Heat a pot of water to boiling. Place a metal or glass bowl on top, with the bottom of the bowl just above the boiling water. Add rhubarb and cook for 25 minutes without stirring.

Meanwhile, dissolve sugar and baking soda in hot water in bowl, stirring now and then.

Pour sugar mixture over the steamed rhubarb in the bowl and cook over boiling water another 2 to 3 minutes.

Pour rhubarb mixture through a mesh sieve into a clean jar; press with a wooden spoon to extract more syrup. Store in the refrigerator.