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chemico
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Identifying an unknown
Good Day Chemistry Aficionados,
Please help me with this problem if you can. We have an unknown compound that we need to identify and a wide array of chemical and potential tests to
identify it with.
The problem is not getting it identified -- the problem is time. We have a limited amount of the time and the quicker and more efficient we get the
identification the better off we score.
We are asking you guys to let us know what you guys believe are the most valuable tests to perform...and any flow charts or materials we should
consult before we officially start this experiment.
Any information or words of advice are greatly appreciated.
Thank you,
Andrew
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Hexavalent
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Do you have any information about your compound?
If it's inorganic, conventional wet techniques may be the way to go. If it's organic, ideally you'd want IR and NMR spectra and a melting point.
"Success is going from failure to failure without loss of enthusiasm." Winston Churchill
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chemico
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We know it is an inorganic compound, for sure. That is pretty much it. We have a list of a few hundred compounds that are all potentially it and all
are inorganic,
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AndersHoveland
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The oxidation of ammonia by hydrogen peroxide is spontaneous, but the reaction is rather slow. Alkaline conditions favor faster reaction rate.
Nitrites can certainly be formed from this reaction, but it should be remembered that the reaction rate for the oxidation of nitrite to nitrate is
much faster than the oxidation of ammonia. And of course, boiling temperatures will just cause the nitrite to be reduced (by the ammonia) to nitrogen
gas.
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amazingchemistry
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Well, what does it look like, feel like or even smell like? that shouldn't take more that 20 seconds to ascertain and would help you at least narrow
it down a bit. I assume you don't have access to anything like ICP? If you do, there are SOPs for that (you'd have to start by dehydrating and
possibly calcinating it). If that's not what you want to do with it you could start by grouping the "hundreds of possible compounds" into broad
categories.
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woelen
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Without more information we cannot really help you. Color of the compound is a strong way to narrow down things. If it is white or colorless, then you
can rule out nearly all transition metal salts (a few exceptions are zinc, cadmium, mercury, silver, titanium).
There are schemes for qualitative analysis, especially in older literature. These schemes in fact are a kind of decision tree, consisting of many
tests, which lead you to the anion and the cation of the inorganic species. The use of sulfide ion, ammonia and weak acids, all in dilute solutions
plays an important role in this decision tree for determining the cation in the tests.
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phlogiston
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Any bit of information you can share on the origin of your mystery compound will also help a lot. It should narrow the range of possible ions. (eg you
would not expect to find a rubidium compound seeping out of the wall of your cellar unless you live in a strange place).
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sonogashira
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I would recommend that you Google (and download) Vogel's Qualitative Inorganic Analysis.
You will need to perform a chromatographic separation to make sure that you don't have a mixture of compounds, or a salt.
It's an interesting problem. It would be more interesting if it was completely unknown. In which case, I would perform a melting point experiment to
crudely determine whether it is organic (charring) or inorganic (high melting point). If organic I would separate it on TLC (or confirm that it is
pure by having it remain un-separated on TLC), then scan the plate with diode array.
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adamsium
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This sounds typical of the practical exams often given for university chemistry courses. I'm pretty sure that chemico is not attempting to identify an
entirely unknown compound found laying around or the like. This means that it can be assumed that the sample is not a mixture and is quite pure
(hopefully  ).
Usually, in order to maximise efficiency, what you want to do is to rule out entire groups of compounds quickly. An example of this would be testing
for chlorides, bromides or iodides with a very quick silver nitrate test. If you're able to wipe out large portions of the list of potential
compounds, you're well on your way. Essentially, it's often a good idea to begin by finding out what you don't have; then you can
start working on deciding what you do have. Because you're given a list of possible compounds, this approach is entirely possible
(and pretty much what they generally seem to expect students to do for these exams).
Also, as woelen mentioned, you can often garner some very useful information qualitatively. This can also help in ruling out large portions of the
list of possible compounds. Woelen also mentioned decision trees, which is exactly what you need for this type of thing if you want to maximise
efficiency.
