| Pages:
1
2
3 |
a.dunn.wa
Harmless
Posts: 21
Registered: 31-5-2012
Member Is Offline
Mood: No Mood
|
|
Identifying Unknown Compounds in a Lab
Hello,
I am looking for assistance from you chemistry maestros, geniuses, and experts on how to handle a 5-day lab of identifying unknown compounds using
limited resources.
We were given four unknown compounds and a list of fifty possibilities. I am not as interested as in you guys helping me identify them (atleast not
yet), but more interested on how you guys would tackle this assignment--no procedure is given to us, we can do whatever we want given the resources we
are provided with.
Resources given are as follows:
Water
Ethanol
Red and blue litmus paper
0.1 M silver nitrate solution*
Bromothymol blue indicator
Phenolphtalein indicator
0.1 M NaOH solution
0.1 M K2SO4 solution*
phenol red indicator
0.1 M HCl
0.1 M H2SO4
And then stuff like hot plates, B.B.s, elec. conductivity meters, thermometers etc. are also provided.
So basically I have two questions:
1.) What would your rough procedure look like and why (what each test would accomplish)
and
2.) What do all of these given testing chemicals do? I know what some do but the ones with asterisks I put next to I am not really sure of, and I hope
you can help me out.
I appreciate your guy's help so much, and I hope you take the time to reply if you can help. It means a lot to me!
Andy
|
|
|
Vargouille
Hazard to Others
Posts: 380
Registered: 16-4-2012
Member Is Offline
Mood: No Mood
|
|
http://cms.imperial.edu/index.php?pid=1205
Check the solubility rules.
|
|
|
Sublimatus
Hazard to Others
Posts: 108
Registered: 8-6-2011
Member Is Offline
Mood: No Mood
|
|
This flowchart will take you pretty far:
http://i.imgur.com/HdQMN.png
Most of the instructions are at the bottom of the page. Just remember to use a new (small) portion of your unknown for each step in the flowchart.
Also, I meant to mention this page is from Introduction to Organic Laboratory Techniques A Microscale Approach, 4th Ed. by Pavia, Lampman,
Kriz, and Engel.
[Edited on 6/1/2012 by Sublimatus]
|
|
|
Mailinmypocket
International Hazard
Posts: 1351
Registered: 12-5-2011
Member Is Offline
Mood: No Mood
|
|
What are the first steps you are planning on trying or what have you already tried? Have you tried testing any of the mystery chemicals with the
silver nitrate and potassium sulfate to see what happens?
[Edited on 1-6-2012 by Mailinmypocket]
|
|
|
Mixell
Hazard to Others
Posts: 449
Registered: 27-12-2010
Member Is Offline
Mood: No Mood
|
|
Water and ethanol are mostly to separate chemicals by solubility.
litmus papers are for checking if the solution is acidic or basic.
Silver cations create insoluble silver halides of the formula AgX (X=Cl/Br/I).
bromothymol, phenolphtalein phenol red are indicators for acid/base when doing titrations (with HCl/H2SO4 or NaOH).
K2SO4 is an indicator to the presence of group 2 metal cations starting from Calcium, which create the insoluble salts of the formula MSO4 (M=
Ca/Sr/Ba).
thermometers are for checking the enthalpy of dissolution of various salts/melting and boiling points and so on.
conductivity meters are useful if you want to determinate if a solution is conductive, if it is, its probably a solution of salt.
Sorry for the lousy format, its 5 AM here, I couldn't bother...
[Edited on 1-6-2012 by Mixell]
|
|
|
a.dunn.wa
Harmless
Posts: 21
Registered: 31-5-2012
Member Is Offline
Mood: No Mood
|
|
UPDATE:
Found some interesting new info on one of the unknowns through a simple solubility in water test. This compound turned yellow when dissolved with
water, although it takes some agitating to get it to dissolve. Also, when initially poured in, some of the compound floated while some sank which gave
us the impression that it's density is very close to 1. If you think you know what this could mean, or if you want to take a stab at guessing the
compound, we would really appreciate your input!
Here's a picture and the list of possibilities:
POSSIBILITIES:
Aluminum hydroxide
Aluminum nitrate
Ammonium chloride
Barium sulfate
Calcium chloride
Calcium nitrate
Calcium sulfate
Iron II sulfate
Iron III sulfate
Lauric acid
Magnesium carbonate
Magnesium hydroxide
Magnesium sulfate
Potassium bromate
Potassium carbonate
Potassium chloride
Potassium iodide
Potassium metabisulfite
Potassium nitrate
Potassium perulfate
Potassium phosphate
Potassium sodium tartrate
Potassium thinocyanate
Sodium acetate
Sodium bisulfate
Sodium borate
Sodium bromide
Sodium carbonate
Sodium chloride
Sodium citrate
Sodium fluoride
Sodium iodide
Sodium nitrate
Sodium nitrite
Sodium oxalate
Sodium phosphate
Sodium sulfate
Sodium sulfite
Sodium thiosulfate
Starch
Stearic acid
Strontium chloride
Strontium hydroxide
Strontium nitrate
Sucrose
Tin II chloride
Urea
Zinc nitrate
Zinc sulfate
|
|
|
a.dunn.wa
Harmless
Posts: 21
Registered: 31-5-2012
Member Is Offline
Mood: No Mood
|
|
Quote: Originally posted by Mailinmypocket  | What are the first steps you are planning on trying or what have you already tried? Have you tried testing any of the mystery chemicals with the
silver nitrate and potassium sulfate to see what happens?
