Environmentally friendly method for determination of ammonia nitrogen in fertilisers and wastewaters based on flow injection-spectrophotometric detection using natural reagent from orchid flower

Crude aqueous extract from the orchid ‘Dendrobium Sonia earsakul’ was utilised as a natural product reagent in flow injection analysis (FIA) incorporating a gas diffusion unit (GD) for the determination of ammonia nitrogen. Sample solution was injected into a NaOH donor stream to generate ammonia gas (NH₃). In the GD unit, NH₃ diffused across a PTFE gas-permeable membrane into the acceptor stream of the orchid extract. As the result, the aqueous orchid reagent became more alkaline and its colour changed from purple to green. The change in the colour of orchid acceptor correlated with the concentration of ammonia nitrogen in the sample and its absorbance monitored by a spectrophotometer at 600 nm. Ammonia nitrogen in chemical fertiliser samples and wastewater samples from agricultural fields were determined and reported as %N (w/w) and mg N L⁻¹, respectively. For chemical fertilisers which contained high content of ammonia nitrogen, a flow rate of 1.0 mL min⁻¹ and injection volume of 100 µL were used with a linear range of 5–40 mmol L⁻¹ and detection limit of 2.12 mmol L⁻¹. However, a higher sensitivity was required for wastewater samples having low ammonia nitrogen content. The flow rate was reduced to 0.3 mL min⁻¹ and the injection volume increased to 1000 µL. As a result, detection limit of 0.76 mmol L⁻¹ was achieved with linear range of 1–5 mmol L⁻¹. The results of our method agreed well with that using the OPA method employing fluorescence detection.