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sonogashira
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One couldn't be
satisfied that there is not a mixture without having tested for it. It's possible/probable that molecular geometry would allow for some sort of
separation/identification (perhaps using electrophoresis - though inorganic analysis is not my area).
Simple chemical tests are all very well, but the smart tortoise picks the right analytical method, and a universal detector, and waits for the
print-out whilst his colleagues are frantically trying to precipitate silver chloride!
[Edited on 24-5-2013 by sonogashira]
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adamsium
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Quote: Originally posted by sonogashira  | | One couldn't be
satisfied that there is not a mixture without having tested for it. It's possible/probable that molecular geometry would allow for some sort of
separation/identification (perhaps using electrophoresis - though inorganic analysis is not my area). |
No; the reason that it can be assumed that this is not a mixture is clearly that it's a laboratory exam. It's (extremely) highly unlikely that a
mixture would be given for this exercise. Also, chemico states in the initial post that they need to identify "an unknown compound". It's safe to
assume that, unless I am completely wrong and this isn't a university lab exam, it is quite reasonable to assume that it is not a mixture.
Edit to add: Talk of specialised tests and such is pointless in the context of an undergrad lab exam. It's all about 'simple chemical tests'. That's
the whole point.
[Edited on 24-5-2013 by adamsium]
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sonogashira
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You may be right.
(If you get cisplatin with an anomalous melting point then don't say you weren't warned!)
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phlogiston
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Another question is how much of the compound do you have to sacrifice to analysis?
If you have many grams, that greatly increases the variety of tests you can do. If you have a few mg only, many of the already proposed methods are
useless.
| Quote: | | We know it is an inorganic compound, for sure. That is pretty much it. We have a list of a few hundred compounds that are all potentially it and all
are inorganic, |
Please post that list.
BTW if this a school assignment feel free to ask for hints, but don't expect anyone to solve it. And ask the mods to move it to 'beginnings'.
[Edited on 24-5-2013 by phlogiston]
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chemico
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Thank you for all the replies and thoughts. The lab is NOT for using complicated/intricate tests like a spectrometer, or a comprehensive analytical
method are unfortunately not allowed.
The compound is going to be pure, barring a freak contamination (very unlikely and unintentional). Once we get the unknown the first time I'll report
back and let you guys know the basic prelim stuff and include a few pictures as well...the 5 senses (sans smell), silver nitrate test, perhaps a flame
test, ammonia reactions, and then everyone will have a better idea of what it can be.
At that point I will also type out the list, as after those tests are done we should be able to get it down to a reasonable amount of options.
Finally, we won't be starting this until Tuesday or Wednesday of next week.
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sonogashira
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If it is definitely one of a known-group of chemicals, then I would just do thin layer chromatography on all of them simultaneously (having found the
best solvent system for the compound that you have). You could eliminate some based upon the information that you have gained from finding the best
solvent system for your unknown (eg. if your unknown is non-polar, don't test any polar substances). You can narrow it down very easily, and it would
take perhaps 30 minutes to run an initial and confirmation chromatogram, using the fact that that they behave identically to confirm that they are
identical. (You could do 2D chromatography if you want to be extra sure). I think that this would be the fastest method by far.
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adamsium
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For these sorts of lab exams, you're generally not given samples of all the possible compounds, only the one they want you to identify. Even if
samples of all few hundred possible compounds were provided, trying to perform TLC on every one of them, including perfecting solvent systems for each
would take..... a very, very long time. It also probably (in fact, almost certainly) wouldn't be all that useful; Rf values are subject to variation,
even from plate to plate in the same solvent system (and even on the same plate) and are a poor way to attempt identification and
characterisation in most cases.
I think some people are missing the point. These lab exams are designed mostly to test a few skills: some (usually fairly simple) actual chemistry,
general lab technique, and reasoning / logic ability.
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sonogashira
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You wouldn't have to perfect the solvent systems for each substance(!), just for the ONE that you are trying to identify. There is nothing wrong with
the method, nor with the accuracy of the method. Comparative chromatography is the most accurate and specific test in the whole of analytical science.
