Abstract: Ammonia nitrogen in water was determined by molecular fluorescence method. Fluorescent substances were formed based on the reaction of phthalaldehyde (OPA) with ammonia nitrogen in boric acid buffer. Under certain pH value, suitable temperature and static time, fluorescence of fluorescent substances was were determined at 425 nm with 362 nm as excitation wavelength. The effects of the ratio of sodium tetraborate, sodium sulfite and OPA, temperature and time on the fluorescence value of the derivatives were investigated. The results showed that there was a good linear relationship between the amount of derivatives and the fluorescence intensity in a certain concentration range. This method has high sensitivity and is suitable for the determination of ammonia nitrogen in low concentration water samples. It could also be used for the determination of high concentration water samples after simple dilution.

Key words: ammonia nitrogen, molecular fluorescence, phthalaldehyde(OPA)