Low-level ammonia nitrogen may be present in water naturally as a result of the biological decay of plant and animal matter. Higher concentrations may be found in raw sewage and industrial effluents, particularly from petroleum refineries where ammonia is a by-product of the refining process.

Ammonia is a major component of fertilizers. High concentrations in surface waters can indicate contamination from waste treatment facilities, industrial effluents or fertilizer run off. Excessive ammonia concentrations are toxic to aquatic life.

The Nessler Method  (Contains mercury.  Dispose according to local, state or federal laws.)  
Ammonia concentrations are determined by direct Nesslerization.  In some waters, calcium and magnesium concentrations can cause cloudiness of the reagent. Adding a few drops of stabilizer solution (Rochelle Salt) will prevent this cloudiness. Results are expressed as ppm (mg/L) NH₃-N.

Shelf-life. Although the reagent itself is stable, its high alkali content attacks the glass ampoule. The resulting precipitate interferes with color comparison below 1 ppm. We recommend stocking quantities of CHEMets^® ampoules and VACUettes^® ampoules that will be used within 5 months. A 2-month supply of Vacu-vials^® ampoules is suggested. Refrigeration will significantly extend the shelf-life of these products.
*References recommend distilling samples prior to analysis.

References: 
ASTM D 1426-03, Ammonia Nitrogen in Water, Test Method A.
APHA Standard Methods, 18th ed., p. 4-78, method 4500-NH₃ C (1992).

The Salicylate Method
Free ammonia reacts with hypochlorite to form monochloramine.  Monochloramine then reacts with salicylate, in the presence of sodium nitro-ferricyanide,  to form 5-aminosalicylate, a green colored complex. The test method measures free ammonia plus monochloramine.  Results are expressed in ppm (mg/Liter) ammonia-nitrogen, NH₃-N.

References:
Krom, Michael D. (1980) Spectrophotometric Determination of Ammonia:  A Study of a Modified Berthelot Reduction Using Salicylate and Dichloroisocyanurate, The Analyst, V105, pp. 305-316.

Methods for the Chemical Analysis of Water and Wastes, March 1979, Method 351.2.