A flow injection multisite detection system was developed for correction of the sample blank in a colorimetric determination. By using detector relocation, a single spectrophotometer is used for the sequential reading of the sample blank and of the colored product; the sample crosses the flow cell to measure intrinsic absorption, the color reagent is subsequently added, and the flow cell is relocated to provide the reading of the resulting plug. The subsequent detector relocation immediately after peak maximum increases the determination frequency. This strategy was tested in the colorimetric determination of iron in soil samples based on the reaction of Fe (II) with 1,10-phenanthroline after reduction of Fe (III) to Fe (II) by ascorbic acid. The results obtained for 15 soil extracts were in good agreement with those obtained by the reference procedure. Relative standard deviations better than 4% were obtained, with a sampling rate of 30/h.