Using a peeler carefully remove the zest from 2 limes.
Juice the limes.
Add the citric acid to the sugar and mix well.
Add one cup of water, sugar and citric acid to a small pot.
Heat over a low heat until sugar is dissolved.
Add the lime zest, turn down to low, cover and simmer for five minutes.
Remove from heat and remove zest from sugar syrup.
Allow to cool and transfer to a heat proof container.
Add lime juice, then top up water until total volume is about 2 and a half cups (600mls)
Freeze for at least 12 hours, preferably 24, to ensure they freeze properly.
To serve remove from molds and enjoy!