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Author Topic: ammonium hydroxide  (Read 9724 times)

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Borek

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ammonium hydroxide
« on: July 04, 2008, 11:43:31 PM »

my chemistry teacher taught about ammonium hydroxide as NH3OH.

Or NH4OH? I have read contradicting reports about NH4OH stability, long ago it was claimed that ammonia solutions are in fact solutions of NH4OH. Then I have read that such compound in fact doesn't exist.

In a way it is similar to the carbonic acid case - hard to tell if it exists or not.

something + water <-> black box <-> products of dissociation

As long as we observe only something and products of dissociation it is hard to tell, what is inside of the black box, and as long as the reaction in the black box is fast enough it doesn't interfere with dissociation.

Anybody with more information? Probably simple query to literature will help, but I have no access.
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Astrokel

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ammonium hydroxide
« Reply #1 on: July 05, 2008, 05:26:02 AM »

Or NH4OH?

Yes, sorry for the typo. Not sure of any literature that discuss about that.

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Valdorod

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ammonium hydroxide
« Reply #2 on: July 05, 2008, 06:33:23 PM »

Or NH4OH? I have read contradicting reports about NH4OH stability, long ago it was claimed that ammonia solutions are in fact solutions of NH4OH. Then I have read that such compound in fact doesn't exist.

I read somewhere ( can't remember where) that Arrhenius himself was the one who proposed that ammonia dissolved as NH4OH so that it would fit with his definition of bases.

In practical terms, there is a lab every semester in which the students test the conductivity of several solutions to classify them as non-electrolytes, weak electrolytes, and strong electrolytes.  1M solutions of ammonia always light up the lightbulb with a dim light.  Without NH4OH I would find it difficult to explain to the students why it conducts.  There are no other ions that I can think of that could be produced to make the solution conducting.

Then again it makes you wonder about the existence of a compound that has no physical properties such as melting point, density, color, etc.

A search through the literature yielded the following

Sing, R.; Rumpf, B.; Maurer, G. Solubility of ammonia in aqueous
solutions of single electrolytes sodium chloride, sodium nitrate, sodium
acetate, and sodium hydroxide. Ind. Eng. Chem. Res. 1999, 38, 2098-
2109.

Reactive Gas Solubility in Water: An Empirical Relation
Islam, M. A.; Kalam, M. A.; Khan, M. R.
Ind. Eng. Chem. Res.; (Correlation); 2000; 39(7); 2627-2630.

Influence of a Single Salt (NaCl/Na2SO4) on the Solubility of Ammonia in Liquid Mixtures of (Water + Methanol)
Schäfer, D.; Vogt, M.; Pérez-Salado Kamps, Á.; Maurer, G.
Ind. Eng. Chem. Res.; 2008; ASAP Article;

Influence of NH4Cl, NH4NO3, and NaNO3 on the Simultaneous Solubility of Ammonia and Carbon Dioxide in Water
Perez-Salado Kamps, A.; Sing, R.; Rumpf, B.; Maurer, G.
J. Chem. Eng. Data; (Article); 2000; 45(5); 796-809.

Solubility of Ammonia in Aqueous Solutions of Single Electrolytes Sodium Chloride, Sodium Nitrate, Sodium Acetate, and Sodium Hydroxide
Sing, R.; Rumpf, B.; Maurer, G.
Ind. Eng. Chem. Res.; (Article); 1999; 38(5); 2098-2109

Koneczny, H.; Lango, D.; Lango, M. The investigations on KCl-NH3-
H2O system. Chem. Stos. 1973, 2, 115-125

Trypuc┬┤, M. The solubility investigations of NaCl in aqueous ammonia
solution. Chem. Stos. 1987, 2, 257-265

The first three all point to the formation of ammonium hydroxide to explain some of their results, the other ones I was not able get hard copies of them yet, so I do not know which way they lean if any.


Valdo
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Borek

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ammonium hydroxide
« Reply #3 on: July 05, 2008, 09:20:36 PM »

n practical terms, there is a lab every semester in which the students test the conductivity of several solutions to classify them as non-electrolytes, weak electrolytes, and strong electrolytes.  1M solutions of ammonia always light up the lightbulb with a dim light.  Without NH4OH I would find it difficult to explain to the students why it conducts.  There are no other ions that I can think of that could be produced to make the solution conducting.

NH4OH by itself is not ionic, so it will not change solution conductivity. Question here is whether ammonia reacts directly with water to give two ions:

NH3 + H2O = NH4+ + OH-

or whether it reacts with water to create undissociated ammonium hydroxide

NH3 + H2O = NH4OH

which then dissociates

NH4OH = NH4+ + OH-

These two cases are very hard to distinguish, as measurements of pH and conductivity will not give definitive answer, you have to find a way to measure concentration of NH4OH. I suppose NMR studies could help.

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Then again it makes you wonder about the existence of a compound that has no physical properties such as melting point, density, color, etc.

That's nothing unusual - there are compounds that are too unstable to be separated, yet we see them using spectroscopy. I think that's the case of carbonic acid. It is a rather stable compound, trick is, it decomposes pretty quickly in the presence of water - so its decomposition rapidly accelerates.
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Valdorod

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Re: ammonium hydroxide
« Reply #4 on: July 07, 2008, 04:56:21 AM »

On more careful reading of those articles that I was able to get my hands on, actually none refer to undissociated NH4OH.

I did seferal searches through academic databases trying to find some sort of analytical measure of NH4OH and found nothing, no spectroscopy, microscopy, potentiometric, or any other type of technique that I could think of.

I am beginning to wonder if anyone can actually argue its existence experimentally.

Maybe it is just like resonance structures, it looks good on paper and it helps get certain messages across, but has no physical basis in reality.

Valdo
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