thriceaseraph
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Registered: 13-7-2010
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Determining the Concentration of Gaseous Ammonia
Hello All,
I am new here, so please accept my apologies ahead of time.
I am having difficulty in determining the concentration of ammonia in a confined volume. As I had set the experiment up, I didn't realize this would
be a difficult thing to do. I was more interested in qualitatively answering whether it could be detected rather than quantitatively. Now that it was
possible, the question of the concentration being detected has arose. I have rather limited resources and hope someone could be of assistance.
We are not set up to do gas phase chemistry- don't have the equipment nor the resourses to purchase the equipment (gas flow cell, various regulators,
flow meters etc).
The experiment was set up using a 4.5 mL square cuvette and adding approximately 0.5 mL of ammonium hydroxide. The enclosed space would then be a
mixture of water vapor, air, and ammonia gas. The cuvette was then placed in front of numerous heat lamps for SWIR detection. I did make a measurement
of the ambient air temperature in front of the lamps.
There were several methods that came to mind, but each seemed to possess several flaws and assumptions. I am not formally educated in gas phase
chemistry asides from undergraduate courses. I don't know whether the assumptions that were made were reasonable or valid.
The first method involved knowing the known w/w percent of the solution as 30%. The assumption was that 50% of the solution would vaporize to form
ammonia. This gave me a concentration of approx 18750ppm (assuming .5mL ammonium hydroxide x .3 x .5 in 4mL headspace). This does not take into
consideration solubility of gaseous ammonia, temperature, or pressure. I suppose it could be figured out with equilibrium constants, but frankly I get
lost. I think realistically I would have to worry about : NH3 (g)+ H2O (g) ↔NH3 (aq)+ H2O (l) ↔ NH4OH ↔ NH4+ + OH-. I don't think
Henry's Law would be valid because the ammonia does react chemically with water, and it is not a dilute solution.
The other method would involve weighing the cuvette before and after venting the gas. The cuvette was weighed, then allowed to vent for 30 seconds,
and weighed again. The assumption that the entire mass loss is due to ammonia alone and does not take into consideration water vapor nor the effect of
opening the cuvette on the liquid volatilizing. The headspace volume is divided by the ammonia gas mass without taking the density of gas into account
and found to be 1,007,779ppm. The other way is to take density into account and calculate concentration to be 1,848,249ppm.
These are drastically different concentrations. I am just not sure if it is possible to accurately determine the concentration without repeating the
experiment (which would be less than ideal) and getting the proper equipment to address this question.
Any help is greatly appreciated!!!
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unionised
International Hazard
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Ammonia barely reacts with water; Henry's law is probably pretty good for this.
More than a million parts per million is an odd state of affairs.
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zed
International Hazard
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Too many variables. Five major gases, no temperature/pressure data. Too small a volume. No instrumentation.
Use a larger vessel, displace the gas, dissolve the ammonia present in water, titrate it. Failing that, titrate your ammonia solutions, before and
after.
Don't really need IR to determine that there is ammonia in an atmosphere.
For generations, students at UC Berkeley have been sabotaging each other with the "old" smoke your colleagues glassware routine.
Professors tend to grade on a curve. Lab time is hard to get. Wanna gain an advantage? Destroy the prospects of the people around you!
A small amount of ammonium hydroxide, is secretly squirted into the vents of other student's lab lockers, this is followed with a few drops of HCl.
When the locker is opened a few days later, all of the glassware therein has a white coating on it.
Gaseous Ammonia, and Gaseous HCl....combine in the air to form Ammonium Chloride "snow". Nasty business.
The glassware all has to be washed and dried. The victims lose a lab period and fall behind.
The saboteurs surge to the head of the class. Grrrr.
[Edited on 15-7-2010 by zed]
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