The concentration of 1,10-phenanthroline in the colour
reagent solution was also studied, using the same conditions
described above. Concentration values between 0.10
and 0.70 g l−1 were tested. For the 0.10 g l−1 solution, absorbance
values increased for Fe(II) standards containing up
to 4mg l−1. Above this value, absorbance values were the
same for all iron solutions. For the other concentration values
of 1,10-phenanthroline, similar linear calibration curves
were obtained for the Fe(II) standards tested. Hence, the lowest
concentration of this group (0.25 g l−1) was chosen for
further studies.