6g tartaric acid (optional, but increases shelf life)
Steps
Heat up water in a pot, add sugar and stir to dissolve over medium heat
Take off the heat and add lime zest and juice
Let it cool down to stop the cooking process (e.g. pour the mixture in a jar, and place the jar in a bowl with iced water, until cool)
Strain with a cheesecloth or a fine mesh, to remove all sediments
Add citric acid and tartaric acid, stir to dissolve
Bottle, and keep refrigerated
Sadly it won’t last for more than 3-4 days, unless you can vacuum pack it. This recipe makes a large batch, as that is what we used in the bar, but you can halve amounts