How To Measure Un-Ionized Ammonia Sampled From An Aquatic System

Article written by: Jason Danaher Ph.D. 

The majority of nitrogen present in the fish feed and added to the culture system will be released as a dissolved form into the aquatic system. Ammonia is the principle nitrogenous waste product excreted by aquatic organisms and is excreted across the aquatic organism’s gills into the water. Ammonia levels have to be monitored frequently and carefully managed because ammonia can be highly toxic to aquatic organisms. The good news is simple water quality test kits and procedures using color discs can help the farmer quantify the concentration of toxic ammonia present in the system so informed decisions can be made, if necessary.

Most ammonia testing procedures available for aquaculture quantify the TAN concentration present at the time of sampling. The TAN consists of ammonium (NH₄+) and ammonia (NH₃). The ammonium (i.e. ionized ammonia) is generally categorized as non-toxic; however, un-ionized ammonia is very toxic to aquatic organisms. These two forms of TAN and their proportion present in water are dependent on pH and water temperature. A greater proportion of un-ionized ammonia exists when the temperature of the water is high and the pH of the water is above 7.0 (i.e. alkaline). The ratio of ammonia:ammonium decreases when pH and temperature decrease. The pH has a greater effect on ammonia toxicity than temperature. The combination of high pH and elevated total ammonia nitrogen concentration can lead to elevated un-ionized ammonia concentrations potentially harmful to fish.

There are two analytical methods used to test TAN concentration in aquatic systems and the method chosen is dependent on the salinity of the water. The Nessler method works best for freshwater samples. The salicylate method can be used for both freshwater, brackish water, and saltwater samples. The test kits offer low-cost, fast, reliable results in an easy-to-carry kit and can detect total ammonia nitrogen in a range of 0.1 to 2.4 mg/L range with 0.2 mg/L increments. To measure the concentration of un-ionized ammonia in a sample follow the steps provided below:

1. First, measure pH and temperature of your water sample.
2. Second, measure the TAN concentration using the appropriate method available in your water quality test kit.
3. Reference the table below adapted from Emerson et al. (1975).
4. Determine the percentage of toxic un-ionized ammonia in the water sample using the table, sample pH, and sample temperature.
5. Multiply your TAN concentration measured from the test kit result by the percent from the table to calculate the concentration in ppm (mg/L) of toxic (un-ionized) ammonia present in the water sample.

Example Calculation:

You collect a water sample from your warmwater system and determine the TAN concentration in the system is 1.8 mg/L. The temperature is 79^oF and the pH is 7.4. What is the un-ionized ammonia concentration? On the table above locate the column for 79^oF or the column closest to the recorded system temperature. Next locate the row for pH of 7.4. Where these two intersect is the number 0.0150. This decimal represents the percentage of TAN that is in the un-ionized form (1.5%). This decimal will be multiplied by your measured TAN concentration in the water sample (0.0150 x 1.8 mg/L) to yield the un-ionized ammonia concentration of 0.027 mg/L. This can be rounded up to the nearest hundredth decimal place, which would be 0.03 mg/L.

Tolerance varies among species, but generally safe levels of TAN range from 0.25 to 2.0 mg/L for most fish grown in aquaculture; however, toxic un-ionized ammonia is capable of killing fish at as low as 0.10 mg/L for coldwater and 0.50 mg/L for warmwater species. The above calculation resulted in a un-ionized concentration of 0.03 mg/L. This concentration is lower than 0.50 mg/L for warmwater species and no corrective action needs to be taken. 

Let’s do another calculation using the example above to observe the effects of pH on un-ionized ammonia toxicity levels. The water sample from your warmwater system had a TAN concentration of 1.8 mg/L with a temperature of 79^oF and the pH has now increased one unit to 8.4. What is the new un-ionized ammonia concentration? 

On the table above locate the column for 79^oF or the column closest to the recorded system temperature. Next locate the row for pH of 8.4. Where these two intersect is the number 0.1326. This decimal represents the percentage of TAN that is in the un-ionized form (13.26%). This decimal will be multiplied by your measured TAN concentration in the sample (0.1326 x 1.8 mg/L) to yield the un-ionized ammonia concentration of 0.2386 mg/L. This can be rounded up to the nearest hundredth decimal place, which would be 0.24 mg/L.

The take home message when comparing the two sample calculations above is that a 1.0 unit increase in the pH level will result in an approximate tenfold increase in the toxic, un-ionized ammonia concentration. This makes sense because pH is logarithmic in scale. Once more, these are sample calculations and production managers will need to confirm un-ionized ammonia tolerance levels for their specific species. 

Corrective Action

If sub-optimal un-ionized ammonia concentrations are observed in the aquatic system then there are a few steps the aquatic system manager can take to mitigate the situation. The first is to immediately reduce or cease feed input until concentrations decrease to safe levels again. Second, ensure the solids filtration is properly functioning to capture and eliminate captured solids wastes from the system and prevent them from entering and overloading the biofilter. Next, measure other system parameters and make sure they are in acceptable ranges to support the biofilter. The total alkalinity concentration may need to be increased to provide a source of bicarbonate ions for the nitrifying bacteria. Exercise caution, because raising the system’s pH through bicarbonate addition may result in a higher concentration of toxic un-ionized ammonia. Then, stabilize the production system’s water temperature and salinity, if applicable, for the nitrifying bacteria in the biofilter. The last option may require a water exchange of 25% to 50% of the total system volume to rapidly decrease un-ionized ammonia levels. A water exchange is not ideal and should be avoided if at all possible.

Feel free to contact Aquatic Equipment & Design, Inc. at 407-995-6490 or email info@aquaticed.com if you have any questions on methods or technologies to measure water quality parameters in your aquatic system.

Sources:

Emerson, K., R.C. Russo, R.E. Lund, and R.V. Thurston. 1975. Aqueous ammonia equilibrium calculations: effect of pH and temperature. Journal of the Fisheries Research Board of Canada. 32:2379-2383.