Date last chaged: 05/Apr/2013 01:01 PM

ACTION OF HYDROGEN SULFIDE

This is normally used as a solution of H2S in acetone (propanone).  The pH of the solution to be tested should be taken into account in interpreting your results.

 

OBSERVATIONS

CATION PRESENT

EQUATION(S)

No precipitate

Cation with soluble sulphide: NH4+, Na+, K+, Ba2+, Ca2+, Mg2+, Al3+, Cr3+

--

Pale yellow or white suspension or ppt. from neutral, alkaline, or acid solution.[i]

Fe3+ (ppt. is sulfur)
(Ions may colour suspension)

2Fe3+(aq) + S2-(aq) ® 2Fe2+(aq) + S(s)

White suspension or ppt. from neutral or alkaline, but not acid, solution[ii].
Ppt. soluble in dil. acid

Zn2+

Zn2+(aq) + S2-(aq) ® ZnS(s)
ZnS(s) + 2H+(aq) ® Zn2+(aq) + H2S(aq)

Grey green ppt[iii]. from neutral or alkaline, but not acid, solution.  Ppt. soluble in dil. acid

Cr3+ (ppt. is Cr(OH)3)

                            H2S

Cr3+(aq) + 3H2O(l) ® Cr(OH)3(s)

 + 3H+(aq)

Cr(OH)3(s) +3H+(aq) ® Cr3+(aq) + 3H2O(l)

Brown ppt. from acidic, neutral or basic solution.

Sn2+

Sn2+(aq) + H2S(aq) ® SnS(s) + 2H+(aq)

Black ppt. from acidic, neutral or basic solution. 

Ppt. insoluble in cold dil. acids, but soluble in hot dil. HNO3(aq) to give a white or pale yellow suspension or ppt.

Original ppt. turns white in H2O2(aq).

Pb2+, Ag+

eg. M2+(aq) + S2-(aq) ® MS(s)

 

3MS(s) +8H+(aq) + 2NO3-(aq) ® 3M2+(aq) + 3S(s) + 2NO(g) 4H2O(l)

MS(s) + 4H2O2(aq) ® MSO4(s) + 4H2O(l)

Black ppt. from acidic, neutral or basic solution.  Ppt. insoluble in cold dil. acids, but soluble in hot dil. HNO3(aq) to give a white, blue or pale yellow, or pale green suspension or white or pale yellow ppt.  Original ppt. also soluble in H2O2(aq) to give a blue solution.

Cu2+

Cu2+(aq) + S2-(aq) ® CuS(s)

 

3CuS(s) +8H+(aq) + 2NO3-(aq) ® 3Cu2+(aq) + 3S(s) + 2NO(g) 4H2O(l)

CuS(s) + 4H2O2(aq) ® CuSO4(s) + 4H2O(l)

Black ppt. from neutral or basic, but not acidic, solution.
Ppt. soluble in dilute acids.

Ni2+, Fe2+

M2+(aq) + S2-(aq) ® MS(s)

MS(s) + 2H+(aq) ® M2+(aq) + H2S(aq)

Pink or dirty white (buff) ppt. from basic or neutral, but not acidic, solution.
Ppt. soluble in dilute acids.

Mn2+, Co2+

M2+(aq) + S2-(aq) ® MS(s)

MS(s) + 2H+(aq) ® M2+(aq) + H2S(aq)

 

Notes:

NH4Cl is sometimes used to produce a slightly acidic solution from which some sulfides precipitate.

Precipitation of sulfides depends on the equilibrium:

H2S(aq) º 2H+(aq) + S2-(aq)

which lies well to the left in acidic solution, giving a very low concentration of S2-, and the solubility product for the metal sulphide, that is the equilibrium constant for:

MS(s) º M2+(aq) + S2-(aq)

Sulfides with very low solubility products (CuS, PbS and Ag2S) will even precipitate from acidic solution.


 

[i] A black ppt. of FeS will form on addition of excess H2S in neutral or alkaline solution, but not in acidic solution.

[ii] Some sources suggest that Al3+ gives a white precipitate of Al(OH)3 with H2S.  This seems unlikely and efforts to produce such a precipitate with either H2S(g) or H2S(acetone) have proved unsuccessful.  It is likely however that such a precipitate is produced by treatment of Al3+(aq) with, for example, Na2S(aq) or NaHS(aq), since these solutions are alkaline.

White or pale yellow precipitates of sulfur may be produced when H2S(acetone) which is contaminated with sulfur is used  This arises when the solution of H2S has been exposed to the air for some time and is due to the reaction:

2H2S + O2 → 2H2O(l) + 2S(acetone)

When a solution of sulfur in acetone is mixed with water, the sulfur precipitates.

[iii] Some sources suggest that Cr3+ gives a grey-green ppt. with H2S.  As with Al3+ (see previous note) this seems unlikely.  A ppt. of Cr(OH)3 would be expected with Na2S or similar reagents.