The colorimetric analysis of arsenic (As) in aqueous solution according to the
molybdenum blue method was investigated. Maximum absorbance reading can be
obtained at λ =880 nm at least 20 minutes after addition of molybdate reagent to the
arsenate, As(V) solution. The absorbance-concentration relationship obeyed the Beer-
Lambert law at low arsenate concentrations (≤5 µgmL⁻¹) but showed a negative deviation
at high (>5 µgmL⁻¹) concentrations. At fixed arsenate concentration, the presence of
phosphate ion gave a higher absorbance reading, which increased with the increase in
phosphate concentration. Analysis of river water and acid mine drainage( AMD) samples
gave total As concentration in the range of 0.013-0.019 µgmL⁻¹ (Mean=0.016f0.003
µgmL⁻¹) and 0.012-0.024 µgmL⁻¹ (Mean=0.017±0.005 µgmL⁻¹), respectively. These
values are higher than the WHO guideline value (0.01 µgmL⁻¹) for arsenic concentration
in water. Arsenite, As(III), and arsenate, As(V), is dominant in river water and AMD,
respectively
