Versatile manifolds for the simultaneous monitoring of selected water quality parameters by flow injection analysis

A flow injection (FI) analyzer with a multi-wave length dual beam detector based on a series of light-emitting diodes and photo diodes was fabricated. Using this instrument, simultaneous detection of two pairs of water quality parameters, phosphate-ammonia and nitrite-nitrate was achieved. A software package, FCS-IP, was used for operation, data acquisition and processing. The FI manifold for the simultaneous detection of ammonia and phosphate was developed using a gas diffusion cell with a single indicator solution and a single mixed reagent. The method for ammonia involved the injection of sample into an alkaline stream in which the ammonium ions present are converted to ammonia molecules. This solution was fed to the cell with a gas permeable membrane in which reproducible exchange of ammonia with a flowing solution of the acidic form of phenol red occurred. The single mixed reagent for phosphate consists of an acidified solution of ammonium molydate with ascorbic acid and antimony ions as catalyst. This solution reacted with the phosphate anion in a 1:1 atomic ratio with antimony. Simplex optimization of the single FI manifolds for ammonia and phosphate was used to find the optimum ratio of reagents, use of which resulted in signal enhancement of ca. 200 percent. Silicate interference of up to 5 mg/L was suppressed by the addition of 0.10 percent tartaric acid incorporated in the mixed reagent. Ammonia was detected in 15S (positive peak) followed by phosphate in 30S (negative peak) from point of injection, leading to a sample throughout of 120 d-derminations per hour. The limit of detection is 10 micro g N/L and 50 micro g P/L with precisions of 0.10 percent and 1.68 percent (n = 7) respectively. The method was applied to the analysis of national waters. Good correlation between the FIA and conventional batch methods was observed