[danago]

Hey. If i had aluminium hydroxide solid and solid zinc hydroxide, how could i distinguish between the two?

The first thing that came to my mind was to mix an excess of each solid into some HCl and then filter off the solutions. Since the Al ion is smaller and more highly charged than the Zn ion, its solution will be more acidic and hence have a lower pH, allowing me to distinguish between the two.

Are there any flaws in my reasoning? Is this a practical method to distinguish between the two?

[leve]

The first thing that came to my mind was to mix an excess of each solid into some HCl and then filter off the solutions. Since the Al ion is smaller and more highly charged than the Zn ion, its solution will be more acidic and hence have a lower pH, allowing me to distinguish between the two.

The acidity depends on the concentrations of Al(OH)3 and Zn(OH)2 and HCl

[Schrödinger]

try adding an acid to both of them (maybe sulphuric or HCl)
Now perform the usual qualitative salt analysis tests on both....

[danago]

The first thing that came to my mind was to mix an excess of each solid into some HCl and then filter off the solutions. Since the Al ion is smaller and more highly charged than the Zn ion, its solution will be more acidic and hence have a lower pH, allowing me to distinguish between the two.

The acidity depends on the concentrations of Al(OH)3 and Zn(OH)2 and HCl

Yea i just realised that after i posted, i completely overlooked the fact that the concentrations would not be the same.

try adding an acid to both of them (maybe sulphuric or HCl)
Now perform the usual qualitative salt analysis tests on both....

When you say qualitative analysis, what type of tests are you talking about? I thought about precipitation tests, but i cant think of any case where one of the ions will precipitate and the other wont.


Another thought i had was to use the mistake i made in my original idea. If i measure out equal masses of each solid and then slowly add HCl until it is all dissolved, i should find that one of the solids will require approximately double the amount of HCl than the other, which will identify it as the Al(OH)3?

[plankk]

I advise to look for a reaction with ammonia.

[danago]

Ohh ofcourse, didnt think of that  The Zn(OH)2 will dissolve due to the formation of the ammonia complex ion, but the Al(OH)3 will not?

Thanks for the help.

[plankk]

That's right.

[ARGOS++]

Dear dango;

Usually the fluorescence of "Morin-solution" is used for "detecting" Aluminium:
          http://commons.wikimedia.org/wiki/File:Aluminium-Morin-Reaktion.svg
Also Alizarin-S is often used: 
          http://www.springerlink.com/content/j58l924712x416mk/ 
Maybe you can search the Net for better references/descriptions.

Good Luck!
                    ARGOS⁺⁺

[aldoxime_amine]

You can use potassium ferrocyanide also, after dissolving both solutions in NaOH.http://en.wikipedia.org/wiki/Potassium_ferrocyanide

The zinc solution will form a bluish white ppt, while the aluminium solution won't.

[danago]

Thanks for the input, ill read up a bit more on those methods

[408]

That's right.

Would not the NH3 complex the Zn, solubilizing it and would not the corresponding OH in NH4OH form aluminate, solubilizing Al?

[Mitch]

The densities should be different.