Here's why:

* Quinine: The key ingredient in tonic water is quinine, which is a bitter alkaloid. However, quinine is added as quinine sulfate, which is a salt.

* Citric Acid: Most tonic water brands also contain citric acid, which is a weak acid.

* pH: The pH of tonic water typically falls between 2.0 and 3.0, indicating acidity.

While quinine is an alkaloid, the presence of citric acid and the overall formulation of tonic water makes it acidic.