The protocol sequence adopted for the determination of iron in soil extracts is given in Table 1. The procedure included four steps. The first step consisted of filling L1 tube with standard/sample by aspiration through syringe 1 while the other syringes were also filled with their respective solutions. Next, the sample plug trapped in L1 was pushed by carrier towards the confluence point (Fig. 1A, T), where it was merged with reducing solution propelled by syringe 3 until it reached valve V7. After that, the mixture between sample and reducing agent was propelled further by carrier,passing valve V7, where colour reagent was added. This addition was performed while valve V7 was commutated between positions ‘on’ and ‘off’ at 7 Hz, riginating a plug where small segments of the previous solutions were intercalated. During this step, the reaction product was measured in the spectrophotometer. Finally, the tubing portions denoted as L1, L2 and L3 inFig. 1were washed with carrier. After this step, the flow system was ready for a new analytical cycle.