Combine 1 cup granulated sugar and 1 cup water in a small saucepan. Heat over medium, stirring occasionally until the sugar is fully dissolved and the mixture just reaches a gentle simmer (do not boil vigorously). Once dissolved, remove from heat.

Allow the syrup to cool for 5 minutes so it is warm but not hot before adding citrus components (hot syrup will alter fresh citrus flavor).

Citrus & flavor

While the syrup is still warm, stir in 2 tbsp fresh lemon juice, 2 tbsp fresh lime juice, 1 tsp lemon zest, 1 tsp lime zest, 1 tsp citric acid, and 1 tbsp light corn syrup (if using). Stir until fully combined. Taste carefully: the mixture should be bright, tart, and slightly sweeter than you want the finished drink because it will be diluted by carbonation.

Let the combined syrup cool completely to room temperature, then chill in the refrigerator for at least 30 minutes. Strain the syrup through a fine mesh sieve to remove the zest solids and any pulp for a clear syrup.

Carbonation & finishing

To serve one 12–16 oz (about 350–475 ml) glass: fill a glass with 1/2 to 3/4 cup ice if desired. Add 2 to 3 tablespoons of the chilled citrus syrup (adjust to taste; 2 tbsp yields a lighter flavor, 3 tbsp stronger).

Slowly pour 1 cup (about 240 ml) very cold carbonated water over the syrup to preserve bubbles and encourage mixing. Stir gently once or twice with a bar spoon to combine. Taste and adjust: add additional syrup (up to 3 tbsp) for more sweetness/tartness, or dilute with more carbonated water if too sweet.

Garnish with lemon and lime slices if desired and serve immediately.