Place the cinchona bark in a saucepan, pour water over it, and bring to a boil.
Turn off the heat as soon as it boils, cover the pan, and let it steep for 20 minutes.
Strain the liquid through a coffee filter.
Add sugar and citric acid, and stir until the sugar is dissolved.
Place it in the refrigerator.
Pour the cold syrup into your sterilized container, and refrigerate.
The syrup can be stored in the refrigerator for 2-3 weeks.
Mixing ratio: 1 part tonic syrup to 3 parts soda water.