Methyl orange was obtained from Sigma–Aldrich (USA). Sodium
dihydrogen phosphate, hydrochloric acid, sodium hydroxide and
methanol were purchased from Merck (Darmstadt, Germany).
The pH buffer solutions were prepared from sodium dihydrogen
phosphate (0.02 M). The final pH was adjusted by the addition of
1.0 M hydrochloric acid or sodium hydroxide solutions. A stock
solution (0.1 M) was prepared by dissolving methyl orange in
methanol. Working solutions at concentration of 105 M were