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Author: Subject: Identifying an unknown
phlogiston
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[*] posted on 24-5-2013 at 07:55


Why do you get the impression there is anything secret about it?

I said observe carefully because there are many things that can happen, some of them subtle but informative.
Perhaps a little condensation in the top of a test tube, indicating release of water.
Or a color change from white to faint yellow (eg. zinc oxide).

Things like that.




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Eddygp
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[*] posted on 24-5-2013 at 08:09


Or a dehydration, like CuSO4·5H2O to CuSO4... for example.



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sonogashira
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[*] posted on 24-5-2013 at 09:30


On second thoughts, the fastest and easiest thing to do would be to take a melting point (or decomposition point) and compare it with known values. That should narrow your "hundreds" of choices down to about 2 or 3. Then get your paper chromatography going!

[Edited on 24-5-2013 by sonogashira]
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chemico
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[*] posted on 24-5-2013 at 09:34


To clarify some points of confusion...

1. This is a chemistry lab for a class. The compound will be pure and is not some random chemical we just stumbled upon.
2. This is about the most efficient simple tests. Which order would most quickly determine the compound's identity. This is a great time for a discussion of the most valuable chemicals (AgNO3, sulfuric acid, ammonia, etc.)
3. We will have a small container full of solution w/ no refills. The bottle size is 3 oz and it is approx 1/2 way full.
4. We have not received the chemical yet! We will get it on Wednesday of next week.
5. We only have access to our unknown chemical not all of them.
6. I am not sure if heating to melting point is safest, fastest, or most accurate route to go. It would take a fair amount of time to heat to melting point and how would you get the most accurate temperature reading?

If anyone has any more questions feel free to ask. Again we won't get the compound until next Wednesday so we won't be able to provide specific information about it until then.

Any comprehensive flow charts for id'ing? Or anyone want to rank the best compounds to use to identify an unknown (eg. sulfuric, ammonia, silver nitrate)
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sonogashira
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[*] posted on 24-5-2013 at 09:37


Quote: Originally posted by chemico  

6. I am not sure if heating to melting point is safest, fastest, or most accurate route to go.
I assure you that it is. Don't you have access to a melting point apparatus?
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[*] posted on 24-5-2013 at 10:18


As far as didactical tools go this is pretty poor, IMHO. Unless you're not presenting it as it will be presented to you... I'd really like to know at what level of chemical education some tw*t has introduced this.

[Edited on 24-5-2013 by blogfast25]




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Eddygp
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[*] posted on 24-5-2013 at 10:38


Quote: Originally posted by blogfast25  
As far as didactical tools go this is pretty poor, IMHO. Unless you're not presenting it as it will be presented to you... I'd really like to know at what level of chemical education some tw*t has introduced this.

[Edited on 24-5-2013 by blogfast25]


Basically, I was going to say that.

[Edited on 24-5-2013 by Eddygp]




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amazingchemistry
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[*] posted on 24-5-2013 at 19:57


Normally, when identifying an unknown we look for specificity. In this case though, your best bet is to go for the most general tests possible as your first step. I echo the comments above about color. React a small amount of it with an acid. Does it fizzle? its probably a carbonate. Take advantage of solution chemistry. Nearly everything dissolves in nitric acid. If it doesn't then you have narrowed down your list quite a bit. If it does, then you start testing solubilities in other compounds. If after, going through your decision tree, your don't get a precipitate, try a flame test. Your compound probably contains sodium, potassium or ammonium ions. Always pay attention to your senses. Your eyes are the most obvious, but if on heating your solution starts smelling like cat pee, then you very probably have ammonium ion. Vogels book has already been mentioned as a reference, you should also have a spot plate handy and read Feigl's "Spot Tests in Inorganic Analysis." Good luck and keep us updated. I love these kinds of problems :)
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[*] posted on 25-5-2013 at 06:51


I'm just going to leave this here.... http://firstyear.chem.usyd.edu.au/SummerSchool/LabManual/E05...
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[*] posted on 2-6-2013 at 08:37


Well we finished day one. Here are our findings and early thoughts. Feel free to chime in with recommended tests, questions, etc.

DAY 1 OBSERVATIONS
--White powder-like compound. Odorless or extremely faint odor.

-- Attempt to dissolve in water was a little bizarre. Not very soluble in water. Appeared like a little dissolved but most just stayed at bottom, water got cloudy color.

