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Total Nitrogen
Total Nitrogen in Water
There is currently no automated United States Environmental Protection Agency (USEPA) promulgated method for the determination of total nitrogen in ambient water and wastewater. There are approved methods for the determination of inorganic nitrogen (NO3-N, NO2-N, and NH3-N), and for organic nitrogen (TKN – NH3-N) . The USEPA Clean Water Act (CWA) requires that all methods used for CWA compliance reporting be EPA approved. While there are no methods for the determination of total nitrogen, it is still a required parameter in many USEPA permits including monitoring of nutrient pollution for ambient water criteria. Because Part 136.3
Table 1b methods are required, laboratories have no recourse but to measure total nitrogen as the sum of TKN, NO3-N and NO2-N. Measurement of total nitrogen by calculation presents several problems. One problem is added work by requiring laboratories to run two methods just to obtain one result. Another, more serious problem, lies with the determination of TKN.
History and Explanation of the TKN method
In 1883, Johan Kjeldahl published “A new method for the determination of N in organic substances” that became one of the greatest achievements in science up to that time. Prior to the Kjeldahl method, also known as Total Kjeldahl Nitrogen (TKN), nitrogen was determined by time consuming combustion methods of Dumas, and of Will and Varentrapp. These methods required trained, expert chemists and were slow and very inconvenient. Only a few tests could be run per day. The new TKN method was faster, applicable to solids and liquids, and the data compared favorably to the combustion methods. Over the next 10 years following Kjeldahl’ s original publication, the method was improved by the addition of a metal catalyst and potassium sulfate salt that combined decreased the digestion time to about 0.5 – 2 hours per sample. The TKN method was standardized and remains virtually unchanged today. The TKN method obtains satisfactory results with almost all nitrogen compounds; however, recovery is incomplete with cyano-compounds, nitro-compounds, and certain alkaloids. Even with the addition of chemicals, such as salicylic acid, that aid in the recovery of samples with nitrate the recovery of TKN in samples with nitrate is usually low.
The TKN method presents problems to the modern high-throughput environmental laboratory attempting to analyze large numbers of samples for trace concentrations of total nitrogen. The TKN method requires a preliminary manual digestion with concentrated sulfuric acid, a metal catalyst, and potassium sulfate. Highest recoveries are obtained using a mercury catalyst but because of the toxicity of mercury many laboratories choose to use copper sulfate instead. The mercury catalyst resulted in a clear digest solution while the copper catalyst results in a green colored solution. The classical TKN procedure distilled ammonia nitrogen (the product of the digestion) separating the analyte from the matrix prior to analytical determination by titration or colorimetry. In a rapid analysis scheme, the added distillation step is very time consuming and severely limits laboratory throughput.
Miniaturized methods for the determination of TKN by semi-automated block digestion
followed by continuous flow colorimetric methods have been developed. These continuous flow methods omit the distillation step speeding the analysis, but suffer from difficulties that result from the color of the sample digest absorbing light at the analytical wavelength, improper matrix matching causing both positive and negative deflections of the baseline due to differences in refractive index between the sample solution and the carrier solution, and excess acid in the digestion solution causing reagents to precipitate with the continuous flow analyzer chemistry cartridge.
The Technicon Instruments Corporation, the manufacturer that developed the first continuous flow analyzer, developed a continuous digestion and analysis system that automatically digested and analyzed TKN. This apparatus/analyzer produced very good comparison data with manual TKN, however, it was never approved by the USEPA for compliance reporting and the instrument is no longer available commercially.
Many laboratories report detection limits as low as 0.01 mg/L by the TKN method, however, the United States Geological Survey National Water Quality Laboratory (USGS NWQL) in Denver Colorado reported that TKN data should not be reported below about 0.2 mg/L.
Ambient water criteria established for the State of Florida recommends maximum total nitrogen concentrations
ranging from 0.24 mg/L – 1 mg/L depending on water type and location. The USGS NWQL is a high throughput specialized laboratory that rarely receives samples from sources other than ambient rivers, streams and groundwater. Commercial laboratories that receive samples with highly polluted matrices will likely suffer higher than 0.2 mg/L N detection limits due to carryover and contamination of digestion vessels. Considering that the precision of measurement decreases (percent relative standard deviation (%RSD) increases) as the concentration approaches the detection limit, methods with lower detection limits than TKN are needed. A method is needed that measures total nitrogen directly, has a low enough detection limit for ambient water quality monitoring, a large dynamic range allowing analysis of clean and polluted samples in one batch, and does not experience a high degree of carryover (contamination) from sample to sample.
Methods using High Temperature Catalytic Oxidation (or Combustion) for TN in Water
These methods couple a High Temperature Catalytic Oxidation (or Combustion) Total Organic Carbon (TOC) analyzer, such as described in ASTM D7531 or Standard Methods 5310B with a chemiluminescent nitrogen detector. The combustion tube of the TOC analyzer is packed with a catalyst (platinum on an alumna support) and capped with a small amount of ceramic fiber. The combustion tube is assembled in a furnace and heated to 720 ºC. Zero carbon air is used as a carrier gas and as a supply of oxygen to the ozone generator of the nitrogen detector. The sample stream passes through a thermoelectric cooler immediately after exiting the combustion tube.
When a sample is introduced into the combustion tube at 720 ºC the nitrogen in the sample converts to nitrogen monoxide (reactions 1 and 2). Nitrogen gas in the carrier gas (air) does not interfere. The carrier gas containing the nitrogen monoxide (NO) is cooled and dehumidified in the thermoelectric cooler. The cooled gas then enters the chemiluminescence analyzer where the NO reacts with ozone (O3) and converts to a combination of nitrous oxide (NO2) and excited nitrous oxide (NO2*) (reactions 3 and 4). As the NO2* returns to the ground state it emits radiation which is measured photo-electrically (reaction 5). The detector signal generates a peak that is proportional to the nitrogen concentration in the sample.
Reaction 1 4NH3 + 5O2 --> 4NO + 6H2O
Reaction 2 2(NH2)2CO + 5O2 --> 4NO + 4H2O + CO2
Reaction 3 NO + O3 --> NO2 + O2
Reaction 4 NO + O3 --> NO2* +O2
Reaction 5 NO2* -->NO2 + hν
The method, ASTM D8083, converts all nitrogen compounds to NO at 720C. The instrument uses an auto-sampler to inject an aliquot of sample onto a platinum catalyst inside a heated combustion chamber. After reaction with ozone, the quantitation is performed by chemiluminescence. The instrument requires calibration to establish a correlation of response with known nitrogen standards. Inorganic nitrogen standards prepared from ammonium sulfate and potassium nitrate can be prepared accurately and are stable in aqueous solution.
