posted by Joe on .
HI, we conducted a qualitative analysis, to identify the cations in an unknown sollution..trying to determine whether it had Al 3+, Fe (3), and Lead ions..i am having trouble understanding some parts of the procedure. before testing for aluminum and iron, hydrogen peroxide was added to the solution, and then enough NaOH to make it a basic solution,( note that the solution had been treated with HCl beforehand), i know hydrogen peroxide is an oxidizing agent but i don't completely understand the function of it and why the solution was made basic.
Furthermore, when testiing for aluminum the solution was treated with acid and base many times, i know aluminum is amphiprotic, but once again i would realy like some clearification. Iv heard that adding an acid or base changes the state of the analyte but can someone please clearify all this properly?
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 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.