Which indicator is used to test alkalinity?

The correct choice for testing alkalinity is phenolphthalein. This indicator is specifically used in titrations to determine the presence and concentration of hydroxide ions in a solution, which correlate with the alkalinity levels. When added to a sample, phenolphthalein exhibits a clear color change from colorless to pink as the pH of the solution rises above 8.2, indicating the presence of alkaline substances.

In contrast, methyl orange is most effective in acidic environments, changing color between pH 3.1 and 4.4, making it unsuitable for measuring alkalinity, as it does not reflect higher pH values typically associated with alkaline solutions. Bromothymol blue is capable of indicating pH changes within a neutral to slightly alkaline range but is not the best indicator for alkalinity tests that require a focus on higher pH values. The universal indicator provides a broad spectrum of pH indication, but it doesn't provide the specific clear delineation that phenolphthalein does for alkalinity testing. Thus, phenolphthalein is the most appropriate indicator for measuring the alkalinity of a solution, making it the correct choice.