TITLE: USDA LTAR Common Experiment measurement: Dissolved ammonia (NH3) concentration
AUTHORS: Oliva Pisani, John L. Kovar, Robert W. Malone, Amy J. Morrow, Kevin J. Cole

[DESCRIPTION]
Ammonia can form naturally from organic matter mineralization or can enter the aquatic environment via anthropogenic sources such as municipal effluent discharge and agricultural runoff. The presence of ammonia (NH3) vs. ammonium (NH4+) in water depends on water pH and temperature. The un-ionized NH3 form is toxic to aquatic organisms, and both forms (NH4+ and NH3) can contribute to eutrophication and hypoxia of surface waters. The reported measurement of ammonia in water is typically the total ammonia N (TAN = NH3 + NH4+). The recommended techniques for measuring NH3 in water are the colorimetric salicylate and phenate methods, whereby an indophenol color complex is proportional to the NH3 concentration of the sample.

[STEPS]
SECTION: Sample collection and filtration
1. Return samples to the laboratory on ice and filter them on collection day if possible.

SECTION: Sample collection and filtration
2. Filter samples through a 0.45 micrometer (µm) pore-size filter to minimize interference from particulates before chemical analysis.

 

SECTION: Sample collection and filtration
3. As samples collected and stored in an autosampler are vulnerable to ammonia loss by volatilization and nitrification, ideally, pre-acidify the samples to pH < 2 (H2SO4) and retrieve from the field within seven days of autosampler collection.

 

SECTION: Sample collection and filtration
4. If pre-acidification is not possible, retrieve the samples as soon as possible and transport them back to the laboratory on ice.

SECTION: Sample storage and preservation
5. As soon as possible after sample collection, filter the water samples through a 0.45 µm pore-size filter to minimize interference from particulates.
SECTION: Sample storage and preservation
6. Analyze samples within 24 hours after collection.

SECTION: Sample storage and preservation
7. To store samples not pre-acidified in the field, preserve the samples with sulfuric acid to pH < 2, cool them at 4 °C, and hold them for up to 28 days (US EPA, 1993).

SECTION: Sample storage and preservation
8. Preserve the samples that are not acidified by freezing to minimize ammonia loss by volatilization.
SECTION: Archiving
9. Store the water samples for NH3 analysis until data certification (QA/QC verification). 

 

SECTION: Sample analysis
10. Direct determination of NH3-N without distillation is possible for water samples with low NH3-N concentrations. 


SECTION: Sample analysis
11. Samples with high NH3-N concentrations (> 5 mg-N/L) may need to be distilled before analysis to minimize interferences and improve precision (APHA, 2005).


SECTION: Sample analysis
13. Salicylate method : Heat the sample with salicylate and hypochlorite in an alkaline phosphate buffer in the presence of sodium nitroprusside (catalyst). The reaction produces an emerald-green indophenol dye proportional to the NH3 concentration of the sample.

 

SECTION: Sample analysis
14. Phenate method : In this method NH3 reacts with alkaline phenol and sodium hypochlorite in the presence of sodium nitroprusside to enhance sensitivity. The indophenol blue dye is proportional to the NH3 concentration of the sample.

 

SECTION: Sample analysis
12. The colorimetric determination of ammonia in water is carried out using the salicylate or phenate method.

 

SECTION: Covariate metrics to be sampled concurrently
15. Dissolved NO3- concentration, total dissolved N (TDN), total dissolved P (TDP), and total suspended solids (TSS). These metrics are described in the following protocols: 

USDA LTAR Common Experiment measurement: Dissolved nitrate (NO3-) concentration
USDA LTAR Common Experiment measurement: Total nitrogen (TN) and total dissolved nitrogen (TDN) concentration
USDA LTAR Common Experiment measurement: Total phosphorus (TP) and total dissolved phosphorus (TDP) concentration
USDA LTAR Common Experiment measurement: Total suspended solids (TSS)
SECTION: Calculations
16. Prepare a calibration curve by plotting the blank-corrected area of each standard peak against its respective NH3-N concentration. 
SECTION: Calculations
17. Compute the NH3-N concentration in the sample by comparing the sample peak area to the calibration curve. Report the concentration as mg NH3-N/L.
SECTION: Recommendations for data collection
18. Table 1. Summary of recommendations for the collection and measurement of dissolved NH3-N
concentration.