Spectrophotometric determination of trace levels of atmospheric ammonia by conversion into indophenol and extraction with quaternary ammonium salts

Indophenol, produced in proportion to the amount of ammonia present, can be quantitatively extracted into chloroform as an ion-pair with tetradodecylammonium (TDA(+)) bromide and determined by spectrophotometry. The molar absorptivity of the indophenol-TDA(+) complex is 2.5 x 10(4) 1.mole(-1).cm(-1) (maximum absorption at 653 nm), which is 25% greater than that for the indophenol-TDA(+) complex in aqueous medium. This procedure is applicable to determination of mul/m(3) levels of atmospheric ammonia, with 1-hr sampling periods.