Simultaneous determination of FeII and FeIII mixtures by spectrophotometric methods is a difficult problem in analytical chemistry because of spectral interferences. By multivariate calibration methods, such as partial least squares (PLS), it is possible to obtain a model adjusted to the concentration values of the mixtures used in the calibration range. The method is based on developing the reaction between the analytes and 1,10 phenanthroline and 5-sulfosalicylic acid as the chromogenic reagent at pH = 4.5. Experimental conditions were established so as to reduce interferences, decrease system complexity and produce a robust procedure that could be used for routine analysis. Spectra should be recorded from 5 to 10 minutes after mixing the reagents. In this study, the calibration model is based on absorption spectra in the 400–600 nm range for 34 different mixtures of FeII and FeIII. Calibration matrices contained 0.1–7.0 and 0.5–14.0 mg cm–3 of FeII and FeIII, respectively. Detection limits were 0.045 and 0.158 mg cm–3 for FeII and FeIII, respectively. RMSEP for FeII and FeIII was 0.1559 and 0.2067, respectively. The procedure was confirmed by FeII and FeIII analyses in pharmaceutical products, and good reliability of the determination was proven
