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March 25, 2017

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Sorry i posted this as an answer after my question had been answered, this is continued from the qualitative analysis question:

That reallly cleared some things up, but I do still have a few questions. Umm im not completely clear on the hyrogen peroxide subject. its added to a solution containing aluminum chloride and iron chloride, and you said it is ther eto make sure the ion is Al3+, can you please explain that bit. Also is it assumed that the chloride ions dissociate in the solution on their own, and can hydrogen peroxide not contribute to making Al ions into Al(OH)3.

Also in the experiment excess NaOH was added like you said, and the precipitate which formed was treated with a little bit of water and H2SO4, before being treated with KSCN. Im wondering whether theprecipitate was treated with H2SO4 simply to dissolve the precipitate , or whether it had another function also.

Lastly, the liquid portion of the solution, from the step where hydrogen peroxide and excess NaOH were added, is suposed to contain the Al(OH)3 like you said. This is then made acidic by the addition of HNO3 before being boiled and treated with NH3. i understand it would precipitate when NH3 was added, but im not totaly clear about why the solution was made acidic first as it was already in liquid form.

  • Chem- follow up question - ,

    Umm im not completely clear on the hyrogen peroxide subject. its added to a solution containing aluminum chloride and iron chloride, and you said it is ther eto make sure the ion is Al3+, can you please explain that bit. Also is it assumed that the chloride ions dissociate in the solution on their own, and can hydrogen peroxide not contribute to making Al ions into Al(OH)3.
    Did I say peroxide was added to make sure Al was +3. If I said that I surely didn't mean it. I think I said peroxide probably was added to make sure iron was in the +3 and not the +2 state. Peroxide has no effect on Al ion. I don't believe peroxide contributes to ppting Al as Al(OH)3 and yes, if AlCl3 was in the solution, it is present as Al^+3 and Cl^-.

    Also in the experiment excess NaOH was added like you said, and the precipitate which formed was treated with a little bit of water and H2SO4, before being treated with KSCN. Im wondering whether theprecipitate was treated with H2SO4 simply to dissolve the precipitate , or whether it had another function also.
    A bit complicated here because I believe you have omitted some of the details. At any rate, USUALLY one adds the NaOH, then to make sure all of the Al is dissolved, one adds an exess of NaOH. The solution is filtered (or centrifuged), washed, and we end up with the filtrate being Al ion dissolved and the ppt being Fe(OH)3 (with no Al contaminating it). Then, we put the Fe(OH)3 in solution by adding a little H2SO4. That gives us the Fe^+3 back in solution and it forms the blood red color when KSCN is added. The blood red color is due to formation of the FeSCN^+2 ion. Fe^+3 + SCN^- ==> FeSCN^+2 [As a side note here: When I was in school the speculation was that the red color was due to some kind of a complex ion. It has been shown now that is actually consists of FeSCN^+2.]

    Lastly, the liquid portion of the solution, from the step where hydrogen peroxide and excess NaOH were added, is suposed to contain the Al(OH)3 like you said. This is then made acidic by the addition of HNO3 before being boiled and treated with NH3. i understand it would precipitate when NH3 was added, but im not totaly clear about why the solution was made acidic first as it was already in liquid form.
    Right, it's already in liquid form but it "may or may not" contain Al^+3; at least you aren't sure of it at the time. It will contain Al^+3 if the original solution contained Al ion but when the Fe ppts you don't really know if Al is there or not. So the trick is to separate it by adding excess NaOH, then filtering and washing, THEN we add a little HNO3. That removes the excess NaOH (remember acids neutralize bases) and the Al ion, if present, re-precipitates as Al(OH)3.

  • chem 12 - ,

    write net ionic equations for each situation in which the precipitate redissolved on the addition of HNO3 or NH3.

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