Total Kjeldahl Nitrogen (TKN) Vs Total Nitrogen (TN)
While the Kjeldahl reaction in theory should recover nearly 100% of the organic nitrogen, the recovery of non-organic forms (nitrate for example) is only partial. This is the reason why TKN cannot be considered a direct measure of TN.
Recovery of Nitrate and Other Oxidized Forms of Nitrogen:
In order to recover an oxidized form of nitrogen, pretreatment with a reducing agent is necessary. The most commonly used is salicylic acid in concentrated sulfuric acid. In this reaction, decomposition of salicylic acid to free carbon results in reduction of sulfuric acid (H2SO4) to sulfur dioxide (SO2) which then produces a reducing environment for the Kjeldahl reaction. Most likely this is the basic rationale used to explain variable nitrate recovery from different samples. The more carbon available to reduce H2SO 4 to SO2 the greater is the potential to recover nitrate. In essence, when adding salicylic acid to the digestion the analyst is providing more reducing power over and above what is provided naturally by the sample.
It is also important to note that the addition of these carbon sources results in additional consumption of available sulfuric acid during the digestion. The total consumption of H2SO4 in the reaction resulting in a dry carbonized sample can result in an explosion if strong oxidants (i.e. hydrogen peroxide) are added. Therefore, if the analyst decides to add an additional carbon source for reduction purposes, then additional H2SO4 should be considered. (Note: A cocktail of zinc powder and potassium chromium sulfate (source for Cr III) in water and concentrated sulfuric acid also produces a reducing environment for N-O recovery.)
Of important note is the temperature of the digestion mixture during the reduction phase. It must be maintained low enough so as not to exceed the temperature at which nitrogenous substances are lost through volatilization or pyrolytic decomposition.