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Complete Article from Edition 48 APG eNewsletter
Article written by Phillip Eleyette, QC Chemist at APG
The Total Kjeldahl nitrogen (TKN) method was developed over 100 years ago by Danish chemist Johan Kjeldahl. Kjeldahl worked for Carlsberg Brewery and was assigned to determine the amount of protein during malt production. The method was designed to find the amount of organic nitrogen in the malt to measure the protein content. Since then, the TKN method has been used in a variety of applications including food, beverages, water, grain, waste water, soils, and other sample to determine nitrogen.
Total Kjeldahl Nitrogen is defined as the sum of free ammonium and of organic nitrogen compounds. TKN is a three step process: digestion, distillation and analysis.
There are two types of process: macro-Kjeldahl and semi-micro-Kjeldahl method. The macro-Kjeldahl is used for low and high concentration but requires large sample volumes. The following table can be used to determine sample size.
TKN in sample, mg/L Sample size, mL
0-5 500
5-10 250
10-20 100
20-50 50
50-500 25
While the semi-micro method is used for high concentrations of organic nitrogen, the sample volume should be chosen to be between 0.2 to 2 mg/L of organic plus ammonia nitrogen. The following table can be used to determine sample size.
TKN in sample, mg/L Sample size, mL
4-40 50
8-80 25
20-200 10
40-400 5
The digestion process involves using a catalyst to convert organic nitrogen into ammonium sulfate. Mercury was first used as the catalyst but due to toxicity and disposal concerns copper has replaced mercury as the catalyst. The solution is digested using heat in a temperature range of 375 to 385oC.
Organic N + H2SO4 → (NH4)SO4 + H2O + CO4 + byproducts
The next step is distillation. In the distillation, excess base is added to convert NH4 to NH3 which is the collected from the distillation. The distillate is collected in boric acid or sulfuric acid. The boric acid is used for titration analysis and the sulfuric acid is used for electrode method. The temperature of distillate should not rise above 29oC.
(NH4)2SO4 + 2NaOH → 2NH3 + Na2SO4 + 2H2O
The final step is the analysis. The analysis can be by titration or electrode method.
The titration method uses 0.020N sulfuric acid as the titrant. The distillate is titrated until indicator turns pale lavender.
The titration procedure can be calculated for TKN with the following equation:
TKN, mg/L= (A-B)N x F X 1,000 / S
where:
A = mL of standard 0.20N H2SO4 solution used in titrating sample.
B = mL of standard 0.20N H2SO4 solution used in titrating blank
N = normality of sulfuric acid solution
F = milliequivalent weight to nitrogen (14mg).
S = mL of sample digested.
If the sulfuric acid is exacty 0.02N the formula is a\shortened to:
TKN, mg/L = (A-B) x 280 / S
References:
Method 4500-Norg - Standard Methods for the Examination of Waste Water. 20th ed.
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