A 6.05 g l−1 of tris(hydroxymethyl)aminomethane solution was prepared for buffering. Working solutions were prepared daily by diluting stock solution in a methanol/buffer
mixture (75:25, (v/v)) and was adjusted to pH 9.1 with 0.2 ml 1 M HCl. Measured solutions were prepared by dilution in he same methanol/buffer solution just before being measured and were deoxygenated for 5 min by ultrasonication.