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Topic: Will acid digestion boil off nitric acid above 83 C?  (Read 2619 times)

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Offline AJW2012

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Will acid digestion boil off nitric acid above 83 C?
« on: August 15, 2012, 02:52:06 PM »
Hello,

Just so nobody has to look this up the B.P of Nitric Acid:

Boiling point of 100% Nitric Acid: 83°C (181.4°C)
Boiling point of Azeotrope (68%) Nitric acid: 120.5°C (248.9°C)

In an acid digestion, we add 1.00ml of concentrated (65-70%) Nitric Acid to 100.0ml of water. The measured temperature of the water once it stabilizes is 97.1°C (206.8°C).

Does that 1.00ml of conc. Nitric Acid boil off at any point during this digestion? If so, to what approximate degree?

All replies are greatly appreciated.

Offline curiouscat

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Re: Will acid digestion boil off nitric acid above 83 C?
« Reply #1 on: August 21, 2012, 10:51:40 AM »
It looks like you are starting out with < 1% HNO3. BP for that ought to be essentially 100 C.

If you look at a HNO3-H2O VLE diagram the only way a mixture has a BP < 100 C is in the 68-100% HNO3 range. There's no way for your mixture to get there unless it somehow jumps above the azeotropic point.

In any case a 97 C BP corresponds to approximately 90% HNO3. What's your final digestion volume? Unless it is close to 1ml this is unlikely from a mass balance.

My suspicions: Check the calibration of your thermometer or else are you at a high altitude location where the BP of H2O itself is close to 97 C?



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