[johnno]

Ello everyone.

Ok, I was given 3 substances without labels although I knew that the three were Zinc Hydroxide, Magnesium Hydroxide and Aluminium Hydroxide. We know that Mg(OH)2 is the only one that isnt Amphoteric so I could get that one easily but Im stuck on what sort of test I should do to distinguish between the Zinc and Aluminium Hydroxides - I would normal use the melting points but Im instructed only to use chemicals and pH indicators.

Any help would be great thanks.

[Bakegaku]

Hmm... I think I'd base a chemical experiment based on the fact that aluminium hydroxide has three hydroxides while zinc hydroxide has two.

[Experte]

The best way to distinguish Zn and Al ( which salt is irrelevant ) is to heat you analysand up on a magnesia-channel until all  Al(OH)3 or Zn(OH)2 was oxidized. Then you have to add one drop of cobalt nitrate CoNO3 ( Note that, if CoNO3 is dropped on a warm substance, it becomes turquoise ) and heat up once more until your substance glows.  In case of the presence of Al the substance becomes blue and in case of Zn it becomes green after cooling down.

This is the best quantitative detection for Zn and beside the detection with Morin even for Al   

 
 

[UnintentionalChaos]

Take samples of each in three test tubes and add NaOH solution. The aluminum and zinc will dissolve. Add HCl in equivalent amounts slowly to new samples in test tubes. The one that dissolves first is zinc, since Zn(OH)2 has a higher molar mass than Al(OH)3. Since the samples weigh the same, there will be a larger molar quantity of Al(OH)3, which already requires more HCl to dissolve per mole. If you scale these up, error in volume of HCl added will mean less and you results will be more reliable. On a test tube scale, a drop or two could give you false results. Or, add HCl to samples until it has just completely dissolved. The pH of the Al(OH)3 will be much lower since it is one of the most powerful lewis acids and zinc is only a decent one.

This last one is a bit suicidal since H2S is extremely toxic. Bubble H2S through mixtures of the two remaining unknown hydroxides dissolved in acids. Zn will precipitate zinc sulfide, while nothing should happen to the aluminum.

[Borek]

Since the samples weigh the same

No such information given, thus assumption is wrong.

This last one is a bit suicidal since H2S is extremely toxic.

Don't scare the people. H₂S is toxic, but if generated in situ from thioacetamide hydrolysis and handled correctly under fume hood it is perfectly safe. In my university times we were doing classic qualiitative analysis of cations so all freshmen labs were stinking of rotten eggs for several weeks. No problems at all.

Somehow I doubt johnno will have acces to most reagents proposed so far.

[UnintentionalChaos]

I meant to take carefully weighed out samples of each and perform the test, or do you not have access to a decent balance? I meant for each test to be done with new samples. The first one is qualitative so mass won't matter much. The other two should be done quntitatively and masses should be carefully measured.

This is still the high school chem board, hence extra caution can't hurt. I think HCl and NaOH are pretty much standard in any lab so I don't think it would be difficult to perform these tests.