Bring about 2 kg of rhubarb to the boil in a large saucepan with the water and simmer for about 30 minutes. Line a sieve with a cotton cloth, pour in the rhubarb and collect the juice. Squeeze the rhubarb well in the cloth.
The juice obtained in this way is used to boil the next portions (again 2 kg of rhubarb each). With 10 kg of rhubarb, this results in approx. 4 liters of rhubarb juice. 750 g of sugar and 30 g of citric acid are added per liter of juice obtained. Boil everything together and fill into sterile bottles.
With 10 kg of rhubarb = 4 liters of juice and thus 3 kg of sugar and 120 g of citric acid, this results in 7 liters of syrup.
The recipe works with almost any amount, only 2 kg of rhubarb are sufficient, it just results in correspondingly less juice and the rhubarb taste is not quite as intense.