One basin of water (about a gallon) to one tablespoon apple cider vinegar. Soak for 10-15 minutes, then rinse in a colander.
Some veg - leeks in particular - are pretty hard to render grit-free, particularly if you buy organic.
So I soak them in the water/vinegar solution first, then chop them how I want them, then rinse them again in the colander w/cool running water.
In the States, I used to use Dr. Bronner's Liquid Castille soap - the unscented, no additives at all whatsoever, with running water.