Analysis of group 4 cations

Here is a resuming graph showing the relationship between pH and precipitation of sulfides (group 4). It is easily observable that the zinc sulfide precipitates way more easily than the other one who need a more alkaline pH, in particular manganese:

 

 

 

 

 

 

 

 

 

 

 

 

The precipitate of the sulphides of the fourth group is "washed" with deionized water and a few drops of Na2S.

Separation: zinc-manganese / nickel-cobalt

The separation of zinc and manganese from nickel and cobalt is based on the chemical phenomenon of the so called "hardening" or "age hardening" of a precipitate. The precipitates of nickel and cobalt sulfides become more dense and then more insoluble; we are now able to separate them!

At the beginning, the group 4 sulfides have more or less the same solubility, except manganese (check their Ksp above). However, over time (around ten minutes), the solubility of nickel and cobalt sulfides decreases. The age hardening of these sulfides can be justified either by a polymorphism (physical process) or by a redox reaction (chemical process).

For instance, it was believed that cobalt sulfide passed from the α form to an allotropic β form, less soluble.

CoS (α) Ksp = 10-22

Cos (β) Ksp = 10-27

It is nowadays considered more likely an oxidation reaction, from CoS to Co(OH)S.

On the basis of this change in solubility it is possible to solubilize zinc and manganese sulfides by mild acid treatment (HCl 2N).   After centrifugation and removal of the supernatant, the precipitate should show up dark (CoS and NiS)

Aging of the nickel and cobalt sulphides

 

 

The graph shows that age hardening makes CoS and NiS less soluble. To solubilize them we would need now a strongly acidic pH. That's why adding diluted HCl CoS and NiS are not solubilized.The result is then:

redissolution of the zinc and manganese sulphides

 

 

The sulfide ion is protonated to hydrogen sulfide, gas at room temperature. Since we have removed sulfide ions, ZnS and MnS, more soluble, are solubilized (don't forget the Ksp!), while CoS and NiS remain as precipitates.

The supernatant should be colorless.

Separation: zinc/manganese

 

 

 

 

 

 

 

We have a solution potentially containing Zn2+ and Mn2+We want to separate  them in order to set up specific identification essays for each one. First of all, the solution is heated to remove sulfide ions. The removal has to be complete;  later in the analysis we will alkalize the solution and an alkaline solution increases the concentration of S2- who might precipitate again the other (and now undesirable) sulfides. As usual, the removal of hydrogen sulfide (gas) is controlled with piece of paper soaked of lead acetate (CH3COO)2Pb.

Now we add 6N NaOH dropwise. The addition is deliberately gradual in order to observe the beginning of the precipitation. Zinc hydroxide should precipitate in white flakes. Manganous hydroxide, (already seen at group 3), is instead white-pinkish.

Then, exceeding with NaOH zinc hydroxide is solubilized as tetrahydroxyzincate ion (Zn2+ also is amphoteric) differently from manganese (not amphoteric) who will reamin as hydroxide. Alkalization is performed rigorously at low temperature. The tetrahydroxy zincate is a quite unstable complex and the heat can cause its composition to give again the hydroxide.

 

 

 

 

 

 

 

The manganous hydroxide is an highly unstable compound too. Manganese at this oxidation state is a strong reducing agent, and could then be easily oxidized by atmospheric oxygen. You could find out oxidation product such as Mn(OH)3 or MnO(OH), dark/black  corresponding to the oxidation state +3.

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