The proposed method for the determination of ammonia comprises three unique techniques, namely, utilization of easily-solidifying solvent, phase separation by cooling centrifugation, and the extraction of association complex. In concrete terms, 1.2.4-trichlorobenzene was mainly used as extracting solvent among the potential easily-solidifying solvents, and phases were separated by the cooling centrifugation at -3℃ and 3000 rpm for 30min after shaking, and ammonia was extracted in the form of ion association complex of indothymol and Capriquat. Under the optimum condition, linear relationship between absorbance of organic phase and ammonia concentration was obtained in the concentration range of 0.25 to 25 ppb NH_4-N with the correlation coefficients of <0.999>_6 for 20 mL of indothymol solution.