Abstract
A flow-injection analysis (FIA) system is presented for simple and rapid determination of ammonia at sub-ppm levels in seawater and concentrated sodium chloride solution. The system consists of a direct on-line coupling of the gas diffusion separation using porous polytetrafluoroethylene (PTFE) membrane tubing and subsequent spectrophotometric detection of ammonia by the formation of indophenol blue dye with sodium hypochlorite and 1-naphthol in a continuous flow system. Sodium citrate added to the alkaline medium which is essential to convert ammonium ion to ammonia proved to be very effective in preventing magnesium and calcium in the seawater sample from precipitating in this FIA system, thus enabling the direct injection of seawater sample. No laborious complicated manual operation for separation and detection is required. The limit of detection was 0.008 ppb and satisfactory precision of 1% relative standard deviation was also obtained. The analytical readout is available within 6 min after sample injection.
