The spectrophotometric detection based on molybdenum blue method either in batch or flow systems is commonly used for determination of P. The flow system provided higher degrees of automation and reduced consumption of the chemicals. A multi-syringe flow injection analysis was proposed for determination of available P in soils [7]. All injection analysis was developed for determination of P in soil and sediment extracts [8]. However, the spectrophotometric detection method suffers potential interference from colored and colloidal substances and silicate. In flow analysis, the effect of refractive index or the Schlieren effect can also be a problem [6] and [8]. These interferences may present in the soil extract at high extents. We have developed the FI amperometric systems, which have higher tolerant to these interferences [9], thus they should be appropriate for soil analysis.
In this work, a new extraction procedure based on off-line column extraction was developed for extracting available P from soil. The collected extract was analyzed for P concentration by using a FI amperometric method which was based on the electrochemical reduction of the on-line formed 12-molybdophosphate, producing an electrical current, which was directly proportional to P concentration. The column extraction was simpler, more convenient to use and provided higher extraction efficiency than the standard extraction procedure based on batchwise shaking. The amperometric detection provided higher selectivity than the spectrophotometric one, as it did not suffer from colored substances, particulates and refractive index effect (Schlieren effect), and could tolerate to silicate at high concentration (up to 1000 mg L−1). The system could be applied to soil extract without the requirement of masking agent and with low consumption of chemicals (2 mL each of KCl and reagent solutions per analysis).