 and would be measuring carbonate alkalinity rather than total alkalinity. Bromothymol blue and litmus aren’t suited for this specific endpoint because their color changes aren’t centered at the precise pH where all alkalinity species have been neutralized.