# What's in your witches' brew? Qualitative analysis



## Lou (Feb 16, 2009)

It sometimes becomes necessary to know what and which base metals are present in a sample. The following information will be helpful for determining what's in what and perhaps what you can do about it.

This is particularly useful for determining what you're sending out for waste disposal.

A useful thing to remember is that the more noble metals will more gladly be reduced.

This is by no means complete, merely how I would go about it. Bear in mind that this is a rough draft done from memory. I'll check it over soon enough.

Reagents and equipment needed:
Concentrated HCl
Concentrated HNO3
A sulfide source/Kipp apparatus. Thioacetamide is preferable.
Potassium thiocyanate
Ethyl alcohol
Dimethyl glyoxime
Distilled water
Sand or water bath or Bunsen burner (least preferable)


Now onto it:

First, take a sample (this is called an _aliquot_) of your unknown and make it acidic with hydrochloric acid (pH 0.5).

If there is a white precipitate, then Ag, Pb, and possibly Hg may be present. Filter this precipitate out, rinsing with distilled water.


Analysis for Ag, Pb, Hg:

This is occurring in solution:
Ag+ + Cl- --> AgCl
Pb+2 + 2Cl- --->PbCl2
Hg2+2 + 2Cl- ---> Hg2Cl2

Not all of the mercury may be in its diatomic divalent state, which means not all will precipitate as calomel. There are ways to remedy this, but most of them will conflict with the other tests.

Lead:
Boil the precipitate. This removes lead (II) chloride as it is markedly soluble in hot distilled water. Filter the residuals and save precipitate for further analysis. The filtrate contains lead. To confirm, add a few drops of concentrated acetic acid followed by crystals of potassium iodide. A yellow precipitate of lead (II) iodide will form. Alternatively, one may add K2CrO4 in lieu of KI.

Silver: 
The precipitate contains silver and mercurous chlorides. Shake the precipitate with concentrated ammonia. Any blue at this stage is strongly indicative of copper contamination (formation of tetrammine copper complex; we'll come back to this very sensitive test). Any silver chloride will dissolve forming the diammine silver (I) complex. Filter the residual precipitate (if existing); this is likely calomel. The ammoniacal filtrate can be acidified with HCl or HNO3 -- if a white precipitate forms, silver is confirmed. 

Mercury:

Dissolve precipitate in warmed 1:1 6M nitric/hydrochloric acid. If it dissolves to give a clear solution, add a few drops of 1M stannous chloride solution. A grayish suspension confirms mercury.

Return now to the filtrate that was saved from filtering all of the insoluble chlorides. What is left in solution will possibly contain Cr(III), Ni(III), Co(III), Fe(II/III), Cu(II), Sb(III), Sn(II/IV), Bi(III), Al (III), Zn (II).

WARNING:_ From here on out, the analysis should be conducted in a draft or out of doors away from neighbors, as H2S and H2 gas are produced. H2S is extremely toxic, flammable like hydrogen, and it reeks. If one finds him/herself not smelling H2S, be concerned--this substance will deaden one's sense of smell as concentration increases, compounding the danger._

One can generate the weak acid H2S in many different ways. Often it is most convenient to use a reagent called thioacetamide as the sulfide source. It is cheap and simple to use and the risks are minimal. Its hydrolysis in acidic (or basic) medium form hydrogen sulfide. Hydrogen sulfide is a popular precipitate for transition metal cation analysis because many metals form insoluble sulfides that are easily separated and characterized. 

If 20 mL of solution were sampled, it should be concentrated with a few mL of 3% hydrogen peroxide (oxidizes all species) down to half its original volume. Check the pH of solution to be tested to ensure that it is ~0.5 pH. Place this solution into a test tube and add 5mL of 1M thioacetamide solution. Boil for 10 minutes. Filter, saving both the solution and the precipitate. Add another mL of thioacetamide to the solution and boil once more, any further precipitate indicates incomplete precipitation. The precipitate contains the sulfides off all the aforementioned metals except aluminum, which is either present as its hydrous oxide or still remaining in its trivalent state in solution.


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## Noxx (Feb 16, 2009)

Nice so far


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## Lou (Feb 16, 2009)

Yeah, I'm getting tired and I have to check my blogs that I write for...


check it out Noxx, http://www.chemistry-blog.com


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## Noxx (Feb 22, 2009)

Nice I like that one.

Thanks for the link.

BTW, are you a fan for Breaking Bad ?
I am since a week  and I can't wait for season two.


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## Lou (Feb 23, 2009)

I have never seen it. I have just heard of it.


Lou


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## LittleJohnnyH2SO4 (May 25, 2009)

Lou said:


> Silver:
> The precipitate contains silver and mercurous chlorides. Shake the precipitate with concentrated ammonia. Any blue at this stage is strongly indicative of copper contamination (formation of tetrammine copper complex; we'll come back to this very sensitive test). Any silver chloride will dissolve forming the diammine silver (I) complex. Filter the residual precipitate (if existing); this is likely calomel. The ammoniacal filtrate can be acidified with HCl or HNO3 -- if a white precipitate forms, silver is confirmed.



What of the textbook disproportionation of calomel in ammonia solution?

Hg2Cl2 + 2NH3 → Hg + Hg(NH2)Cl + NH4Cl

The solution will turn gray or black from a suspension of elemental mercury. Perhaps a diferent complexing agent would be more appropriate, or the use of chlorine gas to convert any mercurous chloride to mercuric. AgCl would remain undissolved. The supernatant (after boiling to expel excess chlorine) could then be tested with stannous chloride.


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## Lou (May 26, 2009)

By the way, I don't know if you've tried the textbook reaction, but it doesn't occur sufficiently quick to pose a problem for qualitative purposes.


Nice resuscitation. To be honest, I was just going to delete this as I didn't feel it would be worth going through an exhaustive list of qualitative tests, many of which won't work unless some particular cations are removed. I think it would be beyond most people's patience, and perhaps the capabilities of a few (at least with regard to reagents).


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## johnon (Apr 4, 2010)

If you can get it, try sodium sulfide aq.soln (aqueous solution). Ad dropwise as the reaction can be vigorous.


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