The routine determination of small concentrations of nitrate in water containing considerable chloride ion is limited to a few special procedures. Indirect reductive methods are usually avoided since natural waters contain other nitrogeneous substances. Direct estimations have been restricted to a few colorimetric procedures.
Zwicker, B. M., and R. J. Robinson. 1944. "The photometric determination of nitrate in sea water with a strychnidine reagent." Journal of Marine Research 5, (3). https://elischolar.library.yale.edu/journal_of_marine_research/620