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.
In the qualitative analysis procedure mentioned, hydrogen peroxide was added to the solution followed by NaOH to make it basic. The purpose of using hydrogen peroxide is to determine the presence of certain cations, such as aluminum and iron.
Hydrogen peroxide acts as an oxidizing agent in this case. It helps to oxidize certain ions, specifically iron(II) to iron(III) and sulfide to sulfate. By oxidizing iron(II) to iron(III), it allows us to test for the presence of iron(III) later in the procedure.
Making the solution basic by adding NaOH serves a couple of purposes. First, it neutralizes any remaining acid in the solution, as it had been treated with HCl beforehand. This ensures that acidity does not interfere with the detection of cations.
Secondly, making the solution basic helps to precipitate certain metal hydroxides. This step is important for the identification of certain cations, including aluminum. When the solution becomes basic, aluminum ions (Al³⁺) react with hydroxide ions (OH⁻) to form an insoluble white precipitate of aluminum hydroxide (Al(OH)₃). This reaction helps in the detection of aluminum.
Now, regarding treating the solution with acid and base multiple times during the aluminum testing, this is done to ensure the complete precipitation and confirmation of the aluminum hydroxide precipitate. Aluminum is an amphiprotic species, meaning it can react as both an acid and a base.
The addition of acid or base changes the pH of the solution and allows us to shift the equilibrium between the dissolved species and the precipitate. By repeatedly adding acid and base, any dissolved aluminum hydroxide can be converted to the precipitate and vice versa.
This ensures that the reaction reaches equilibrium, maximizing the chances of observing the precipitate and confirming the presence of aluminum ions.
In summary, hydrogen peroxide is used as an oxidizing agent to help detect certain cations, and making the solution basic helps in the precipitation and identification of specific metal hydroxides, such as aluminum hydroxide. Treating the solution with acid and base multiple times ensures the completeness of the reaction and the confirmation of the presence of aluminum ions.
In qualitative analysis, the purpose of adding hydrogen peroxide (H2O2) and making the solution basic using NaOH is to create specific chemical reactions that help identify certain cations.
1. The addition of hydrogen peroxide:
Hydrogen peroxide is an oxidizing agent, meaning it can cause other substances to undergo oxidation reactions. In this specific case, hydrogen peroxide is likely added to the solution to oxidize any potential interfering species that could interfere with the identification of the desired cations (Al3+, Fe3+, and Pb2+). Oxidizing these interfering species can help prevent false positive or false negative results.
2. Making the solution basic using NaOH:
By adding NaOH to make the solution basic, the purpose is to create a suitable environment for specific chemical reactions to occur. In this case, it likely helps to precipitate certain hydroxides that can aid in the separation and identification of the desired cations.
Regarding the treatment of the solution multiple times with acid and base when testing for aluminum, it is done to distinguish between aluminum hydroxide and aluminum oxide species.
Aluminum is amphiprotic, which means it can act as both an acid and a base. When an acidic solution is added to the mixture containing aluminum, it will react with the acid to form an aluminum complex. On the other hand, when a basic solution (such as NaOH) is added to the mixture, it will react with aluminum to form aluminum hydroxide.
The repeated addition of acid and base is done to find the point at which the reactions of aluminum complexation and aluminum hydroxide formation are balanced. This helps identify the presence of aluminum in the solution.
In summary, the addition of hydrogen peroxide helps eliminate potential interfering species, while making the solution basic aids in the precipitation of certain hydroxides. Treating the solution multiple times with acid and base is used to distinguish between aluminum hydroxide and aluminum oxide species.
It's important to note that the specific procedure may vary depending on the qualitative analysis protocol being followed. It is always recommended to refer to the specific procedure or consult a chemistry expert for accurate guidance.