Here is the summary of reactions in which cadmium has been involved through the group 2 analysis:
Cadmium recognition when copper is absent
Copper is the main interferent cation in cadmium recognition assays. When is absent, it is much easier to deal with Cd2+.
- Treatment with sodium sulfide (Na2S)
As you can see from the scheme, cadmium should be now present as Cd(NH3)42+. To search for it, we must first eliminate the amino complexes and then add the precipitating agent. To remove cadmium from amino complexes is sufficient to acidify the solution: ammonia,NH3, moves to ammonium ion, NH4+. Doing so we can rely on the availability in solution of Cd2+ ions, and therefore precipitate cadmium as one of its unsoluble salts. Indeed, in this case, we're going to precipitate the unmistakable cadmium sulfide CdS.
From the practical point of view, the alkaline solution for NH3 is acidified with concentrated acetic acid CH3COOH up to neutral or even slightly acidic pH. Subsequently we add the precipitating reagent, Na2S, which releases quantitatively sulfide ions, S2-.
Cadmium recognition when copper is present
The identification is complicated somewhat when in solution we have both the amino complex of cadmium and that of copper. The copper interferes in any way with cadmium specific recognition assays, and indeed, usually generates much less soluble species (as the copper sulfide itself).
If we found cadmium in a solution that contains copper then, we must first get rid of copper. There are at least a couple of ways to do this:
1) Masking copper with KCN and precipitation of Cd2+ as CdS
Copper forms incredibly stable complexes with the cyanide ion. What we therefore do is adding to the KCN to the solution, without acidification (we avoid having to deal with the really unpleasant HCN).
Initially, the copper is reduced by the cyanide ion to copper cyanide (I). Afterwards Cu(I) forms complexes with 3 or 4 CN- groups.
2Cu2+ + 2CN- 2Cu+1 + (CN)2
Cu+1 + 4CN- CuCN43-
The complex between Cu+1 and CN- is incredibly stable, and has K equal to 10 -28.
In fact it forms a complex also identical for cadmium, but with a dissociation constant equal to 10-17 . While Cu2+ ions are sequestered so efficiently from the solution, a certain amount of Cd2+ ions which is enough to exceed the Kps value of CdS subsequently the addition of the precipitating reagent, that's once again sodium sulfide (Na2S).
The reaction is, therefore, once again:
2) Treatment with dithionite (S2O42-)
Another way to eliminate the interference of Cu2+ is to reduce it to copper metal, Cu°. To this aim it is usual used a strong reducing agent, sodium dithionite (or hydrosulphite), Na2S2O4. It is a very unstable and potentially dangerous substance, that easily decomposes and must be therefore handled with dry spatulas and away from heat sources like the flame of the Bunsen burner.
Copper (II) is reduced firstly to copper (I) and subsequently to metallic copper.
2Cu(NH3)42+ + S2O42- + 4OH- 2Cu+ + 2SO32- + 8NH3 + 2H2O
2Cu+ + S2O42- + 4OH- 2Cu° + SO32- + 2H2O
As you can see from the reaction dithionite carries out its reducing activity (gives electrons to other molecules) in a basic environment (4OH-).
From a practical point of view:
Our solution contains copper and cadmium as amino-complexes. When amino complexes of copper are present in the solution thiswill be colored in blue more or less small intestely depending on their concentration.
To the solution we add 1 spatula (few mg) of dithionite and we bring the test tube in boiling hot water (bain-marie) stirring with the glass rod. We should note two things: the discoloration of the solution because of the reduction of Cu2+ and the formation of a dark precipitate, which should consist of metallic copper and probably also other metals, such as Hg or Pb, also reduced by dithionite.. The precipitate is separated quickly from the supernatant and the operation is repeated on the latter. The process must be carried out as long as the residue forms.
Later we add the precipitating reagent, Na2S, in order to precipitate the cadmium sulphide, CdS.
The precipitate may be dirty (and thus not the usual canary yellow). If this happened we just need to wash with 3N sulfuric acid, which bring back in solution the cadmium sulfide but not the other sulfides adsorbed on the latter. After centrifugation and separation of the supernatant from the precipitate, we restore the neutral or slightly acid environment byaddition of ammonia and afterwards is added again a source of sulfide ions (Na2S).
The presence of copper could be known as early as we analize group 1. Indeed, Cu2+ can give variously colored solutions. Already since the hydrochloric attack, we could have noticed a greenish color due to chlorocuprates. In any case, its presence becomes obvious to the second group, after treatment with ammonia NH3. The amino complex of copper is indeed intensely colored in blue, unless the concentration of Cu2+ is not extremely low. This, already in itself, represents an assay that allows to identify copper.
Anyway, starting from the colored ammoniacal solution more or less intensely colored in blue, you can carry out specific recognition assays. Cadmium does not interfere.
You can perform a very simple confirmatory test:
- Treatment with K4Fe(CN)6
We treat the ammoniacal solution with few drops of concentrated acetic acid, to bring Cu2+ in solution by dissolving the amino complex. The pH iS around 4-5. We avoid more acidic pH values to avoid the decomposition of the ferrocyanide, which among other things would product hydrocyanic acid.
The precipitate has a very strong color, red-brick, which covers the light yellow precipitate (if there was any) of cadmium ferrocyanide.