Determination of ammonium in seawater based on the indophenol reaction with o-phenylphenol (OPP)

A method for the determination of ammonium in seawater is described. The method is based on the indophenol (or the Berthelot) reaction with o-phenylphenol [(1,1′-biphenyl)-2-ol, abbreviated as OPP] as an alternative of phenol. Unlike phenol, OPP is available as stable flaky crystal with no caustic odor. This nature of OPP can eliminate much of the inconvenience associated with the use of phenol. The sensitivity of detection is comparable to that of the common phenol-based methods. The indophenolic compound obtained from the reaction with OPP is more hydrophobic than that from the reaction with phenol. Thus the indophenolic compound can be extracted directly from the alkaline reaction solution of the present method either with organic solvent or with reversed-phase column. The extracted indophenol can be used to recover ammonium nitrogen for the determination of 15-N isotope ratio. The experimental procedure of the extraction of indophenolic compound is relatively simple compared to the other existing methods.