The samples were mixed with 0.7 ml of 0.5 M HCl then 0.7 ml of 0.1 M barium chloride–polyethylene glycol reagent was added and mixed again. After 10 min, the samples were mixed again and the absorbance read at 600 nm using a spectrophotometer. The readings were compared to a standard solution of 0–2 mM Na2SO4.