--Flame test orange

-- COMPOUND + HNO3 --> no reaction (rules out carbonates)

-- COMPOUND + AgNO3 --> cloudy weak/light ppt

-- COMPOUND + H2SO4 + MnO2 --> exothermic rxn but stayed black color of manganese dioxide.
** on heating, colorless, odorless gas emitted
** bleached/turned red the blue litmus paper
** suspected chloride

-- COMPOUND + HNO3 + HCl --> no ppt BUT addition of HCl cleared up solution's foggy color before (does this support our compound being a chloride?)


That is all we were able to do in ~20 minutes we had with it. We are planning on doing the following tests next lab day (Tues.):

  • hydrogen sulfide test to test for cation (acidic hydrogen sulfide can ID Cd/Sn/Cu)
  • verify chloride presence w/ compound + managanese dioxide +sulfuric (then shake/stir), followed by some hexane (should produce pure halogens, wafting should give bleachy/chloride smell w/ exo rxn)


And from our list of 300+ compounds here are a list of plausible ones:


  • aluminum hydroxide monohydrate
  • aluminum phosphate
  • antimony sulfate
  • barium bromide
  • barium chromate
  • barium phosphate
  • barium sulfate
  • barium sulfite
  • cadmium hydroxide
  • cadmium phosphate
  • calcium carbonate
  • calcium sulfite dihydrate
  • chromium (iii) bromide
  • chromium (iii) chloride
  • cobalt (ii) hydroxide
  • cobalt (ii) phosphate
  • copper (ii) carbonate
  • copper (i) carbonate
  • copper (i) iodide
  • copper (i) sulfite monohydrate
  • iron (ii) hydroxide
  • lead (ii) bromide
  • lead (ii) carbonate --- unlikely
  • lead (ii) chloride
  • lead (ii) chromate
  • lead (ii) hydroxide
  • lead (ii) iodide
  • lead (ii) nitrate
  • lead (ii) phosphate
  • lead (ii) sulfate
  • lead (ii) sulfite
  • lithium phosphate
  • magnesium phosphate
  • magnesium phosphate tetrahydrate
  • manganese (ii) hydroxide
  • manganese (ii) sulfide
  • mercury (i) bromide
  • mercury (ii) bromide
  • mercury (i) chloride
  • mercury (ii) chloride
  • mercury (ii) iodide
  • strontium hydroxide
  • zinc sulfide



Also, it's important to note that there are a lot of other plausible compounds. This is just a sampling to show you guys what we are dealing with.

Right now we have a hunch it may be mercury (i) chloride but it's just a hunch..

[Edited on 2-6-2013 by chemico]
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[*] posted on 2-6-2013 at 09:35


Chemico, firstly, why do you rule out sodium, potassium and rubidium?? Secondly, it should contain chloride or bromide ions (silver precipitate?) and a less electronegative cation than Ag.



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[*] posted on 2-6-2013 at 10:20


Eddygp, wouldn't virtually all sodium compounds readily dissolve in water? This was very slightly soluble in water. If anyone knows of any sodium compounds that don't really dissolve in water, I would be welcome to test further for sodium.

I agree with the second part. I'll take a second look at the list and refine at a later point.
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[*] posted on 2-6-2013 at 10:29


Potassium and sodium should be ruled out at the end of the decision tree, as nearly all sodium compounds are soluble in aqueous sol. Excuse for the seemingly obvious question, but when you said, "no reaction" with HNO3, did you mean "no bubbling, but dissolution" or "inertness"? I'm inclined to think of the second one, as I'd be surprised if your compound was unreactive toward HNO3
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[*] posted on 2-6-2013 at 10:32


Amazingchemistry, you are correct. By no reaction I meant no bubbling but dissolution.
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[*] posted on 2-6-2013 at 11:42


I'm sorry chemico. I forgot that it was insoluble!



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[*] posted on 2-6-2013 at 12:52


No problem, do you have any recommended tests we should perform? We really need a way to test for mercury or a test for lead.
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[*] posted on 2-6-2013 at 13:48


Remember your flame test results: lead is reported to give a blue/white flame. You can try to reduce a mercury salt, and differentiate between similarly colored metals using your flame test results. You can also rule out sulfites, since you didn't note a sulfurous odor. The carbonates are gone too, not just unlikely, since it didn't off-gas with nitric acid. Many transition metal salts can be ruled out as well: a white salt is unlikely to be an iron, cobalt, or copper salt. You should really look through your salts and cross the colored ones off the list, as well.
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[*] posted on 2-6-2013 at 18:03


It doesnt seem to be a carbonate. If im not mistaken, treating with HCl should tell you if you have a Pb compound. Flame tests are your surest bets for sodium, unless you have access to zinc uranyde acetate :P (uranium sodium salts are insoluble)

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[*] posted on 2-6-2013 at 19:59


What about sodium rhodizonate which tests for some heavy metals, IIRC.



