Combine the grapes, sugar, water, lemon zest, orange zest, and salt in a medium saucepan. Bring to a boil over medium-high heat, then reduce the heat to a simmer. Using a wooden spoon, crush the grapes against the side of the pan. Cook until completely soft, about 20 minutes. Remove from the heat and strain the mixture through a fine mesh sieve into a large bowl. Stir in the lemon juice. Transfer to a container and chill.

For each soda, pour 1/4 cup of syrup into a glass with ice. Top with club soda or seltzer.

The syrup will keep for up to a month in the refrigerator.