We are trying to analyse the nitrogen content in soil, and we are trying to use the Kjeldhal method. Yet we are running into problems with the heating (due to insufficient equipment) and therefore would like to replace the H2SO4 with another acid which one does not have to heat up to high temperatures. We are currently thinking about HCl, would this be advisable to use? Note, this is for the beginning part of the method (digestion), so not the titration.
Hello Julie,
Based on what you report to us, I think that you may need to heat up the mixture. You have alternatives to sulphuric acid such as alkaline persulfate but you need to heat the mixture. I'm puzzled on HCl digestion, I think that it is more dangerous than efficient but experiences from others are welcome.
Did you consider the use of elemental analysis (if you have the equipment) ?
if u dont want to use high temperature, i suggest you may increase the digestion time until the digestion complete . Purpose of using high temperature to fasten the digestion to be complete. You may try to adjust temp and time. Adding catalyst and oxidizing agent can help to fasten the digestion as well.
perchloric acid is a suitable oxidat, but dangerous and difficult to handle!
Do not use perchloric acid to analyze for N!!! I have run thousands of pechloric acid digests as well as kjeldahl digests and have had extensive experience in both. The perchloric acid procedure that I am most familiar with uses an 3:7 perchloric acid to nitric acid ratio. Other methods may include approximately 10% sulfuric or hydrochloric acid. In any case, the addition of nitric acid renders the digest unuseable for N analysis even though most of the HNO3 is boiled off. In addition, without properly vented chemical hoods, there is a danger of perchlorates forming in the hood which can detonate when exposed to volatile organic solvents.
Younes mentioned the use of alkaline persulfate not perchloric acid! This is a possibility but i don't have experience with it but persulfate and perchloric are not the same! I'm not sure what equipment or facilities you have available for analyzing for N. But, Younes suggestion of using a CN or CNS instrument for high temperature combustion is an excellent one if you have access to the equipment or a laboratory with it.
As far as running the Kjeldahl analysis with minimal equipment, some years back, Hach Chemical, Loveland, CO, USA had a very simple system of conducting kjeldahl digests with a Pyrex volumetric flask on a hotplate of heater. I don't know if this is still available but I believe that they also offer prepackaged chemicals and catalysts for kjeldahl digestions.
I hope that this is helpful.
We used a domestic radiant hotplate with an output of 2kW as our heating source. On this we placed an aluminium block drilled with holes 2.7 cm diameter and 3.5 cm deep. Digestion tubes were "Exelo" 50 ml calibrated test tubes (cat. No. T2/30).
This catalyst enhanced Kjeldahl method worked in the Pacific Islands and New Zealand if nitrate is present
I suggest first to take inspiration from the Mannheim and Berlin Processes
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