Ammonia

Ammonia determination is one of the most common uses for automated chemistry analyzers and there are several methods to choose from. Ammonia samples must be handled with care and are prone to interference from ambient conditions in the lab. It is recommended that the analysis be performed as soon as possible.

Salicylate Method

This method is used for the determination of ammonia in drinking, surface and saline waters, domestic and industrial wastes, plants and soils. The low range of this method is 1 to 100 µg/L as Nitrogen. However, this method is also applicable to other ranges. 

Summary

Ammonia reacts with sodium salicylate, sodium nitroferricyanide, and sodium hypochlorite in a buffered alkaline solution to form the salicylic acid analog of indophenol blue. The blue-green color produced is directly proportional to the concentration of ammonia present and is measured at 660 nm.

Limitations

Precipitation of calcium and magnesium hydroxides is eliminated by the addition of potassium sodium tartrate in the working buffer. Samples that are turbid must be filtered or centrifuged prior to determination. Background color may interfere. Contamination of samples due to ammonia in the laboratory atmosphere is very common. Sample handling and analysis should be performed where there is no possibility of ammonia contamination. See Operating Notes in method document for tips on minimizing contamination due to ammonia absorption from the laboratory surroundings.

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Phenol Method

This method is used for the determination of Ammonia Nitrogen as N in drinking, surface and saline waters, domestic and industrial wastes, plants and soils. The EPA range of this method is 0.01 to 2.0 mg/L. However, this method is also applicable to other ranges.

Summary

Ammonia reacts with alkaline phenol and hypochlorite to form indophenol blue. Sodium nitroferricyanide intensifies the blue color formed which is measured at 660 nm.

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Ammonia in Sea Water

Fluorometric Method

This method is used for the determination of ammonia in seawater. The applicable range of this method is 0.05 to 5 μM. The sample is mixed with o-phthaldialdehyde and sodium sulfite in a borate-buffered solution at 75°C. After sufficient mixing, the sample concentration is measured by fluorescence spectroscopy using 360nm excitation and 420-470nm emission wavelengths. The increase in fluorescence is directly proportional to the ammonia concentration.

Limitations

Inorganic salts can have a depressive effect1; Copper 6% (>300μg/L), Iron 0.5% (1-3mg/L), Mercury 5% (10mg/L), Sulfide 2.3% (>10μM). Particulate matter should be removed or centrifuged to prevent clogging in the system.

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 Gas Diffusion Method

This method is used for the determination of Ammonia in seawater and is applicable to many ranges. The sample is mixed with an alkaline complexing reagent to liberate ammonia gas. The ammonia gas then diffuses across a Teflon membrane into a slightly acidic stream. The ammonia reacts with alkaline phenol and hypochlorite to form indophenol blue. Sodium nitroferricyanide intensifies the blue color formed which is measured at 640 nm. The absorbance is directly proportional to the ammonia concentration.

Limitations

Precipitation of calcium and magnesium hydroxides are eliminated by the addition of sodium citrate complexing reagent. Turbid and colored sample interferences as well as varying refractive index effects are eliminated by dialysis.

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