Mix together pineapple juice, water, vinegar, sugar, ketchup, pepper flakes, salt and soy sauce in a medium saucepan. Set the pan over medium-high heat and bring to a boil.
¾ cup 6 ounces pineapple juice, 1/3 cup water, 1/3 cup white or rice vinegar, ½ cup granulated sugar, 3 tablespoons ketchup, 1/8 teaspoon crushed red peppers, ¼ teaspoon kosher salt, 2 tablespoons soy sauce