Repetitive determination of chloride using the circulation of the reagent solution in closed flow-through system

The precipitation reaction of silver chloride (AgCl) is carried out in a large amount of ammonia (NH3). This makes possible to adopt a closed-loop flow injection (FI) system and to determine chloride repetitively. A solution of 30 mmol l−1 silver nitrate and 80 mmol l−1 NH3 in a single reservoir (250 ml) is continuously circulated through the flow cell at a flow rate of 2.0 ml ml−1. The chloride containing sample (100 μl) was introduced into this reagent solution by means of six-way valve. AgCl precipitates formed in the sample zone are monitored spectrophotometrically (at 500 nm) in the flow system. After passing through the flow cell, the excess NH3 in the circulating reagent solution dissolves AgCl precipitates and the stream then returns to the reservoir. Various variables of the FI system were optimized and a study of interfering ions was also carried out. A linear calibration graph was obtained from 3.0 to 30 mg l−1 chloride. Two hundred repetitive injections of 5.0 mg l−1 chloride into the circulating reagent solution have shown unchanged base-line and good reproducibility. The method was successfully applied to the determination of chloride in tap, natural and the reference waters.