Grapefruit Soda Syrup

Place sugar, grapefruit zest and juice, lemon juice, and salt in a medium saucepan along with 2 cups water. Bring to a boil over medium-high heat. Remove from heat and let steep for 1 hour. Strain and discard solids. Transfer syrup to an airtight container and keep refrigerated until ready to use, up to 2 weeks. To serve, fill a 16-ounce glass with ice cubes and add 1 1/2 ounces syrup; top with seltzer and stir to combine.