unionised
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Total nitrogen determination
At work I have been asked to measure the total nitrogen in something that's mainly polyurethane (based on MDI).
If this were a protein I know I could use the Kjaeldahl method (boiling sulphuric acid etc to convert N to ammonium sulphate).
Has anyone out there got any experience of using this method for other nitrogen compounds- in particular aromatic amines?
I know I can use a surrogate compound to check the method but it would be nice if anyone can offer tips or experience.
Thanks
(MDI is the industry's name for bis (4 isocyanatophenyl)methane aka methylene diphenyl diisocyanate)
[Edited on 3-9-09 by unionised]
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Inoxia
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In a book i read another method is fusing the nitrogen containing compound with Sodium metal, forming Sodium cyanide then forming Prussian blue from
the cyanide. The amount of Prussian blue obtained gives you the amount of Nitrogen in the compound.
I have never tried the H2SO4 for aromatic compounds, but i never realy do organic chemistry.
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DJF90
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You might want to check Vogel's qualitative organic analysis to see if it has any suggestions or experimental procedure.
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unionised
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The Lassaigne's saodium fusion test is great but it's nothing like quantitative. I really need to measure the amount of nitrogen present.
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Paddywhacker
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Yes, check with the literature.
Kjaeldahl is probably the method of choice, with added K2SO4, CuSO4 and selenium dioxide for the difficult digestion.
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UnintentionalChaos
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If the (plastic?) sample isn't already powdered, you should probably consider cryogrinding it so the Kjeldahl goes smoothly.
Department of Redundancy Department - Now with paperwork!
'In organic synthesis, we call decomposition products "crap", however this is not a IUPAC approved nomenclature.' -Nicodem
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monoceros4
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You may be interested in this article, Analytica Chimica Acta 463 (2002) 283–293. It describes a procedure for total nitrogen (and phosphorus)
determination by oxidation of the sample with potassium persulfate and microwave assistance or autoclaving, comparing the results with conventional
Kjeldahl digestion. The procedure takes somewhat less time and gave accurate results for five model substances but which gave low results compared to
the Kjeldahl method for natural samples (turbid waters and sediments.)
If I ever try an experiment in nitrogen determination I intend to try this method but then I'm not a professional.
"You must lead your opponent into a deep, dark forest where two and two make five, and the path leading out is only wide enough for one." Mikhail Tal
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unionised
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Thanks. I will get the library to send me a copy of that paper.
The sample is a foam and I think it will dissolve in the acid with no problem. What I'm not sure about is how well the nitrogen will be cleaved from
the ring and turned into ammonium sulphate. I have a feeling that I read somewhere that Kjaeldahl isn't so good with aromatic amines.
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monoceros4
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Quote: Originally posted by unionised  | Thanks. I will get the library to send me a copy of that paper.
The sample is a foam and I think it will dissolve in the acid with no problem. What I'm not sure about is how well the nitrogen will be cleaved from
the ring and turned into ammonium sulphate. I have a feeling that I read somewhere that Kjaeldahl isn't so good with aromatic amines.
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If you can't get a copy, I could perhaps help you with that *cough*. For what it's worth, one of the model compounds in the study was nicotinic acid,
indicating that the method might work even with compounds that give the Kjeldahl procedure trouble.
"You must lead your opponent into a deep, dark forest where two and two make five, and the path leading out is only wide enough for one." Mikhail Tal
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entropy51
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It's hard to imagine your polymer resisting hot H2SO4, especially if a catalyst such as H2O2 or Se were present, but who knows.
Check out this handy, dandy guide to Kjeldahl digestions:
http://www.scancotec.com/publicfiles/brochures/Foss_Scanco_H...
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UnintentionalChaos
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Relevant paper on microwave kjeldahls using H2O2. Plain old H2SO4 is definitely not going to work for what you want.
Attachment: microwavekjeldahl.PDF (207kB)
This file has been downloaded 1664 times
[Edited on 9-5-09 by UnintentionalChaos]
Department of Redundancy Department - Now with paperwork!
'In organic synthesis, we call decomposition products "crap", however this is not a IUPAC approved nomenclature.' -Nicodem
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unionised
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Thanks folks.
One of those reports says you can use acetanilide as a standard which is nice because it's fairly structurally similar to the stuff I want to digest;
the other says you need to add H2O2 to get tryptamine to react (tryptamine is also fairly similar).
Since the first paper adds copper sulphate as a catalyst and K2SO4 to raise the reaction temperature, I guess that's what makes the difference.
I will try stewing something like acetyltoluidine up with acid, copper and K2SO4 and see if I get the right answer then try the polymer.
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unionised
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Just to let you know.
0.1g of polymer, 5 g of KHSO4, 0.38 g CuSO4, and 6 ml of H2SO4 and heated to about 350C ( heating block temp) for about 4 hours with a glass funnel in
the top to act as a reflux condenser seemed to work. The material went black at first and frothed a bit then became paler. Rinsing the stuff from the
sides of the flask with a little more H2SO4 was necessary because some of it spattered.
Ammonium sulphate was measured by ion chromatography.
Recovery of N from acetyltoluidine (as a surrogate standard) was 95- 98%
Would probably have been better with the right glassware.
I don't know the true value for the resin I looked at so I can't know if it was quantitative but there was no evidence of the polymer surviving (as
measured by HPLC).
Anyone who needs to be told that very hot conc sulphuric acid is hazardous isn't going to live very long anyway.
[Edited on 23-11-09 by unionised]
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JohnWW
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That reminds me: the Kjeldahl method for total nitrogen is still used as the standard method of analysis for total protein in milk and milk solids.
Unfortunately, it does not distinguish between protein nitrogen and other organic (and inorganic) nitrogen. This was how the $cammers in that San-Lu
joint milk venture in China, in a company part-owned by New Zealand's Fonterra dairy company, were able to get away with watering-down milk and then
adulterating it with melamine (2,4,6-triamino-1,3,5-triazine), a cheaper high-nitrogen organic chemical which is polymerized to make melamine plastic,
to make an infant food formula. They pocketed the extra value of the adulterated milk. The $cam came to light earlier this year, after about 6 infants
died, and thousands more hospitalized with liver or kidney problems due to the stuff being unable to be eliminated in the quantities that were
ingested. The director of the company, a Communist Party boss, was sacked and imprisoned, and two Chinese dairy farmers involved in the $cam were
executed a few days ago.
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