Ammonia (NH3) is one of the many form’s nitrogen takes as it makes its way through the environment. In high enough concentrations, it is toxic to fish and everyone is aware of the unpleasant odor it spreads. In nature ammonia occurs from the decomposition of organic compounds that contained nitrogen (almost all of them) or the hydrolysis of urea (the compound that gives urine its name). Generally speaking, natural sources of ammonia are dispersed far and wide enough that we would never notice it. However, if enough of those natural sources of ammonia are concentrated into one stream (a sewer) it can become a problem in the water source.
For NPDES/CWA reporting there are several methods available for ammonia analysis. Most of the time a preliminary distillation is necessary – EPA 350.1, SM4500-NH3 B, or AOAC 973.49 all contain this step. There are some requirements to document when you choose not to distill. Consult your local regulations to see if you qualify. The analysis of ammonia is accomplished through the following methods:
The distillation consists of heating an alkaline buffered solution and condensing the vapors into a mildly acidic trapping solution. The ISE analysis utilizes a standard ammonia ISE to detect ammonia in solution. The phenate method reacts alkaline phenol and hypochlorite with ammonia. This forms indophenol blue with an intensity proportional to the amount of ammonia present. The color intensity is measured photometrically to determine the final concentration.
To distill or not to distill is always the biggest question with this method. The current (as of this writing) table in 40 CFR part 136.3 has the following note about distillation: “Manual distillation is not required if comparability data on representative effluent samples are on file to show that this preliminary distillation step is not necessary: however manual distillation will be required to resolve any controversies.” There are two things to note about this:
Additionally, if you are using ISE, you will have to prepare your standards in a matrix to match the level of dissolved ions in your samples. If you are using the phenate method you will need to correct for turbidity and/or color as well as pH match your standards to your samples.
The best way to ensure consistency is to simply distill your samples. Your choice of analytical methods will determine your trapping solution. You would use boric acid if you are titrating and H2SO4 if you are using the phenate method or ISE. Distillation works because of the properties of ammonia. When samples are collected for ammonia analysis, they are typically acidified to a pH of less than 2.0. The acid conditions and subsequent cooling not only minimizes microbial activity but also converts all NH3 (a gas) in solution to NH4 (an ion). Generally, NH4 will not leave the solution under ordinary conditions, giving us the 28-day holding time. Buffering the solution at alkaline conditions forces the NH4 to convert back to NH3. This is a volatile gas that can be removed from solution through simple heating. It is important to ensure an alkaline pH when distilling. Theoretically, anything basic will suffice. However, at the increased temperature of distillation, cyanates or organic compounds containing nitrogen can hydrolyze to form ammonia where none previously existed. A pH of 9.5 minimizes these rates of formation, thus the specific pH buffer requirement.
Traditional distillations have required distilling long enough to collect water along with the ammonia to bring your trapping solution up to volume. This also ensured sufficient time to maximize the recovery of ammonia. Recent advances in distillation technology use hydrophobic frits in the assembly. This not only eliminates potential boil-over but also reduces the volume of non-analyte that can distill over. The simple fix is to distill long enough for complete removal and then add the needed distilled water yourself to bring up to final volume. Additionally, the SimpleDist system from Environmental Express has the added benefit of eliminating the need for cooling water.
One of the nice things about distillation is that because it is a separation technique, spiking and reagent additions do not affect the concentration calculations. You are moving the actual analyte of interest from one place to another and leaving the rest of the original solution behind. Accurate measurement of the actual sample aliquot and an accurate final volume of distillate are all that are required to measure properly. This means that if you overshoot your pH in either direction a few times, the only thing it affects is your time spent and possibly gives you a larger volume in your distillation tube.
Note – This is not intended to be a standalone method and does not address all safety or quality control aspects that may be required. Please consult your local regulations to comply with all requirements.
Don’t forget your calibration standard.