Chemical Forums
Chemistry Forums for Students => High School Chemistry Forum => Topic started by: Rutherford on February 16, 2012, 03:46:34 PM
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I was wondering if there are better ways for proving some cations (Mg2+,Ca2+,Ba2+,Al3+,NH4+) in a quialitative analysis. I know these ways:
Ca2+: Ca2++C2O42--->CaC2O4 white precipitate
Ba2+: Ba2++CrO42--->BaCrO4 yellow precipitate
Mg2+: Mg2++2NH4OH-->Mg(OH)2+2NH4+
Mg(OH)2+2NH4++HPO42--->MgNH4PO4+NH4OH+H2O I don't know the colour
NH4+: NH4++NaOH-->NH3+Na++H2O red litmus is going to be blue
Al3+: Al3++3NH4OH-->Al(OH)3+3NH4+ white precipitate
Are some of these ways even wrong?
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Google for "qualitative cation analysis".
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These procedures have a long history in beginners chemistry courses. The best way is to work systematically, from HCl to remove the so-called "Group I" elements, 3 common ones with insoluble chlorides (you recall those, right?) and then separating the remaining elements into groups by precipitating them as sulfides or hydroxides, then working on the remaining groups using subtle chemical differences. If you can, to work with your example, add an excess of hydroxide to a mixture of Ba and Al, the aluminum will form a soluble complex, and you can work on them separately.
That's really the whole point of these sorts of experiments is to teach problem solving, and proper following of directions. Historically, Feigl's spot tests were developed to test for just about any ion. You can check a university library for a copy of Fritz Feigl's work. It will use the most obscure reagents and procedures, but will cover everything.
Its a good read, if you want to fill your head with this trivia. But procedures exist to analyze just about any common solution of ions, via ICP-MS instrumental analysis, and identify all practically all elements qualitatively and quantitatively to four decimal places.
FWIW, I have a book of Feigl's spot tests I bought when my local library was having a book sale. I rarely read it 'tho. Its so old, its spine is broken. I really should have it re-bound, I did buy it after all.
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Look at this webpage here: http://www.bpreid.com/cations.php It contains an applet that should visually show you what I tried to describe.
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I knew the ways for Ag and Pb with HCl but I didn't understand this:
add an excess of hydroxide to a mixture of Ba and Al, the aluminum will form a soluble complex, and you can work on them separately.
You mean if only Al and Ba left, than I have to add an excess of hydroxide so I can guess what cation of these two is in the solutions?
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Oh, I found on wikipedia a text. It's great! http://en.wikipedia.org/wiki/Qualitative_inorganic_analysis#1st_analytical_group_of_cations
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The only thing I need now is the reaction of NH4+ with Nessler's reagent.
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I knew the ways for Ag and Pb with HCl but I didn't understand this:add an excess of hydroxide to a mixture of Ba and Al, the aluminum will form a soluble complex, and you can work on them separately.
You mean if only Al and Ba left, than I have to add an excess of hydroxide so I can guess what cation of these two is in the solutions?
Yes. That was the trick they tried to teach me in undergrad chemistry. Try to build a flowchart of your own, using your list of ions above, and the info from the Wikipedia page you found. Put the list of ions, draw a box for the reagents, and two arrows from the box left and right, showing how the reagent separates them. Repeat until each ion is unique. And do try that applet, its fun.
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Thanks for the info, I will try that for sure. I only need now the reaction of NH4+ with Nessler's reagent (because I have to write it), and I didn't understand the proving of 2nd analytical group of cations. I only need for Cu2+,Hg2+,Pb2+, there is a part that I don't understand:
"To distinguish between ions in the black or reddish brown precipitate, it is first boiled in diluted HNO3. If it is insoluble, then Hg2+ is present. If it is soluble, then Cu2+, Bi3+ or Pb2+ may be present; sulfuric acid is then added to the resulting solution. If a white precipitate forms, then Pb2+ may be present; if no precipitate is formed, then a new solution is made by adding an excess of ammonium hydroxide in the original salt solution."
What is the original solution?
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As you've been going through the procedure, you've been adding reagents -- HCl, HNO3, thioacetamide (assuming you don't bubble H2S to form sulfides, and maybe even the sodium, potassium and ammonium salts of the anions you need to get a reaction. So at worst you've added cations, and at best you've diluted and lost while transferring much of your solution. So the later stages of the analysis are done on a small portion of the original solution you left behind at the start.
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So, the original solution is the one on the beginning, and I have to make some reserves of it. Thanks.
Why do I need thioacetamide?
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What to do if I get a salt that is insoluble in water? How to prove what cations and anions is the salt made of, like BaSO4?
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First of all, you do anions and cations separately, since the reagents you're using to precipitate have to be compounds that contain some of the anions and cations you're looking for. You have to have an idea of what you're looking for, a typical method is the different reactions of sulfides with excess of base -- some dissolve, others don't. There are other methods, like some hydroxides dissolve in an excess of base, because they form a soluble hydroxide complex. You'll want to build a flowchart for this reason, when you're done, you'll be able to distinguish them all.
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I am a little confused now. Let's say that I get to prove BaCl2, so I can't do the test for Ag and Pb, because the anion is Cl-, this makes things much more complicated. I tried to make a chart but I didn't understand what you said, this part:"two arrows from the box left and right showing how the reagent separates them".
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What you've just described, is a different experiment than the one you started with. ;D If you have only one unknown, and you just have to test it, you won't need a flowchart trying to separate a bunch of other cations. What is the real question? You have to test a white crystalline powder for, what exactly? It is either barium chloride, or ... what, sodium chloride, barium nitrate, barium bromide, barium iodide, barium fluoride? Or a mix of some of those?
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I get a salt and I have to:
1.write in what it is soluble
2.determine the cation and the anion
3.write the equations of the proving reactions
I won't get just any cation or anion, only some of these:
Cation: Ag+,Pb2+,Cu2+,Hg2+,Fe3+,Al3+,Ba2+,Ca2+,Mg2+,NH4+
Anions: CO32-,SO42-,Cl-,NO3-,CH3COO-
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OK, for the anions, do some searches for some spot tests. They have very different properties, when mixed with certain reagents. You won't need a flow chart for anions. For the cations, you will need to make a flow chart, you will find some online after a search. Here's something useful: you will be given a soluble salt (I'm assuming.) What possible combinations are insoluble? Once you've excluded some pairs, based on the anions, you may be able to trim your cation flow chart and you'll learn a useful test for determining the identity, if it is there.
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Then I think that it's better to find the anion first (because there are only 5) and then to think about how to find out the cation. Found some flow charts on google, I hope that I will figure out how to determine the cation.