SPECTROPHOTOMETRIC DETERMINATION OF AMMONIA-NITROGEN AFTER PRECONCENTRATION AS INDOTHYMOL ON A GLASS-FIBER FILTER IN THE PRESENCE OF A CATIONIC SURFACTANT

Ammonia-nitrogen, in the range 0.06 to 2-mu-g in 20 ml of water sample, is treated with hypochlorite and thymol to form an intensely blue anionic dye called indothymol. The indothymol is collected on a glass-fiber filter as the ion-pair with benzyldimethyltetradecylammonium ion. The indothymol collected on the filter is dissolved in a small volume of dimethylformamide (DMF) and the absorbance due to the indothymol is measured at 675 nm against the reagent blank. An unexpectedly high sensitivity was obtained because of the increased molar absorptivity of indothymol in DMF (3.2 x 10(4) l mol-1 cm-1, lambda-max = 675 nm), approximately 3 times larger than that in water (1.16 x 10(4) l mol-1 cm-1, lambda-max = 660 nm). The detection limit, defined as three times the standard deviation of the blank, is better than 1-mu-g l-1 of ammonia-nitrogen with 5-fold preconcentration.