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A simple and sensitive extraction-spectrophotometric method for the determination of total dissolved ammonia and ammonium ion content is reported. The ternary complex DC18C6-ammonium-Orange II, (DC18C6–NH4–OR II), was quantitatively extracted into dichloromethane and its absorbances is measured at 483 nm. Linear calibration graph is obtained over the concentration ranges of 0.05–3.00 μg mL−1 for ammonium. The relative standard deviations for 1.0 μg mL−1 of ammonium was 1.10%. The method was applied to real samples such as sewage, agricultural soil and Persian golf shrimp. The results showed high potential of the recommended method for the determination of total dissolved NH3 and NH4+ content in different samples.