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Total Nitrogen Measurement and "TKN"

By Mark Heyda


A Danish chemist by the name of John Kjeldahl originally developed his method in 1883 to determine analytically, the quantitative organic nitrogen level in a liquid.  The method which bears his name: Total Kjeldahl Nitrogen or "TKN" is defined as total organic nitrogen and ammonia nitrogen.  The level of organic nitrogen is then determined by subtraction after first determining the ammonia component.  His method basically converts organic nitrogen to ammonia and then tests for total ammonia (naturally occurring ammonia contributed a positive error.)   However, this method is very time consuming and requires a distillation and an apparatus to trap the organic nitrogen that is converted to gaseous ammonia.  Other forms of oxidized nitrogen which also exist as NO2 and NO3 (nitrite and nitrate) are not addressed.


Now in the 21st century, Hanna Instruments reactor model 9800 and TN test kits offer the analyst with a more complete analysis:  all organic and inorganic nitrogen forms are converted to nitrate with a per-sulfate digestion.  The nitrates are photometrically quantified to yield Total Nitrogen.  Again, by subtraction (of NO2, NO3, and NH3), the level of organic nitrogen is determined.  This method has the advantages of speed and cost while giving the analyst more usable data.  The tests are based on those specified using “Standard Methods for the Examination of Water and Wastewater” and EPA Methods for Analysis of Water.  The TN/COD multi-parameter photometer (C214) has the advantage of being able to test for N03, NH4, TN, and COD among others using a variety of wavelength lights and mathematically predetermined algorithms to quantify the specified species.  These instruments make the analysis much faster by using specially designed pre-measured reagents.  Hanna even offers a recycling program for your used cuvets.  The bottom line is an inexpensive, fast, straightforward way to achieve high quality analytical results which are approved by the EPA.

Click here to link to COD - Nitrogen instruments






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Last modified: 29-Oct-2008