[Edited on 1-6-2012 by Mailinmypocket] |
Not yet we are planning on doing that Monday/Tuesday. Today (Friday) was our first day being able to play with the unknowns.
|
|
|
White Yeti
National Hazard
Posts: 816
Registered: 20-7-2011
Location: Asperger's spectrum
Member Is Offline
Mood: delocalized
|
|
Strange, they don't give you any sodium bicarbonate. Sodium bicarbonate can be useful in identifying certain substances.
"Ja, Kalzium, das ist alles!" -Otto Loewi
|
|
|
Amm
Harmless
Posts: 10
Registered: 1-6-2012
Member Is Offline
Mood: No Mood
|
|
Here is a picture of the compound that made the yellow solution before it was dissolved in water.
|
|
|
Amm
Harmless
Posts: 10
Registered: 1-6-2012
Member Is Offline
Mood: No Mood
|
|
Photo:
|
|
|
Vargouille
Hazard to Others
Posts: 380
Registered: 16-4-2012
Member Is Offline
Mood: No Mood
|
|
Just wiki all of the compounds for solubility and colors, both in solid and in solution.
|
|
|
barley81
Hazard to Others
Posts: 481
Registered: 9-5-2011
Member Is Offline
Mood: No Mood
|
|
You cannot measure the density that way. First, the solid may have floated due to surface tension. Second, air is incorporated into the powder, so the
density of the solid is not the same as the density of the solid.
This is kind of puzzling because none of the compounds would normally make a yellow solution. It could be anhydrous (or weakly hydrated) iron II
sulfate, which is white-ish, and forms a brownish/yellowish solution due to oxidation to iron III...
|
|
|
Mixell
Hazard to Others
Posts: 449
Registered: 27-12-2010
Member Is Offline
Mood: No Mood
|
|
I suggest ignoring the floating part if its a fine powder. Fine powders of materials that do not dissolve readily in water (require agitation at
minimum) tend to float because of surface tension of water. For example very fine nickel flakes float on the surface of water like a charm, I
literally couldn't get them to sink.
Now, all of the first and second (tin and zinc too) group metal cations are colorless in solution, so you can rule them out (the anions you specified
are also colorless).
Iron II is light green in color, and iron III is yellow (a good candidate).
All of the other chemicals either do not dissolve in water, or are colorless (not so sure about urea though, heard it can be yellowish).
Anyway, check the pH, if its acidic, its most definitely Iron III.
Too bad you don't have some barium nitrate to check for the sulfate ion to make a 100% determination.
[Edited on 2-6-2012 by Mixell]
|
|
|
Mixell
Hazard to Others
Posts: 449
Registered: 27-12-2010
Member Is Offline
Mood: No Mood
|
|
Barley,
I think quite a lot of oxidation is required for the color of the solution to shift to this shade of yellow, it would not happen immediately using
distilled water.
Also, according to a Wikipedia picture, Fe2(SO4)3 can have a pretty off-white color (especially if the photographing conditions are shite): http://en.wikipedia.org/wiki/File:S%C3%ADran_%C5%BEelezit%C3...
Anyway, make sure that the solution is fresh, the water is distilled and check the pH!
|
|
|
barley81
Hazard to Others
Posts: 481
Registered: 9-5-2011
Member Is Offline
Mood: No Mood
|
|
Iron II is oxidized fairly rapidly in distilled water (neutral solution) and dissolved oxygen can certainly produce a yellow color from iron III
(hydrolyzed species) in this solution. Plus, the solid may have oxidized in the air, producing more iron (III) compounds. You're right that it could
be ferric sulfate.
Both ferrous sulfate and ferric sulfates are not yellow! Does this mean that basic iron III compounds are present in the solid? I have some ferrous
sulfate heptahydrate that I left out for a short while, and it has become tinged yellow/brown even though some excess sulfuric acid was trapped in the
crystals.
|
|
|
Mixell
Hazard to Others
Posts: 449
Registered: 27-12-2010
Member Is Offline
Mood: No Mood
|
|
Define "fairy rapidly" please, because I've dissolved ferrous sulfate, and it didn't develop any noticeable yellow tinge (certainly not the one shown
in the picture) during the several minutes when the solution was not in use.
And according to Wikipedia and google pictures, ferric sulfate can be off-white to yellow.
May be the topic starter can clarify whether the yellow color appeared right away or after some time. And the exact color of the solid compound.
