Phosphate buffer solution (PBS) 0.1 M pH 6.8 and 7.4 were prepared from stock solutions A and B then adjusted to the desired volume using sodium hydroxide. Solution A consisted of 0.2 M solution of monobasic sodium phosphate (27.8 g/l water), while solution B consisted of 0.2 M solution of dibasic sodium phosphate (53.65 g of
Na2HPO4.7H2O in 1 l water). PBS pH 6.8 was prepared from 51 ml of solution A and 49 ml of solution B and diluted to 200 ml.