Limon tuzu, or lemon salt, is the Turkish name for citric acid crystals, and it is not a salt in the seasoning sense. This coarse-grain version from Bodrum comes in a 100 g stand-up pouch with a tear-and-reseal strip and a window showing the clear crystals. The front label says 'Citric Acid Crystals' and 'No artificial colouring'.
In Turkish kitchens it is a pantry staple for adding sourness without the liquid of fresh lemon. Use a small pinch in pickling brines and in jam and fruit preserves, where the acidity helps the flavour, in tomato-based soups and olive oil vegetable dishes for a bright finish, or dissolve it in cold water with sugar for quick homemade lemonade. Coarse crystals dissolve more slowly than fine powder, so stir until clear and add a little at a time, tasting as you go.
It is very sour and is used by the pinch, never like table salt. Close the pouch tightly after use and keep it dry, as the crystals will clump if they take up moisture.