Posted by Joe on Sunday, February 28, 2010 at 12:38pm.
The question is a little vague but I can help get you started. First, the peroxide probably is to make sure that the iron is in the +3 state. Iron(II) hydroxide is insoluble as is iron(III) hydroxide but the Fe(OH)3 is a little better test AND it is a little more insoluble. Then the addition of NaOH to make it basic is to precipitate Al as Al(OH)3 and iron as Fe(OH)3. Al(OH3 is a gelatinous white solid ppt and Fe(OH)3 is a reddish brown (rust colored) ppt. Both are exceptionally insoluble in a basic solution. However you can have both ions in the same solution an more definitive tests must be run to make sure which is which. Also, USUALLY the Fe(OH)3 ppt will mask the Al(OH)3 ppt. Somewhere in the procedure, you will add an excess of NaOH. Fe(OH)3 ppts in a basic solution and doe not dissolve in an excess of NaOH but Al reacts differently. Al(OH)3 ppts in a basic solution but dissolves in an excess of base (due to its amphoteric nature). So the color and the different reactions in NaOH and excess NaOH is how to differentiate between Al and Fe ions. There are confirming tests that usually are done for each after the first "call" is made. Don't be shy about following up with questions.
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 hte precipitate , or whether it had another function also.
Lastly, the liquid portion of teh 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.
Related Questions
Chemistry - (Qualitative Analysis) I know this is vague, but if i had a solution...
Chemistry - The experiment that I did in lab consisted of performing a ...
CHEMISTRY - What is the reagent responsible for the formation of H2S in the ...
Ochem - What would be a newer more efficient method to do the qualitative ...
Chemistry - you were asked to determine the melting point of an unknown compound...
Project Management - Evaluating the Two Methods of Risk Analysis Explain how the...
Chemistry - Lab: Titration of NaOH (0.1 molar)to unknown solid acid. Trying to ...
Chemistry - In studying the chemistry and qualitative analysis of anions, why ...
AP Chemistry - I need help with figuring out an appropriate way to do this sort ...
Chemistry - Compound Identification How can I identify the anion of an unknown ...
For Further Reading
