Zinc is sought to fourth analytic group as Zn 2+. It is precipitated as sulphide insoluble at pH 9.8 for ammonium / ammonia buffer. The precipitating reagent is sodium sulfide, Na 2 S (preferable to ammonium sulfide and TAA)
The zinc sulphide, together with the manganese sulfide, is separated from sulfides of cobalt and nickel; these latter undergo a chemical-physical process known as "aging", ie are transformed into densified forms much less soluble.
The separation is made by addition of hydrochloric acid (HCl) 2N. The sulphides of zinc and manganese, more soluble, is risolubilizzano according to the following mechanism:
The sulfide ion is shifted to hydrogen sulfide from HCl. Given that decreases the concentration of S 2-, which appears in Kps, (Kps = [Zn 2+] [S 2-]) the equilibrium shifts toward the dissolution of the salt. In this way it is also solubilized the manganese sulfide. The aged sulfides of cobalt and nickel, if present, are so poorly soluble that the treatment with HCl 2N leaves them as precipitates.
We now have Zn 2+ in solution with, if present, Mn 2+. The separation of these two cations is done by treatment with 6N NaOH (we previously eliminate the sulfide ion for heating, controlling soaked with map of lead acetate).
Zinc initially precipitates as hydroxide:
Zn 2+ + 2OH - Zn (OH) 2 white precipitate
but in excess base is risolubilizza as tetrahydroxy galvanized:
Zn (OH) 2 + 2OH - Zn (OH) 4 2-
The manganese instead, if present, remains as a precipitate of manganous hydroxide, Mn (OH) 2, white-rosino, not presenting the anfoterismo that is precisely zinc.
The treatment with soda must be carried out cold. The galvanized is very unstable, and the heat subsides balance to give the hydroxide.
In the solution thus obtained, it is made of the specific recognition assays for zinc.
-precipitazione as hydroxide Zn (OH) 2
We brought in the zinc solution by this reaction:
Zn 2+ + 2OH - Zn (OH) 2 Zn (OH) 2 + 2OH - Zn (OH) 4 2-
Now, starting from galvanized, we aim to make reprecipitated zinc hydroxide. The essay is simply done by adding H 2 O deionized drip and bringing in a water bath. The heating causes the hydrolysis, as we mentioned before, the zinc hydroxide to give back. Dilution, for the principle of Le Chatelier, also pushes the balance to the left to give the zinc hydroxide.
It may be easier to understand the dilution effect by writing the equlibrio as:
Zn (OH) 4 2- ZnO 2 2- + 2H 2 O → Zn (OH) 2 + 2OH -
Dilute equivalent to adding water, that being one of the two reagents, leads to the equilibrium shift to the right.
If necessary, you can help the reaction by addition of a few drops of hydrochloric acid diluted. Obviously, you should not get too acidic pH.
-precipitazione as sulfide ZnS
The solution is acidified by addition of diluted HCl. It is restored Zn 2+ in solution. We can not use the so obtained solution to precipitate the sulphide. Indeed, at acidic pH the concentration of sulphide is too low to allow the precipitation of zinc sulfide. It is added concentrated NH 4 OH to obtain an alkaline environment in which zinc is present in the form of amino complex. Subsequently it adds the source of sulfide ions, and then Na 2 S, which now may precipitate, if present, zinc sulfide, black.
Zn (OH) 4 2- + 4H + Zn 2+ + 4H 2 O
Zn 2+ + 3 xNH Zn (NH 3) x 2+
Zn (NH 3) x 2+ + S 2- ZnS ↓ + 3 xNH
You might ask a question. Having started from a basic solution for soda, how come we have acidified and returned then again at alkaline pH (ammonia)? We could not acidify just enough to get the pH 8-9 to precipitate sulphide which we desire?
Well, there are various indications. First of all, this type of operation, would turn this essay recognition in what we have seen before. For slight acidification would precipitate zinc hydroxide. In addition, rather than recede from pH 13-14 to pH 8-9 is much less difficult to acidify advance and then get to the pH of the ammonium-ammonia buffer. By treating with ammonia addition, we have zinc in the form of amino complexes, and so we are sure that no precipitate zinc hydroxide.
-Precipitazione As zinc ferrocyanide
The solution was added a few drops of potassium ferrocyanide, K 4 Fe (CN) 6. If this Zn is obtained a celestial white precipitate of zinc ferrocyanide, Zn 2 Fe (CN) 6. If the precipitate is not formed when trying to acidify with CH 3 COOH, so as to ensure a higher concentration of Zn 2+. Moreover it avoids the decomposition of ferrocyanide, unstable in alkaline environment.
It can not be easy to appreciate the color in solution, and therefore more accurately you can wash and centrifuge the precipitate.
An excess of potassium ferrocyanide may precipitate mixed ferrocyanide of zinc and potassium, white.
- Essay with dithizone (difeniltiocarbazone)
The presence of zinc can be detected with a colorimetric method via the dithizone. The compound is presented as a green solid. The zinc-dithizone complex is colored red; the coloring is instead yellow-orange in the absence of copper.
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