Carefully clean the chokeberries of their leaves, stems and wash them thoroughly under running water (best if in a colander).
Put the fruits in a pot and pour in enough water to cover them. Wash the lemon well because you'll be cutting it along with the rind.
Put it with the the chokeberries and blend everything to a sludge, add the citric acid as well. If you'd like a greater quantity of syrup, add more water. Put thus prepared mixture in the fridge for 16 hours (12 is OK as well).
The next day, pour the entire mixture into a cheesecloth and carefully squeeze out the juice. Measure out how much you have. Add 2 lb (1 kg) of sugar for every 4 1/5 cups (1 L) of juice. Put it to boil on the stove and then immediately distribute into dry jars or bottles, close their caps and flip upside down.
Prepared this way, the syrup has a long shelf life (at least a year) but if you're worried about it you can sterilize it for 5 min. too.
It's amazing when dissolved in water or carbonated water and if you add slices of lemon it's practically irresistible!