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This paper reports a novel method for the rapid determination of ammonia nitrogen in sewage by headspace gas chromatography (HS-GC) through a chemical reaction. During the experiment, formaldehyde, initially in excess, was reacted with NH 4 + in a headspace bottle to produce a strong acid with equal molarity. Thereafter, NaHCO 3 was reacted with the newly generated acid to produce CO 2 . In the final step, the released CO 2 was determined using a gas chromatograph equipped with a thermal conductivity detector. Good linear correlation between the ammonia nitrogen content and the gas chromatographic signals was achieved. The observed correlation coefficient was 0.999, and the limit of quantification (LOQ) was 0.786 mg/L (measured based on the mass concentration of N). The relative standard deviation of reproducibility was less than 2%, and the standard recovery was between 95% and 105%. Compared with Nassar's reagent method, the relative deviation of this method is within ±5%. This method has high detection efficiency and is suitable for the rapid batch detection of ammonia nitrogen in sewage.