budullewraagh
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Qualitative Tests
i am a member of a competitive team of physical and natural scientists at my high school. we compete in various events and i serve as the chemist.
one of our events involves qualitative analysis, which isn't terribly difficult usually, but given the restrictions they put on us, it appears to
be quite difficult.
if you could give me feedback on my work and help me with things i am missing, i would be greatly appreciative.
equipment/reagents available:
-test tubes, stirring rods
-deionized water
-any ONE of the following: 3M HCl, 3M NaOH, 0.1M AgNO3 or phenolphthalein
we will have to identify any number of the following solids:
-NaHCO3, Na2SO3, Mg(OH)2, CaCO3, NaH2PO4*2H2O, NH4Cl, ZnCl2, KI, KOH, Al(NO3)3*9H2O
CaCO3 will be easy nomatter which reagent i choose, as it is insoluble in water
if i use the HCl, i will notice bubbling with CaCO3 and NaHCO3, and feel an exothermic reaction with Mg(OH)2 and KOH (and also Na2SO3??)
if i use the NaOH, i will notice a reaction with NaH2PO4*2H2O, NH4Cl, Al(NO3)3, (ZnCl2 perhaps?)
if i use the AgNO3, i definitely will precipitate the halides, so count out NH4Cl and ZnCl2, KI, and i'm not sure but perhaps Na2SO3, Mg(OH)2,
KOH
the phenolphthalein would indicate Mg(OH)2, KOH, and perhaps CaCO3?
so i think the silver nitrate would be the best reagent to use, although i may be wrong in its capabilities. if i am, please let me know. the
problem is that nomatter which reagent i choose to use to aid in my analysis, i will have a lot of precipitates that will be less than easy to
identify and a lot other unaffected chems.
i suppose i could use solubility, although i do not believe we will be given a balance and i know we are given only 10g of each of the solids. thanks
in advance
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neutrino
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Do you get the use of any kind of heat source? Only six of those are heat stable. Of those, one would sublime, and one would eat your glass. Sodium
dihydrogen phosphate would melt or boil at low temperatures, depending on the hydration.
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budullewraagh
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no heating source at all
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BromicAcid
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Magnesium hydroxide is almost insoluble in water so I guess you could mix it up with CaCO3 if you try to tell them apart based on solubility, with
acid they will both go into solution but the carbonate will of course release carbon dioxide gas giving it away.
ZnCl2 and KOH are hygroscopic, just sit them out and see if they 'melt' you could always differentiate between the two of them 'by
taste' although I would mildly recomend against it.
AgNO3 is going to precipitate almost everything so there is nothing selective to it. I would be wary of it for that reason although some of the
precipitates have certain colors, and most are light senstive so those physical characteristics could guide you.
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HNO3
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You can identify ammonium salts by adding NaOH and noting the smell. *gag
cough*
\"In the beginning, God...\" Wait a minute, God doesn\'t exist!!!!!!!!!! \"OK, in the beginning, ummm, hydrogen...\" Wait a minute, what about the
laws of thermodynamics? \"OK, in the beginning, ummm.....UMMMMM, what\'s left to choose from?
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cyclonite4
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@neutrino: I'm just curious, which of the compounds would sublime? And is it correct that hot bases are responsible for the glass eating??
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neutrino
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You are correct. I’d specifically bet on KOH, as molten NaOH is notorious for destroying… well… everything it touches. Ammonium chloride would
sublime.
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Magpie
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I recently aquired some ore concentrate from the Red Dog zinc mine in Alaska. This inspired me to do some qualitative tests to determine what other
metals might be in the ore besides zinc. Before I knew it I had decided to take the big step of assembling all the liquid reagents needed to do a
complete qualitative analysis for Group I [Ag, Pb, Hg(I)], Group II [Pb, Bi, Cu, Cd, Hg (II), As, Sb, Sn], and Group III [Co, Ni, Mn, Fe, Al, Cr, Zn]
cations. Eventually I will add the reagents needed for Group IV [Ba, Sr, Ca] and Group V [Na, K, Mg, NH4] cations. Tests for many common anions also
exist. Most of the liquid reagents I placed in 60 mL dropper bottles.
The text I used for this is the Semimicro Qualitative Analysis section of "College Chemistry with Qualitative Analysis," 7th ed, 1984, by Holtzclaw et
al. It may be available on the used book market for those interested in this disappearing art.
At one time this was a required 5 credit course for chemistry majors as well as others. My brother was required to take it as a zoology major. As a
chemical engineering major, however, I was not. I think the course may be disappearing in colleges and universities. After all, who wants to do
these wet chemistry tests when ICP and ion chromatography are available. But for the home chemist I think the tests are a lot of fun, and much
chemistry can be learned from doing them.
Shown below is a picture of the reagents that I placed in 60 mL dropper bottles. Hopefully the labels next to the bottles are readable.
[Edited on 21-2-2009 by Magpie]
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len2
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Here's a free book on the Web on this very interesting subject (though not the one Magpie was referring to)
Introduction to Semimicro Qualitative Analysis
(edited to fix broken link)
[Edited on 2-21-2009 by Polverone]
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Magpie
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Thanks len2 for posting that link. It looks to be very complete and gives detailed instructions. Those details will be quite helpful to the learner
in this subject.
One thing I noted was that the Google book specifies the use of H2S as the reagent for Group II and Group III. Modern texts, such as the one I cited
above, specify the use of 5% thioacetamide instead. This is very handy and precludes the generating and handling of H2S directly. By adding the
specified number of drops of 5% thioacetamide to the acidified sample and heating, H2S is generated in situ:
CH3-CS-NH2 + 2H2O -----> H2S + NH4+ + CH3CO2-
Thioacetamide is a bit expensive but well worth it IMO.
Also, to do these tests efficiently, a small electrical centrifuge is needed. I picked up one off eBay for not too much. I suppose a hand
centrifuge, or even filtering, could be substituted, but I wouldn't recommend it.
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smuv
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In some cases you can use acidified sodium thiosulfate as a source of H2S; something I will let you research on your own as I don't have much more
info about it.
LATE Edit: Thiourea can probably directly be replaced for thioacetamide in those reactions. See this monograph for more details.
[Edited on 2-22-2009 by smuv]
"Titanium tetrachloride…You sly temptress." --Walter Bishop
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