Mix 240mL of hot water, 45mL lime juice, 45mL microplaned lime zest (from the peels of roughly 4 limes), 250g of sugar, and 30mL of citric acid (food-grade quality of course). Blend until everything is well dissolved. Pass through a filter and store. [This is much a much better recipe, by Morgenthaler. The acidity is a big part of it. Citric acid can easily be bought on Amazon or health-food stores.]