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[*] posted on 2-6-2013 at 22:42


Any decent text will tell you that if a precipitate is present when treating a solution containing your ions of interest (say, your nitric acid + compound solution) with HCl it can either be PbCl, Hg2Cl2, or AgCl. Pb is sufficiently soluble in hot water to test this wash by adding chromate ions (or some other ions whose Pb salts have a smaller water solubility than Cl) to get a precipitate. Hg2Cl2 should precipitate with ammonia. AgCl should dissolve and re-precipitate when acidifying. Given that this is standard procedure (that some cursory amount of reading would have uncovered) I'm starting to feel like we are just giving you the answers to what is supposed to be your puzzle. I apologize if this is a mistaken impression.
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[*] posted on 2-6-2013 at 23:01


Quote: Originally posted by amazingchemistry  
Any decent text will tell you that if a precipitate is present when treating a solution containing your ions of interest (say, your nitric acid + compound solution) with HCl it can either be PbCl, Hg2Cl2, or AgCl.

CuCl would get dismuted on solution, and so would AuCl. But how about TlCl?
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[*] posted on 3-6-2013 at 03:38


Quote: Originally posted by chornedsnorkack  
CuCl would get dismuted on solution, [...]


If you meant 'disproportionate' then no: in the absence of oxidisers CuCl is quite stable. It's also insoluble.

[Edited on 3-6-2013 by blogfast25]




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phlogiston
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[*] posted on 3-6-2013 at 05:22


You are not using all the information you already have.
On the basis of color alone you can already exclude all the chromium, copper, iron, cobalt compounds that you have listed and then some (lead (ii) iodide, mercury(ii)iodide for instance which are well known to be brightly colored), with the possible exception of copper(i)iodide which appears to be a bit off-white.

Did I understand correctly you observed complete dissolution upon adding HNO3?
An important hint! Oxides and hydroxides are likely candidates then.
The solubility of chlorides doesn't normally change upon addition of HNO3.

Also upon addition of HCl you would expect chlorides to become less soluble, not more. (look up "common-ion effect"), so again, your observations argue against a chloride.

Was there any noticeable heat effect upon mixing with water?
Solubility in hot water?
Effect of dry heating of the compound?
Given the flame test, perhaps you should include a few more calcium compounds on your list. So far, your observations are consistent with calcium hydroxide for instance, which isn't on your list.
You should repeat your flame test in the presence of chloride (dip wire in HCl, then in powder and heat in colorless flame). This will give you the well known green for barium/red for strontium/blue for copper if they are present.

So, there are a lot of simple things you can do to narrow down the list before resorting to very specific and complex tests for lead/mercury/chloride etc.





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[*] posted on 3-6-2013 at 05:49


I wouldn't get too carried away with looking for mercury and cadmium unless you really start ruling out pretty much everything else (in a methodical manner). The likelihood of them giving salts of mercury, cadmium, etc to undergrads (especially without telling you what it is... but even just at all) these days is probably infinitesimally minute. They used to just about throw that stuff around like table salt, but it's a pretty big 'no go' to use those things now, both for student safety and environmental reasons. I'm not saying that you should just totally assume it can't be that (although, really, part of doing science is using common sense, and this is common sense). Just know that the chances of it being anything like that are minimal, at most. Of course, I could be entirely wrong and it could be a mercury salt, but I'd be very, very surprised.

An orange flame test would perhaps indicate something like Ca2+. You also said that when you did the halide test, a gas was evolved, but it was odourless. Chlorine would certainly not be odourless. Don't discount the fact that there could just be halide contamination somewhere giving a slight cloudy precipitate.

"addition of HCl cleared up solution's foggy color before (does this support our compound being a chloride?)" No. I'd expect most chloride salts to be precipitated by addition of HCl, not dissolved, unless a complex is forming (which is unlikely, as you don't seem to have a d-block salt). Consider Le Chatelier's principle and, specifically, the common ion effect.

I'll leave it at that for now.

Edit: I started this and left it for some time. Looks like phlogiston was typing up much of what I was saying in the meantime :) Also, phlogiston, I was considering Ca(OH)2 as a very strong contender based on what has been presented, but stopped short of saying so... so I definitely agree with your assessment.

Honestly, forget about mercury (and anything else remotely nasty) for now. All that stuff is there simply to 'pad out' the list.


[Edited on 3-6-2013 by adamsium]
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[*] posted on 3-6-2013 at 06:29


That's funny adamsium, we got nearly exactly the same points. We clearly think alike.



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