Qualitative Analysis

Cyrus

Hello, I have to perform a qualitative analysis on an unkown salt for lab as a test. The problem is that one of the test viles will have two salts mixed together. I want to be clever and find a way to separate them due to their differing density and then test each one indivdually. The problem is that they are all soluable, so the intial thought I had about separating them in a liquid that they are both not soluable in would not work. Too many unknown salts to choose a viable liquid for separation, some might dissolve. Any suggestions on separation? or am i going to have to test both of them mixed together.

Do you think I could centrifuge the two salt + water mixture to separate them out by density? If they are nearly equal, this might now work though. I might give it a shot though. Oh whoops, they will both be clear liquids, I dont see how this will help either.

poolwin2001

Hi
Why should you separate the two salts?In qualitative analysis,the idea is to find cations & anions.This is done by pption ,colour change or evolution of some gas by a reaction.(so changes can be noticed)
During cation analysis,You can ppt out one cation completely(almost)then you have only one cation and you can do the test for that one.You neednt use any physical methods to separate the salts.ALso some cations like Ammonium doesnt interfere with the tests of other cations.

chem_tr

I agree with poolwin; just learn analytical chemistry... It's fun !

poolwin2001

I think ,when you have a mixture,you will have problems with flame test and ash test if you dont ppt out a cation first.
So you cant find a cation the easy way.You have to do Group analysis.
Yes,_tr analytical chem is fun

Cyrus

But if I have two salts arent they two anions that I am interested in? How would I ppt them out? Could you explain how that is done?

chem_tr

You may try evaporating the solution to dryness, and work with the precipitates with the classical flame test, especially. You may also try fractional crystallization, but it is not easy as it sounds. If you are very careful, you might notice the point where crystallization of only one substance starts, and wait for the entire compound ppts out. Filter it, and evaporate the filtrate to dryness, it's over. They'll be individual compounds then.

Cyrus

But then I will have filtered out both salts wont I? Plus, I am already given the two in powder form mixed together. So what would be the point of filtering them to obtain what I already have?

chem_tr

Oops, I have not understood that you are given the solid mixture (as well as the binary solution you're preparing). Sorry.

Then it will be relatively easy, just try to find the cations with standard procedures, i.e., flame testing for alkali metals, etc. If you need a special detecting technique, just let me know. I can help you about the reaction.

poolwin2001

What colour do you get in a flame if cations where Ba,Cu????
or ash test when Zn and Al???
I think straight forward group analysis will do the job.
regards

chem_tr

Yes, but cyrusabdollahi doesn't seem very eager to follow the standard group analysis.

I remember that barium gives pale greenish flame on platinum wire. Copper should be identified instead by classical ammonia test: a deep blue solution with concentrated ammonia suggest that your sample likely contains copper ions.

Zinc and alumina have their selective and specific identification methods; both are identified as cobalt oxides. First, prepare cobalt(II) oxide on a filter paper soaked with a solution of cobalt(II) ions, by heating it. Then, add some of your sample and go on heating. After several minutes, you'll get Rinmann Green for Zinc (CoZnO₂), and Thénard Blue for Aluminum (CoAlO₂).

poolwin2001

Thanks.
I know this,I was asking what will we get if we did flame test on a salt which has two cations which gives colour to flame like Ba,Ca(sorry not Cu)
or ash test on a sample containing both Al and Zn.Whether we could make out anything.
Thanks anyway.

chem_tr

Well, Ba and Ca may even be detected in a binary mixture, depending on the volatileness rate, increasing with increased molecular mass. I am trying to say that barium salts will give green color in the last stage; firstly it is possible to see calcium's sparkling brick-colored response.