When CA was determined, 1 mL of the filtrate was added to a10 mL flask, 0.5 mL 5% NaNO2and 0.5 mL 10% Al (NO3)3added, leftstanding for 5 min at room temperature, then 2 mL 1 mol L−1NaOHsolution added. The final volume of 10 mL was made up with 80%ethanol. CA was measured at 530 nm by a spectrophotometer, with80% ethanol as the blank. CA content in the sample was calculated with a standard curve (Nie et al., 2010).