Development of a new gas-permeation system and its application to the spectrophotometric determination of ammonium ion by FIA

A new on-line gas-permeation system was developed to enhance the efficiency and stability of gas permeation for a long time, and was applied to the de termination of ammonium ion in river-water samples by die flow-injection me thod. The proposed gas-permeation system consists of a newly designed gas-p ermeation unit, a sample injection valve, a reaction coil and two six-way r otary valves in a constant-temperature oven. The gas- permeation unit was a ssembled using an inner microporous polytetrafluoroethylene (PTFE) membrane tubing and an outer glass tube, which could be easily connected with one a nother using connectors with ferrules and o-rings. A sample solution was in jected into a carrier containing 0.02 M sodium hydroxide, in which ammonium ion was converted to gaseous ammonia. The gaseous ammonia generated in the carrier passed through the microporous PTFE membrane, was absorbed in a re agent solution and changed the pH of the reagent solution, which resulted i n a color change to cresol red. The absorbance change of cresol red at 550 nm was measured using a visible detector with a now cell (8 mu l, 10 mm pat h). After the measurement, two six-way rotary valves were switched over fro m the flowing state to the drain state of the gas-permeation unit, and the residual solutions in the gas-permeation unit were removed and the PTFE mem brane tubing was dried. By keeping the temperature of the oven at 40 degree s C, the sensitivity and the reproducibility for the determination of ammon ium ion was improved. Furthermore, the efficiency of gas permeation through the inner tubing could be kept high and constant for a long period without any procedure for reactivating the porous PTFE membrane tubing.. Calibrati on graphs for ammonium ion were liner over ranges of 0 to 10 ppm and 0 to 1 .0 ppm of N-NH4+, and the sampling rate was 30 samples per hour. The detect ion limit for N-NH4+ was about 0.01 ppm when the effective length of the me mbrane tubing was 5 cm. By using the proposed FIA system, ammonium ion in r iver-water samples was determined. The analytical results obtained by die p roposed method showed a good correlation with those obtained using the indo phenol derivatization/FIA method. The relative standard deviations of ten i njections were 0.51% and 0.83% with concentrations of 2.73 ppm and 4.11 ppm , respectively. The recoveries of ammonium ion were found to be 97 similar to 98%.