Despite the fact that citrus fruits - especially grapefruit - are so acidic, the effect they have on your body is the opposite; alkaline producing. The only drawback is that citrus and tropical fruits are generally lower on the ORAC scale when compared to berries and even most apples.
ORAC Source
USDA Database for the Oxygen Radical Absorbance Capacity (ORAC) of Selected Foods, Release 2 - Prepared by Nutrient Data Laboratory, Beltsville Human Nutrition Research Center (BHNRC), Agricultural Research Service (ARS), U.S. Department of Agriculture (USDA) - May 2010