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woelen
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Especially if the compounds on the list are salts (which is very likely if you are sure it is something inorganic) then having a list brings down the
number of options very much. You can separately do the analysis for cations and for anions.
The list may have a few hundreds of compounds, but the number of anions and the number of cations probably is in the order of 20 each. With 20 anions
and 20 cations in theory you can make 400 compounds, in practice probable a little less (some combinations of cations and anions may lead to unstable
salts, such as ammonium nitrite or copper(II) iodide). But by splitting analysis of anions and cations in separate problems you can tremendously
reduce your problem.
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adamsium
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| Quote: Originally posted by sonogashira | | You wouldn't have to perfect the solvent systems for each substance(!), just for the ONE that you are trying to identify. There is nothing wrong with
the method, nor with the accuracy of the method. Comparative chromatography is the most accurate and specific test in the whole of analytical science.
|
Fair point regarding perfection of the solvent system. However, even perfecting one solvent system can be very difficult and time consuming.
Also, TLC is only as useful as the resolution you get. It's just not a practical approach in this situation, even if they were to be provided with
samples of all possible compounds (which they almost certainly won't be). Let's say you run your sample and get an Rf value of 0.51. Then you run the
other (several hundred) known samples... when you finish a couple of days later (  ), how do you then decide if it's the one with an Rf of 0.50, or the one with an Rf of 0.49, or 0.52, or even 0.51? And, unless you use extremely
long plates, you may well end up with more than one compound with the same Rf value. It's just not practical in this scenario.
That said, I am interested in knowing more about TLC for inorganic substances. I've only personally used it for organic compounds.
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Fantasma4500
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i would say... first off colour.. then you can say alot of things its not
does it react giving off gas with acid? probably carbonate or something similar
is it soluble?
does it melt?
does it oxidize by any means (test with bad fuel then with really really good fuel.. magnesium super fine dust perhaps)
and can you make a compound with it that somehow have a specific colour? oxides are usually not looking very much alike each other, or well at least
many oxides are different.. perhaps turn it into metal with thermite reaction?
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blogfast25
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| Quote: Originally posted by chemico |
Please help me with this problem if you can. We have an unknown compound that we need to identify and a wide array of chemical and potential tests to
identify it with.
|
Problems where there is no a priori knowledge about the substance at all are very rare and extremely difficult to solve. You must
'know' something about the substance/compound before you even start testing, surely? Or did it come falling out of the blue sky onto your desk? 
[Edited on 24-5-2013 by blogfast25]
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binaryclock
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| Quote: | | Or did it come falling out of the blue sky onto your desk? |
They found it on a mirror in an abandoned hotel room.
[Edited on 24-5-2013 by binaryclock]
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phlogiston
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TLC for inorganic compounds would not be the first thing to try IMO.
1. note color
2. heat as dry powder. Observe carefully.
3. solubility in water (cold, then hot).
4. If insoluble: solubility in nitric acid.
5. If not soluble in nitric acid: test solubility in concentrated ammonia and sodium hydroxide solutions
6. If soluble in nitric acid: split the nitric acid solution and test for precipitations after adding solutions of:
sodium chloride
sodium carbonate
sodium sulphate
7. If you get a precipitate, heat to boiling, and allow to cool down.
8. In addition, if you got a precipitate upon adding chloride:
- allow to stand in light for a while
- add concentrated ammonia
Be careful when adding the bases to the nitric acid solution (add slowly and don't use highly concentrated solutions).
I think these tests should yield sufficient information to distinguish between many simple inorganic salts.
[Edited on 24-5-2013 by phlogiston]
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binaryclock
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| Quote: |
1. note color
3. heat as dry powder. Observe carefully.
4. solubility in water (cold, then hot).
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all good suggestions, but why is #2 so secret?
[Edited on 24-5-2013 by binaryclock]
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ScienceSquirrel
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#2 could a be a flame test.
This will reveal the presence of sodium, potassium, etc.
Or if it burns, goes black an organic material.
[Edited on 24-5-2013 by ScienceSquirrel]
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