Sprinkle some water on the floor and see if any of it is absorbed. If there is a lot of old adhesive or other contaminants on the floor and water isn't absorbed, the mortar won't stick. An acid wash may not help...usually it takes mechanical work to get it off - either a razor scraper or if really bad, a scarcifier (basically a big grinder to take the top layer off). If the concrete has a really smooth surface, etching it can help. The better scarcifiers come with dust collection. They can also help take off high spots to level the floor.
If water gets absorbed and doesn't just bead up, use of a thinset mortar specified for use over cutback will suffice. This will usually be one of the better, modified (laytex addatives) thinsets.
Are there any cracks in the existing floor? They need to be handled, or they may telegraph into the tile. If any of the cracks (if there are any) are at different heights (one side either rose or fell in relation to the other), generally, the recommendation is to not tile it.