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Topic: Confirming Cations of a salt  (Read 1589 times)

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Offline xinch3

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Confirming Cations of a salt
« on: March 21, 2013, 04:29:28 PM »
I am in Gen Chem doing qualitative analysis for soluble salts.
Green Salt: XCl2
Blue Salt: XSO4

I know my anions are correct. However, I am unsure about my cations.
What can I add that will give a distinct color to confirm whether what cations I have?

I have deduced that I may have the following cations for my anions:
Green Cl: Ni, Cr, Cu, Fe
Blue SO4: Fe, Cu, Co, Cr

I have HCl, NaOH, NH3, BaCl2, H2S, HNO3, H2SO4, AgNO3, KOH, H2O, (NH4)2C2O4, DMG, H2OH that I can add to a solution of my salt dissolved in water.


Thank you!

Offline Borek

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Re: Confirming Cations of a salt
« Reply #1 on: March 21, 2013, 06:45:52 PM »
You have to show your attempts at solving the question to receive help. This is a forum policy.
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Offline xinch3

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Re: Confirming Cations of a salt
« Reply #2 on: March 21, 2013, 10:57:51 PM »
Oh my bad!
I used NH3 first to see if it formed an ammonia complex for copper.
It did indeed turn dark blue (looks just like the copper ammonia complex I've seen from identification practices)

However, I do not know if other cations form a dark blue (copper-like) complexes.
If none of the other possible cations produce a dark blue complex then I'm okay.

Otherwise, I haven't tried anything else yet.

I was just confused because CuCl2, I thought would have been a blue salt instead of a green salt. Thank you!

Offline 9-92-6-19

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Re: Confirming Cations of a salt
« Reply #3 on: March 21, 2013, 11:36:57 PM »
Color (when regarding ionic crystals) is a result of hydration; CuCl2·2H2O is a green solid, while CuCl2 anhydrous is a brown substance. Similarly, CuSO4·5H2O is a bright blue crystal, while CuSO4 anhydrous is a very light brown/white.

Furthermore, if CuCl2 were to be dissolved in different concentrations of Cl- (such as from HCl) then there will be different colored solutions, which is a direct result to how much Cl- is in excess. This can be represented by the following reactions:

CuCl2 + Cl-  ::equil:: CuCl3-

CuCl2 + 2Cl- ::equil:: CuCl42-

In the latter reaction Cl- is in a much larger excess compared to the former reaction; hence, the color difference that is caused by the different concentrations.
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