Povidone Nitrogen Content - Kjeldhal method

Discussions about sample preparation: extraction, cleanup, derivatization, etc.

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I'm having problems performing the USP procedure for Nitrogen content method 2 by steam distillation.

I don't have access to the automated set up and have resorted to putting together my own steam distillation apparatus.

I can't seem to get recovery though.

Is there any tips for this procedure?
You have to get your procedure dialed in. I believe Lysine is a pretty popular AA to practice on. It's not the toughest one to get ammonia out of, but it's not too easy either. If your cook, quench and steam/titration rig gets good, consistent recovery on lysine, you should be ready for "real" samples.

It has literally been nearly 30 years since I did this, but if memory serves, the acid cook temp can be too hot, and if your NaOH addition is not close to a perfect quench, that can cause problems too. You also have to be careful to match the amounts of sample, salt, acid and base so the NH3 production will be "complete".

There are many ways to mess this up, even with the automated still/titration app.
Thanks,
DR
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It’s funny you mention that. The usp method
Says to add 7ml of sulfuric acid. At first I was adding more sulfuric acid to wash down the sides of the flask to get carbonized material down but I think this makes the digestion too acidic. Then when you add the naoh it’s not enough to render the solution alkaline enough to generate the ammonia.

I’ve since stuck exactly to the 7ml amount so when I add 30ml of 10N naoh it should be alkaline enough.

I also saw online as you mentioned, that if the digestion gets too hot you can generate NO2 and will produce lower than expected nitrogen contents. When performing the digestion, I’m using a heating mantle and I was cranking it as high as it would go. I looked at USP’s general method for nitrogen content method 1 And they state that during the digestion to keep it below the boiling point. I think this is to prevent you from generating the NO2. I might try that on Monday. It means that the digestion step will take longer.
The usp procedure for providone says to use a powdered mixture of potassium sulfate, curriculum sulfate, and titanium dioxide (95:2.5:2.5) roughly. I’ve seen elsewhere digestion tablets you can buy where the cupric sulfate content is much higher (almost 10%). Supposedly the potassium sulfate is meant to raise the boiling point of sulfuric acid, cupric sulfate and titanium dioxide are catalysts to drive the liberated nitrogen to form ammonium sulfate? Maybe if I increase the curpric sulfate content and remove the titanium dioxide? Or even double the amount of powder I add to the digestion mixture?

Another problem is that for my condenser I have a recirculator but it doesn’t have a cooling element and I’ve resorted to using ice to get the temp down. I can get the temp down to 8c or so but the temp at the still head where the condenser is is still very hot to the touch. I’ve tried wrapping the still head and round bottom with aluminum to keep that hotter than the top of the condenser to prevent condensate from getting stuck in the still head and round bottom.

Any other recommendations would be appreciated as I’ve done this a few times and I got close to the lower limit of the spec once or twice but other times I was way off or I would titrate my condenser and it would turn color immediately.
As far as the tablets are concerned, you just need "enough" CuSO4.
The total molarity of the resulting solution will be what helps you set the boiling point of the solution (remember, freezing point depression and boiling point elevation are colligative properties).
If memory serves, we ran our heating mantle at about 400°C, and the tablets weighed about the same, or more than our samples did, and they were probably about 65% moisture!
Also, I think we may have used 50% caustic (which is close to 19N).

If you're using 7mL conc H2SO4, you should need a bit less than 13mL to quench it.
Thanks,
DR
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