An Expeditious and Sensitive Microprecipitation Approach Using Chrome Azurol S and Cetyltrimethylammonium Bromide for Visual Detection of Fe(II) and Fe(III) in Water

The aim of this work is to develop a simple, sensitive, and reliable colorimetric test method for the visual detection of Fe(II) and Fe(III) in water. The sample solution of Fe(III) was initially complexed with chromeazurol S (CAS) followed by ion-pair association with cetyltrimethylammonium bromide (CTAB) in the presence of perchlorate ions. This led to the formation of a bulky and insoluble fluffy precipitate involving the physical entrapment of the ternary complex of Fe-CAS-CTAB, which floats on the top of the solution. Depending on the concentration of Fe(III), this type of ternary complex produces an impressive color change from reddish magenta to purple and then to cyan-blue, easily detectable with the naked eye without the need for expensive instrumentation. The ternary-colored complex was formed instantly and remained stable for at least 8 h with constant absorbance. For the determination of total iron and Fe(II), complete oxidation of Fe(II) to Fe(III) was carried out using 100 μL of KMnO₄ (0.005%). The minimal response with the test method was found to be about 5 ng/mL of Fe(III) in 2 min of the total sample testing procedure. However, Fe(III) could be quantified using a spectrophotometer after dissolving the fluffy precipitate in toluene solvent. The calibration graph was linear in the range of 5–30 ng/mL of Fe(III) with a correlation coefficient of 0.9931 and a limit of detection of 0.9 ng/mL. The relative standard deviation was found to be less than 6%. At the end of the study, the efficacy of the method was evaluated by its application to groundwater and thermal power plant coolant waters.