The total nitrogen estimation from the plant sample is a very common analysis often performed by the students and researchers of plant science. However; there are many major and minor alterations of the most commonly used method of nitrogen estimation i.e, using Kjeldahl digestion and distillation assembly. For example; the acidic media (often 4% boric acid) which entraps the liberated ammonia; contains an indicator for facilitating titration. Some literature suggests using methyl red+ bromocressol green while some other suggests either methyl red+ methylene blue (Tashiros indicator) or congo red. Which is the most appropriate mixed indicator for this kind of acid-base titration? Again how much NaOH should be used to neutralize the H2SO4 within the Kjeldahl tube? Can anybody suggest a well explained and detailed procedure of analysis of total nitrogen which is accepted by the majority of the good journals? I have found ISO standard procedure of available nitrogen analysis from the soil sample however plant analysis procedure was enlisted there.
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You need a compound to test digestion recovery, e.g. TRIS base and (say) NH4Cl to test distillation recovery. Digestion rate is T dependent and if a T controlled block is available it can be set near 400 C with a K2SO4 /H2SO4 ratio ~ 1 g/mL. Se and Hg are the most efficient catalysts.
Several good books are available on this subject. I find the attached one very useful. It describes the methods in a very easy to follow way.
Dr. Bhattacharya,
Nitrogen analysis from agricultural systems doesn't have any confusion with the analytical methods available and as given reference by Dr. Srivastava, Paul or Dr. Bijay Singh. Also can have it from ISRIC'S publication.
With regard to the boric acid trapping procedure, I have used the following successfully for many years:
20 g boric acid dissolved in 1 l. DI water and buffered at pH 5.0. I then add 2-3 drops of mixed Bromcresol green-Methyl red indicator solution. I use a commercially available product but it can be made by dissolving 0.099 g of bromcresol green and 0.066 g of methyl red in 100 ml ethanol. The solution color is greenish blue at pH 5.4 and light pink at pH 4.5. Around pH 5.0 the color will be a gray color. When you add the indicator to the boric acid you should have a gray green solution color.
The key to using this method is to titrate to the same color end point each time. If it is more green or pink at the end point it should not make any difference as long as you titrate your blanks (very essential to have blanks) to the same color as your analyses. This is critical because you use the blanks (0 N) to correct for background N in your reagents in this way. The greatest error with titrating is not so much in your titration, but too high of acid normality when analyzing samples with low quantities of N. low normalities of acid often results in high volumes of acid needed to complete a titration and time required for titration. Thus one needs to have acid normalities that are compatible with the range of N values of your samples for best results.
Boric acid does not dissolve very rapidly in water and I'm not sure how easily one can get a 4% solution.
As far as amount of boric acid trapping solution needed for a samle this will be related to the amount of N in your sample. Trapping solution volume or sample aliquot size can be adjusted to optimize the trapping acid volume needed per sample. I usually use 5-7 ml per sample across a wide range of sample types and N contents in the samples.
Some of the references provided above are excellent references on this subject.
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