Role of citric acid in delaying gypsum precipitation

The comprehension of the mechanisms of nucleation and growth that regulate the gypsum (calcium sulfate dihydrate) precipitation is fundamental to prevent scaling problems. The present paper reports measurements of the induction period for gypsum nucleation when citric acid is added in solution as an additive to retard scale formation. The supersaturation ratio was varied in the interval 2.3-6.0, the concentration of citric acid ranged from 0.01 to 0.30 g/L, while in all experiments the temperature was fixed to 25 degrees C. The induction period values were compared to those previously obtained, by using the same experimental technique, in the absence of foreign ions in the mother liquor, showing that the citric acid has a strong retarding effect towards gypsum nucleation.