Citric acid enhancement of copper solubility and toxicity in bicarbonate solutions

Copper solubility and cupric ion toxicity to photosynthesis of isolated chloroplasts and algal cells in bicarbonate solutions were enhanced by the addition of citric acid. The toxicity of cupric ions to algal growth during the first 2 days of incubation was increased by the presence of citric acid in the growth medium. No adverse effects of citric acid on cupric sulfate as an algicide were detected. Sulfhydryls were oxidized in the presence of cupric ions to disulfides with one mole of oxygen taken up for every 4 mol of sulfhydryls oxidized. In bicarbonate solutions, this reaction was stimulated by the addition of citric acid. A four to one ratio of citric acid to cupric sulfate pentahydrate on a weight to weight basis, 5.2 mol citric acid per mol copper, appeared to be the most efficient ratio. The citric acid influence on copper solubility and cupric ion toxicity was not due to a pH effect.