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ammonium hydroxide and aluminium


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"Logically, something that does not exist does not attack aluminium."

 

Right... so ammonia solution doesn't have basic properties towards water then? You are saying hydroxide does not exist in access this solution at all are you? I prepose that if you remove the oxide layer and place in ammonia solution I expect you would see the same reaction occur- albeit more slowly.

 

Secondly just because hydroxide attacks it, doesn't mean hydroxide is the only thing that attacks it- so ammonia might still participate in or catalyse a reaction despite its basic (or lack of, as you imply) properties.

 

I agree that ammonium hydroxide is not an accurate physical description. But I disagree that it is not a useful identifier on the grounds of: It still decribes accurately it's basic properties and ideal existance, and it is widely recognised by both the scientific and non-scientific communities alike. It is not an illogical name- it's just that in a real life situation the "ideal" properties are not realised.

 

And saying that "ammonium hydroxide doesn't exist" without any further explaination does not answer anything at all- especially not if the person was to have little understanding in chemistry. It is a closed, dismissive statement with no constructive purpose and quite honestly wrong as aluminium does react with ammonia (or so called ammonium hydroxide) solution (apparently producing hydrogen gas).

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OK.

I was wrong.

I checked and found that aqueous ammonia does react with Al.

A nice squeaky pop confirmed the production of hydrogen.

However it remains the case that aqueous ammonia doesn't contain nearly as many hydroxide ions as you would expect from the name and so the name is still misleading.

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"and so the name is still misleading."

 

Yes- the name can be misleading and more-so in certain contexts. I completely agree. I just don't feel that a complete withdrawal of the name from use is required, or that jumping down peoples throats about it is appropriate. Most chemists recognise that the solution is in equilibrium and recognise that the term "ammonium hydroxide" is used to refer specifically to an ammonia/water solution despite its inaccurate description of the actual molecular species present and the possibility of misinterpretation.

 

My grounds for keeping the name is not because I think it is accurate (I don't), but because it is well known and the ambiguity that is present on a theoretical level (i.e. what molecular species is present) disappears when employed physically (i.e. However it is labeled you can only ever physically have an ammonia solution with a few ammonium and hydroxide ions present in equilibrium).

 

Basically, I'm saying that the theoretical misinterpretation is unlikely to transpose to practical error- especially as the 'misinterpretation' better describes the properties of its solutions at first glance, e.g. pH.

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I accept it as a legitimate name. Also I never stated that I personally reference ammonia solutions as ammonium hydroxide, just that I accept that some people do and that the name is valid- so me calling water hydronium hydroxide is technically going to be inconsistent.

 

So are you finished, or is there another smart comment on the way?

_______________

Oh, also a post targeted at the original OPs question.

I've been thinking about this and browsing the web a bit.

 

When NaOH reacts with aluminum the following reaction takes place:

 

2Al+ 2NaOH+ 6H2O -->2NaAl(OH)4 +3H2

 

So, I was thinking that maybe -OH attacks and removes the oxide coating of the aluminium first:

 

2-OH + Al2O3 + 3H2O --> 2Al(OH)4-

 

Then the aluminium surface is free to react with water:

 

Al + 3H2O --> Al(OH)3 + 1.5H2

 

Al(OH)3 reacts with excess -OH to produce Al(OH)4-

 

Al(OH)3 +2NaOH --> 2NaAl(OH)4-

 

If the above is correct then in ammonia solution, the -OH could be sufficient to remove the oxide layer and start the reaction with water but not concentrated enough to convert the hydroxide to aluminate leaving you with the aluminium hydroxide. I haven't looked for any papers to support this hypothesis yet- it's just based on stuff I found online. I'll have a bit of a browse in my downtime (Just started at a new uni- trying to work out their database system).

Edited by Suxamethonium
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  • 6 years later...

I recently prepared ammonia water (likely pure NH3 (aq) ) by placing distilled water (DW) in a large mouth vessel together in a container (which was twice the volume of the DW) loaded with yellow colored household ammonia rich in surfactants, etc. I carefully placed a plug, prepared by wrapping layers of Reynold's Al foil (Al 98% with remainder Fe and Si) around my finger full of NaCl, into the household ammonia and sealed the entire chamber.

The logic is that as NH3 has much reduced solubility in seawater, I expected with time the DW would amass a good amount of the liberated ammonia from the now salt rich household ammonia. A day latter, I opened the vessel, I was surprised to see the amount of bubbling/attack performed on the Al plug, with no heating and no long waiting time (under 12 hours). Favorably, the latter Al attack added H2 gas to increase pressure in the vessel, which to reduce stress in the system, likely further increased the  ammonia content of the DW.

Note, if one already have some ammonium sulfate,  there  could try adding it to the vessel with the impure household ammonia/aluminum plug/NaCl. As the formed aluminum hydroxide is amphoteric it may ( Al(OH)3 can be a difficult salt to work with ) actually react with the ammonium sulfate to produce aluminum sulfate and further liberate ammonia. 
 

I viewed this as an inexpensive and successful path to pure aqueous ammonia.

It also indicates that aqueous ammonia is pretty reactive with an Aluminum alloy/NaCl with possibly a Fe presence. Here is a study confirming the attack by chloride ions  on industrial Aluminum alloy (see http://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.429.4364&rep=rep1&type=pdf ). Even without the transition metal, this result appears to parallel the attack of Al with HNO3 in the presence of NaCl (see https://www.researchgate.net/profile/Vladimir_Petrusevski/publication/215904100_REACTION_OF_ALUMINIUM_WITH_DILUTE_NITRIC_ACID_CONTAINING_DISSOLVED_SODIUM_CHLORIDE_ON_THE_NATURE_OF_THE_GASEOUS_PRODUCTS/links/0f4f8e0daf9ae43bd314f60e/REACTION-OF-ALUMINIUM-WITH-DILUTE-NITRIC-ACID-CONTAINING-DISSOLVED-SODIUM-CHLORIDE-ON-THE-NATURE-OF-THE-GASEOUS-PRODUCTS.pdf ). This last paper makes the strong statement that Al 'dissolves in practically all  strong acids' in presence of chloride ions. Likely a similar truth occurs with bases, including aqueous ammonia. Given the electropositive nature of Aluminum metal, some electrochemistry/surface chemistry is likely afoot.

To ever assume that Aluminum is inert with an acid or base in real world applications replete with salt particles, seawater,..., is as foolish as the US Navy was in investing a billion dollars in a fleet of dissolving aluminum hull warships (see https://www.wired.com/2011/06/shipbuilder-blames-navy-as-brand-new-warship-disintegrates/).

Edited by AJ999
Grammar, Conclusion, Grammar
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  • 2 weeks later...

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