Can A New Test Method Improve Nitrogen Measurement In Water?
By William Lipps
Editor’s note: Nitrogen is concern for treaters of drinking water, wastewater, and water used in industrial facilities. This article reviews current approaches to measuring nitrogen in water, and reports on work by the American Society for Testing and Materials (ASTM) through its D19 Committee on Water to improve the accuracy of testing for nitrogen in water. The new approach will be covered in the ASTM’s D8083-16 Test Method for Total Nitrogen in Water by High Temperature Catalytic Combustion (HTCC) and Chemiluminescence Detection.
The Clean Water Act (CWA), which the U.S. Environmental Protection Agency (EPA) administers, requires national approval for all methods used for CWA compliance (1). There are approved methods for the determination of inorganic nitrogen (nitrate-nitrogen [NO3-N], nitrite-nitrogen [NO2-N], and ammonia-nitrogen [NH3-N]), and for organic nitrogen (TKN – NH3-N) (2). There are no EPA-approved methods for the determination of total nitrogen (TN), although it is a required parameter in many EPA permits, including monitoring of nutrient pollution for ambient water criteria (3). Because Part 136.3 Table 1b methods are required, laboratories have no recourse but to measure TN as the sum of the Total Kjeldahl Nitrogen (TKN), NO3-N and NO2-N (4) N (4), or as Simplified TKN (s-TKN™) (5).
TKN Method History and Explanation
In 1883, Johan Kjeldahl published “A New Method for the Determination of N in Organic Substances” (6) that became one of the greatest achievements in science up to that time. The new TKN method was faster, applicable to solids and liquids, and the data compared favorably to the previous methods. The TKN method remains virtually unchanged today and obtains satisfactory results for the measurement of 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 retrieval of TKN in samples with nitrate (NO3-N) 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. It requires a preliminary manual digestion with concentrated sulfuric acid, a metal catalyst, and potassium sulfate. Mercury is the best catalyst, but many laboratories choose to use copper sulfate instead. The mercury catalyst results in a clear digest solution, while the copper catalyst results in a green colored solution. The classical TKN procedure distills 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.
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