[Edited on 2-6-2012 by Mixell]
|
|
|
barley81
Hazard to Others
Posts: 481
Registered: 9-5-2011
Member Is Offline
Mood: No Mood
|
|
If you leave a sample of solid ferrous sulfate out, it becomes brown-colored in a while (hours/minutes ?).
| Quote: | | If for some reason you need ferrous sulfate with very low acid content, you'll have to take your chances with huge losses to oxidation by trying to
grow the crystals at a higher pH (near neutral). Such a solution can be made by adding slightly more steel wool to the acid than it can theoretically
dissolve. It may then become necessary to do the whole experiment under an inert atmosphere (nitrogen or argon). There will probably still be traces
of H2SO4 in the crystals despite this extra effort. |
Fairly rapidly = fast enough that crystal-growing becomes difficult due to formation of rusty-crap.
http://www.crscientific.com/ferroussulfate.html
[Edited on 2-6-2012 by barley81]
|
|
|
Mixell
Hazard to Others
Posts: 449
Registered: 27-12-2010
Member Is Offline
Mood: No Mood
|
|
I think hours is the more accurate time frame.
And I don't think he messed with the solution for hours before taking a photo (or dropped it into a hydrogen peroxide solution).
|
|
|
AJKOER
Radically Dubious
Posts: 3026
Registered: 7-5-2011
Member Is Offline
Mood: No Mood
|
|
First, closely examine the structure of the salts. Color, of course, and smell.
You may also be able to determine the precise solubility in water. Also, using H2SO4 (or NaOH ), precisely how basic (or acidic) the salt is using a
litmus test.
Heat the salt and record the temperature at which it starts to melt (or boil) and/or decompose. Capture some fumes and dissolve in water and note
smell. If carbon remains, you have an organic compound. You may be able to determining density of melted salt.
Heat with HCl, observe any gas colors and also carefully note smells again. For example, nitrates will produce fumes of Nitric acid, sulfites SO2,
acetates smell of vinegar, citrates smell of citric acid, bromides HBr (caution), iodides I2 vapors,...
If no reaction with HCl, do NOT react with H2SO4 as you may have NaF and heating with H2SO4 can produce deadly HF fumes.
Do a flame test to isolate Na, K, Zn, Fe, Ba, Mg, Sn and Sr.
[Edited on 2-6-2012 by AJKOER]
|
|
|
a.dunn.wa
Harmless
Posts: 21
Registered: 31-5-2012
Member Is Offline
Mood: No Mood
|
|
| Quote: Originally posted by Mixell | Define "fairy rapidly" please, because I've dissolved ferrous sulfate, and it didn't develop any noticeable yellow tinge (certainly not the one shown
in the picture) during the several minutes when the solution was not in use.
And according to Wikipedia and google pictures, ferric sulfate can be off-white to yellow.
May be the topic starter can clarify whether the yellow color appeared right away or after some time. And the exact color of the solid
compound.
[Edited on 2-6-2012 by Mixell] |
The yellow color did not appear immediately, it appeared after we agitated the solution for maybe 5-10 seconds. We ran the test 2x with this unknown,
due to it turning yellow we wanted to observe it again, and it does turn a shade of yellow once some of the compound starts to be dissolved; and once
it is all dissolved it reaches the shade photographed.
The solid compound's color is a yellowish-whitish color. We also noted that it''s initial appearance is granular. I will try to get a better picture
with decent quality on Monday of the compound again.
Thanks everyone for the input! It is really helpful
|
|
|
Mixell
Hazard to Others
Posts: 449
Registered: 27-12-2010
Member Is Offline
Mood: No Mood
|
|
Well here you go, yellow directly on dissolution, yellowish as a solid.
Just check the pH, and may be with barium nitrate to be 100% sure, and if the results are positive (acidic solution and sulfate present), it is
Fe2(SO4)3 (Fe III sulfate).
|
|
|
Amm
Harmless
Posts: 10
Registered: 1-6-2012
Member Is Offline
Mood: No Mood
|
|
Hi I'm the lab partner for this experiment, we don't have barium nitrate any other suggestions
|
|
|
Vargouille
Hazard to Others
Posts: 380
Registered: 16-4-2012
Member Is Offline
Mood: No Mood
|
|
Strontium nitrate also does it, but it's not quite as good, because SrSO4 is more soluble in water than BaSO4. Even worse is calcium chloride or
calcium nitrate, but if you really have no soluble strontium salt, it could work.
If you don't have any of those, there might be some way of identifying a sulfate that I'm forgetting.
|
|
|
Amm
Harmless
Posts: 10
Registered: 1-6-2012
Member Is Offline
Mood: No Mood
|
|
We have k2so4 and h2so4, I will post photos of the outcomes of mixing each thing with those when I have access to a computer
|
|
|
Hexavalent
International Hazard
Posts: 1564
Registered: 29-12-2011
Location: Wales, UK
Member Is Offline
Mood: Pericyclic
|
|
The guys doing the experiment . . .do you have to carry out your work in the fumehood?
"Success is going from failure to failure without loss of enthusiasm." Winston Churchill
|
|
|
| Pages:
1
2
3 |
![[*]](images/xpblue/default_icon.gif){kind=icon}
