Hydrogen sulfide group

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A hydrogen sulfide group (H 2 S group) is a group of elements whose salts with hydrogen sulfide solution form sparingly soluble sulfides even in the presence of acid . These include cations of the metals with the element symbols

  • Hg and Pb (both also in the hydrochloric acid group , but here residues of these cations are still precipitated as mercury (II) and lead (II) sulfide),
  • Bi , Cu and Cd (together with Pb and Hg referred to as "copper group") and
  • Sb , Sn , As (“arsenic-tin group”) and possibly also Se and Te .

The sulphide precipitation in the acidic range (ideally with acetic acid / sodium acetate buffer solution at pH values ​​around 4 to 5) is used in the cation separation process for qualitative analysis (in inorganic chemistry ) for the separation and detection of the cations mentioned above. The cations of the remaining separation groups are then in the filtrate:

Group felling

Almost all heavy metal cations are insoluble in water in combination with sulfide anions. The hydrogen sulphide group, however, consists of heavy metal salts or cations, which are particularly sparingly soluble and therefore form sparingly soluble sulphides from the filtrate of the hydrochloric acid group in a precipitation reaction even if this is not neutralized, but is mixed with the separating agent hydrogen sulphide directly in the acidic range (which Sulphide precipitation ).

CuI (white), Cu (NO 3 ) 2 + (NH 4 ) 2 S (no reaction), CuS (black), Cu (OH) 2 (blue), [Na 2 Cu (OH) 4 ] (blue), [Cu (NH 3 ) 4 ] 2+ (blue), CuCO 3 (blue)

For example, when hydrogen sulfide is introduced into a solution of copper (II) nitrate , copper sulfide also precipitates in a hydrochloric acid environment:

Other heavy metal sulfides such as B. Zinc and manganese sulfide, however, still remain dissolved, as they are only insoluble in the neutral to alkaline pH range ("acid-soluble sulfides"). It is possible to separate the hydrogen sulfide group from the cations of the following ammonium sulfide group via the pH value and the sulfide concentration. At a pH value of 4 to 5, the following therefore occur:

In the so-called extended cation separation process, Ge , Mo , Ti , Se and Te are also taken into account, with selenium and tellurium being reduced to the element.

Group segregation

The precipitation of the hydrogen sulfide group is followed by a separation of the cations from one another so that they can then be found undisturbed with the help of detection reactions. This group is first divided further into copper and arsenic-tin groups . For this purpose, the sulphide precipitate is leached out in ammonium polysulphide solution. The precipitates of the arsenic-tin group dissolve in the form of thiosalts, while the copper group remains undissolved and can be separated by filtration.

Copper group

There are Bi 2 S 3 (brown), CuS (black), CdS (yellow) and as residues from the foregoing acid group also: PbS (black), HgS (black).

Arsenic-tin group

Soluble as thio salts are Sb 2 S 3 / Sb 2 S 5 (orange), As 2 S 3 / As 2 S 5 (yellow), SnS / SnS 2 (brown / yellow), MoS 2 and GeS 2 .

When the arsenic group is acidified, the thiosalts break down and orange antimony sulfide precipitates.

literature

  • Michael Wächter: chemistry laboratory . Verlag Wiley-VCH, Weinheim 2011, ISBN 978-3-527-32996-0 , pp. 235-236.
  • Gerhart Jander: Introduction to the inorganic-chemical internship , 13th edition, S. Hirzel Verlag, Stuttgart 1990, ISBN 3-7776-0477-1 .
  • Bertram Schmidkonz: Practical course in inorganic analysis . Verlag Harri Deutsch, Frankfurt 2002, ISBN 3-8171-1671-3 .

Web links

Wikibooks: Practical Inorganic Chemistry / Hydrogen  Sulphide Group - Learning and Teaching Materials