Students determine the iron mass
in dietary supplements using four
colorimetric assays. These colorimetric assays were selective for
Fe3+. It was complexed by thiocyanide, salicylate, gallic
acid, and Fe(CN)6
4–, forming blood-red,
red, violet, and blue complexes, respectively. The λmax of these complexes were 447, 480, 570, and 700 nm, respectively.
These assays were scaled-down, where 1 mL of chromogenic reagent was
mixed with 0.1 mL of sample. Salicylic acid, gallic acid, and K4[Fe(CN)6] were harmless and cheap chromogenic reagents
that could be used instead of traditional chromogenic reagents such
as KSCN and phenanthroline. Quantitative analysis was carried out
using absorbance measured at λmax of each assay (standard
method) and 96-well-plate digital images obtained with a desktop scanner
(proposed method). The matrix effect was investigated by comparing
the standard addition curves with the external calibration curves,
where no matrix effect was found. Results obtained using the four
assays were compared using hypothesis tests such as one-way ANOVA,
post-hoc tests, and plots. Investigation of interactions between samples
and assays was carried out using two-way ANOVA. Statistical tests
were carried out using open-source and graphical user interface programs
(JASP and R Commander). The student learning goals of this experiment
include comparing multiple assays using hypothesis tests. Learn about
the principles of green chemistry, method scale-down, and matrix effect.