Ammonia/Ammonium

Ammonia is a colorless, pungent gaseous compound of hydrogen and nitrogen (one nitrogen atom and three hydrogen atoms, NH₃) that is highly soluble in water.

Ammonia is formed naturally as a product of the microbiological decay of nitrogenous organic matter (animal and plant protein). It can also be produced for use in fertilizers, or for use in the production of plastics, pharmaceuticals and other chemicals.

Ammonia in groundwater is normal, due to microbiological processes. However, the presence of ammonia nitrogen in surface water usually indicates domestic pollution. Excess ammonia can damage vegetation and is incredibly toxic to aquatic life, especially at elevated pH and temperature levels.

Aqua Ammonia

For various applications, ammonia is dissolved in water to produce “aqua ammonia.” Bulk aqueous ammonia (aqua ammonia) solutions are unstable and the ammonia concentration may decrease during transportation or during storage. Therefore, delivered solutions are billed based on the ammonia concentration delivered to the facility or end-user.

Ammonium is a compound containing one nitrogen and four hydrogen atoms (NH₄⁺). While ammonia is a neutral non-ionized molecule (weak base), ammonium is an ion carrying a positive charge. In addition, ammonia emits a strong odor, but ammonium does not smell at all.

The major factor that determines the proportion of ammonia to ammonium in water is pH. The activity of the ammonia is also influenced by ionic strength and temperature of the solution. It is important to remember that while molecular ammonia can be harmful to aquatic organisms, ammonium ion is basically harmless. In the water industry, it is important to know the concentrations of hydrogen bound nitrogen. Therefore, terms ammonia and ammonium are used interchangeably, depicted as NH₃^-N or NH₄^-N correspondingly, and normally expressed in mg/L or PPM of N.

The chemical equation that drives the relationship between ammonia and ammonium is: 
NH₃ + H₂O <-> NH₄⁺ + OH^-

When the pH is low, the equilibrium is driven to the right and when the pH is high, the equilibrium is driven to the left. In general, at room temperature with a pH less than 6, the portion of ammonia‐N as NH₃ is very low and almost all ammonia nitrogen is present as NH₄⁺. At a pH around 8, the portion of NH₃ is 10 percent or less, and at a pH slightly above 9 it is about 50 percent. Once the pH is > 11, all ammonium ions in solution will be converted to the molecular form of ammonia. The activity of aqueous ammonia is much lower at low temperatures.

Ammonia nitrogen can be measured using a spectrophotometer (or colorimeter) or an ammonia ion selective electrode (ISE). Provided below are accepted or equivalent United States Environmental Protection Agency (USEPA) methods for wastewater analysis.

Nitrogen-Ammonia Nessler Method 8038 - USEPA accepted for wastewater analysis (distillation required), Method 350.2.
Nitrogen-Ammonia TNTplus Method 10205 - Equivalent
Nitrogen Ammonia Ion Selective Electrode (ISE) method 10001 and 10002 - These procedures can be used for standard methods for the examination of water and wastewater 4500-NH3 E for USEPA NPDES reporting.

The ammonia TNTplus reagent sets TNT830, TNT831, and TNT832 are considered equivalent for United States Environmental Protection Agency (USEPA) wastewater analysis and reporting. The ammonia reagent sets TNT Amver are not considered equivalent for USEPA reporting.

Hach has created an EPA formatted method with side-by-side support data to follow the MUR and state equivalency for the TNTplus^® ammonia salicylate reagents. This has not been done for the older TNT Amver salicylate reagents. If a user wants to consider the older TNT ammonia tests for regulatory compliance, they can still discuss with their regulatory authority.

Reference EPA formatted method, "TNTplus Ammonia-Spectrophotometric Measurement of Ammonia Nitrogen and Total Kjeldahl Nitrogen in Water and Wastewater" for information on USEPA equivalency to 350.1 and 351.1.

Both the Nessler and salicylate methods are based on a reaction with molecular ammonia in a basic solution. If the original sample contained ammonium ions, the basic reagents would convert it to molecular ammonia and then it will react and contribute to the final test result. However, these simple ammonia tests do not include the ammonia that is in organically bonded amino groups. A true total ammonia test would include chloramines and require heating the sample in an acidic solution to digest the organic ammonia.

Since the test result is a sum of both molecular ammonia (NH₃) and ionic ammonium (NH₄⁺), the preferred unit for reporting is as nitrogen (NH₃^-N). Most Hach colorimeters and spectrophotometers have the option to convert the test results between nitrogen, ammonia, and ammonium. This does not change the chemistry of the test; it only changes the units. It should be noted that while there is the same amount of nitrogen in ammonia and ammonium, the stoichiometric ratio of N to NH₃ and NH₄⁺ are not exactly equal due to different number of the hydrogen atoms.

• To convert from mg/L NH₃^-N to mg/L NH₃, multiply by 1.216.
• Calculated from the mass of NH₃ divided by the mass of N (17.034 ÷ 14.01 = 1.216).
• To convert from mg/L NH₃^-N to mg/L NH₄, multiply by 1.288.
• Calculated from the mass of NH₄⁺ divided by the mass of N (18.042 ÷ 14.01 = 1.288).

When the measurement is displayed on the Amtax sc, the units are "NH₄^-N" or "NH₄", which means that the instrument measurement is expressed as ammonium and reports the values as nitrogen (also described as "NH₄ as N"), or as ammonium. There is no difference between expressing ammonia concentration ("NH₄^-N" or "NH₃^-N") because in both cases it is calculated as nitrogen, which amount is the same in both either form of ammonia.

The Amtax sc analyzer adds sodium hydroxide (NaOH) to adjust the pH and convert ammonium ions (NH₄⁺) to gaseous ammonia (NH₃), which passes through an ammonia gas selective membrane and causes a pH shift of an electrolyte solution. This pH shift in the electrolyte is measured as a mV signal that is proportional to the concentration of ammonia (NH₃) in the sample.

Ammonia exists in water as either the ammonium ion (NH₄⁺) or the non-ionized ammonia (NH ₃). Non-ionized ammonia is toxic to fish, while the ammonium ion is non-toxic, except at extremely high concentrations. At neutral pH 7 and ambient temperature, almost all the ammonia exists as NH₄⁺. As the pH and temperature increase, the amount of NH₃ increases and the amount of NH₄⁺ decreases.

To measure the concentration of non-ionized ammonia in a sample, follow the steps provided below:

1. Measure the ammonia concentration using any ammonia method, except the free ammonia method.
2. Measure pH and sample temperature. Reference the table titled “Percentage Un-ionized Ammonia in Aqueous Solution by pH Value and Temperature Calculated from data in Emerson, et. al*” on page 11 in the FF2 Freshwater Aquaculture Test Kit Manual.
3. Determine the percentage of NH₃ using the table, sample pH, and sample temperature.
4. Multiply your ammonia concentration by the percent from the table and then divide by 100.