Sulfides insoluble in basic solution

Sulfide precipitates are generally grouped as sulfides insoluble in acidic solution and sulfides insoluble in basic solution. Explain why there is a difference between the two groups of sulfide precipitates. 

Zinc is one of the most mobile of the heavy metals. The zinc compounds formed with the common anions found in surface waters are soluble in neutral and acidic conditions. In reducing environments, zinc sulfide (ZnS) is a relatively insoluble and stable compound, which may oxidize in the presence of dissolved oxygen. Zinc carbonate (ZnCOj) is assumed to be less stable than zinc sulfide, though still relatively insoluble. Zinc ions are dominant up to pH values of about 9 in simple aqueous systems. In basic solutions, zinc hydroxide (Zn(OH)2) precipitates if the concentration of zinc is 10.4 M. Zinc hydroxide shows minimal solubility at pH 9.5 and dissolves at higher pH values as the zincate anion, Zn(OH) . 

Group 3. Base-insoluble sulfides and hydroxides After the solution is filtered to remove any acid-insoluble sulfides, it is made slightly basic, and (NH4)2S is added. In basic solutions the concentration of is higher than in acidic solutions. Thus, the ion products for many of the more soluble sulfides are made to exceed their K p values and precipitation occurs. The metal ions precipitated at this stage are Al +, Cr Fe +, Zn +, Ni +, Co +, and Mn + (The Al +, and Cr ions do not form insoluble sulfides instead they precipitate as insoluble hydroxides, as Figure 17.23 shows.)... 

The last reaction is slow in neutral, fast in basic solutions. The daik-biown Pb02, on a basic-lead-acetate paper, is a good test for O3. Many other insoluble sulfides—e.g., of Cu, Zn, Cd and Sb— behave like PbS. 

Like acetals, thioacetals are stable in basic solution. However, thioacetals also survive under the acidic conditions that would hydrolyTe an acetal. Thus, they protect a carbonyl group and allow us to react many other functional groups under acidic or basic conditions. Because thioacetals are stable in acid, their hydrolysis requires use of mercuric chloride in aqueous acetonitrile. The formation of an insoluble mercury(II) sulfide provides the driving force for the reaction. 

Nickel peroxide is a solid, insoluble oxidant prepared by reaction of nickel (II) salts with hypochlorite or ozone in aqueous alkaline solution. This reagent when used in nonpolar medium is similar to, but more reactive than, activated manganese dioxide in selectively oxidizing allylic or acetylenic alcohols. It also reacts rapidly with amines, phenols, hydrazones and sulfides so that selective oxidation of allylic alcohols in the presence of these functionalities may not be possible. In basic media the oxidizing power of nickel peroxide is increased and saturated primary alcohols can be oxidized directly to carboxylic acids. In the presence of ammonia at —20°, primary allylic alcohols give amides while at elevated temperatures nitriles are formed. At elevated temperatures efficient cleavage of a-glycols, a-ketols... 

Compounds of Tl have many similarities to those of the alkali metals TIOH is very soluble and is a strong base TI2CO3 is also soluble and resembles the corresponding Na and K compounds Tl forms colourless, well-crystallized salts of many oxoacids, and these tend to be anhydrous like those of the similarly sized Rb and Cs Tl salts of weak acids have a basic reaction in aqueous solution as a result of hydrolysis Tl forms polysulfldes (e.g. TI2S3) and polyiodides, etc. In other respects Tl resembles the more highly polarizing ion Ag+, e.g. in the colour and insolubility of its chromate, sulfide, arsenate and halides (except F), though it does not form ammine complexes in aqueous solution and its azide is not explosive. 

Group 3 Cations. At this stage, sodiirm hydroxide is added to the solution to make it basic. In a basic solution, the above equilibrium shifts to the right. Therefore, the more soluble sirlfides (CoS, FeS, MnS, NiS, ZnS) now predpitate out of solution. Note that the Al and Cr ions actually predpitate as the hydroxides Al(OH)3 and Cr(OH)3, rather than as the sulfides, because the hydroxides are less soluble. The solution is then filtered to remove the insoluble sulfides and hydroxides. 

Thus the most insoluble sulfide salts, such as CuS (Ksp = 8.5 X 10-45) and HgS (Ksp = 1.6 X 10 54), can be precipitated from an acidic solution, leaving the more soluble ones, such as MnS (Ksp = 2.3 X 10-13) and NiS ( Ksp = 3x10 21), still dissolved. The more soluble sulfides can then be precipitated by making the solution slightly basic. This procedure is diagrammed in Fig. 8.12. 

If you are still in doubt about your cation, the following tests might be helpful. To 10. drops of an aqueous solution of your sample (not an HCl solution) in a 10 x 75-mm test tube, add 1 drop of 6 M hydrochloric acid, HCl, solution. Does any precipitate form What color is it If so, it is an insoluble chloride salt of your cation. If no precipitate forms, add 5 drops of 1 M sodium sulfide, Na2S, solution. Does any precipitate form What color is it If so, it is an insoluble sulfide salt of your cation. If no precipitate forms, add 6 M aqueous ammonia, NH3, solution dropwise until the solution is basic to litmus paper with 3 drops in excess. Does any precipitate form What color is it If so, it is an insoluble sulfide salt of your cation or an insoluble hydroxide of Al(III) or Cr(III). 

Another class of compounds whose cations may not be precipitated by the addition of hydroxide ions are the most stable complexes of metal cations with Lewis bases, such as ammonia, amines, and tertiary phosphines. Because of the large number of these compounds and their wide range of properties, it is not possible to give a general procedure for separating the cations. In many cases, metal sulfides can be precipitated directly from aqueous solutions of the complexes by the addition of aqueous sodium sulfide. If a test-tube experiment shows that other measures are needed, the addition of hydrochloric acid to produce a slightly acidic solution will often decompose the complex by protonation of the basic ligand. Metal ions that form insoluble sulfides under acid conditions can then be precipitated by drop wise addition of aqueous sodium sulfide. 

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