Cation separation pathway

from Wikipedia, the free encyclopedia

The classic cation separation process (also called "cation separation process" for short) is a qualitative process in inorganic analytical chemistry, along with other separation processes, for the wet-chemical separation of cations that are in an analysis substance ("sample", "original substance"). The aim of this analysis method is to determine which cations are contained in an unknown sample (salt solution) ( qualitative analysis ). The analysis result obtained at the end of the cation separation Ganges in performing detection reactions for individual cations without similar interfere with the detection reagents reacting substances.

The chemists Carl Remigius Fresenius (1818–1897) and Frederick P. Treadwell (1857–1918) developed this method for qualitative analysis. In this context, the Swede Bergmann investigated the behavior of the cations towards hydrogen sulfide (hydrogen sulfide). The further development finally led to the classic separation pathway, which is a reliable method for separating the cations into individual separation pathway groups and elements.

Beginnings and development of the cation separation process and its detection reactions

The aim of many experiments has always been to get information about the presence of certain substances (e.g. poisons) in a substance sample. Chemical analysis methods also existed with regard to cations and metal salts even before chemistry established itself as a natural science (and separated from alchemy) - Pliny already knew how to detect iron sulphate in verdigris by using gall apple juice (which forms with iron II ions a black iron compound). And with regard to copper salt solutions - manufactured e.g. B. from separating water (nitric acid) and bronze - taught Andreas Libavius (approx. 1550–1616) how to detect this in water with the help of ammonia ("ammonia spirit"): copper salt solutions turn deep blue "through the ammonia spirit" ( complex formation reaction ).

In 1685 Robert Boyle developed the following, first "analysis" to examine the quality of a body of water without harmful taste samples:

  1. Measurement of temperature and determination of density (measuring volume and weighing)
  2. Careful determination of color, odor and effect on the skin
  3. Determination of moving particles in the water with the help of magnifying glasses ("microscope") and the effect of the air on the water sample,
  4. Test with gall apple juice (if the water contains iron salts, it turns black, with copper salts it turns red and / or cloudy),
  5. Test with violet or red cabbage juice (alkaline solutions color the juice green)
Friedrich Hoffmann

Friedrich Hoffmann expanded the "analysis process" in 1703 to include the detection of table salt (detection means: "Höllenstein" ( silver nitrate , AgNO 3 ), a salt that he obtained by dissolving silver in separating water) and sulfur compounds (with the help of mercury and / or mercury salts ), and at Bergman's time (around 1780) the “analyst's set of reagents” already comprised litmus, violet and gall apple juice, sulfuric acid , oxalic acid , potash, lime water, hellstone, “lead sugar” (= lead acetate) and “alcohol” (ethanol ).

In the 19th century, after the discovery of more and more elements, a repertoire of detection methods and detection reactions that was soon barely manageable for laypeople developed . And in order to prevent certain substances from interfering with specific detection reactions (through coloration, cloudiness, etc.), chemists finally developed a separation system : With the help of certain precipitants (group reagents ), they separated the metal salts ( cations ) to be detected into groups of precipitates and solutions classical wet chemical cation separation process emerged. This was based on precipitations and acid-base reactions and the methodologically more targeted use of the same, more efficient precipitation and detection agents in laboratories.

Qualitative inorganic analysis: separations and evidence

Iron (II) sulphate (slightly yellow-greenish) and iron (III) chloride (yellow-brownish) and their detection with blood liquor salts

The Qualitative Analysis work technique comprises the following steps:

  1. Preliminary samples and external description of the sample,
  2. Attempts at dissolution,
  3. Detection reactions for anions (from stock and soda extract),
  4. Separation (cation separation process) and detection reactions for cations,
  5. Possibly. Digestion of insoluble components.

Carrying out the detection reactions for cations requires the previous cation separation process, as otherwise interference from cations with similar reactions can occur. This systematic analysis is based on the possibility of precipitating related groups of cations together as precipitates and filtering them off (example: as sulfides , see figure). The individual separation groups ( groups of substances ) are then further separated and analyzed until detection reactions for individual cations are possible without being disturbed.

The five major separation gang groups

As a rule, in the simplified cation separation process, the following separation pathway groups are first precipitated and separated for individual analysis:

  • The hydrochloric acid group (HCl group, heavy metal chlorides, precipitated in a strongly acidic environment: lead, silver, mercury cations; see under hydrochloric acid group),
  • The hydrogen sulfide group (H 2 S group, heavy metal sulfides that precipitate even in a weakly acidic environment: the cations of elements such as bismuth, copper, cadmium, residues of lead and mercury as well as arsenic, tin and antimony - possibly divided into copper and arsenic-tin -Group; see under hydrogen sulfide group),
  • The ammonium sulfide group ((NH 4 ) 2 S x group, compounds that only precipitate in an alkaline environment, such as heavy metal sulfides of the cations of elements such as cobalt, nickel, manganese and zinc, as well as iron-III and chromium-III hydroxide - possibly also in addition with cations of the urotroping group ; see under ammonium sulfide group),
  • The ammonium carbonate group ((NH 4 ) 2 CO 3 group, alkaline earth carbonates of the cations barium, strontium and calcium which precipitate in an alkaline medium with carbonate anions; see under ammonium carbonate group) and
  • The soluble group (magnesium and alkali metal cations).

After further experimental separation of the separation pathway groups up to the qualitative individual detection of the cations, an analytical result can then be determined. The sum of the test results of the detection reactions then provides information about which ions are contained in the analysis substance.

A separation path that has been expanded to include cations that are important for studying chemistry contains z. B. the following segregation groups:

Acid-sparingly soluble group Hydrochloric acid group , Al 2 O 3 , highly annealed oxides , alkaline earth sulfates , silicates , Fe 2 O 3 , TiO 2 , WO 3 , Cr 2 O 3 , FeCr 2 O 4
Reduction group ( hydrazine ) Platinum , palladium and other noble metal ions as well as Se and Te are reduced to the metal
Hydrochloric acid group (HCl) Ag + , Hg 2 2+ , Pb 2+ , Tl + / 3 +
Hydrogen sulfide group (H 2 S) Copper group: Pb 2+ , Bi 3+ , Cu 2+ , Cd 2+ , Hg 2+
Arsenic group: As 3 + / 5 + , Sb 3 + / 5 + , Sn 2 + / 4 + , Mo 2+ , Ge 4+
Urotropine group (= hexamethylenetetramine ) Fe 2 + / 3 + , Ti 2 + / 4 + , La 3+ , Al 3+ , Cr 3 + / 6 + , Zr 4+ , Ce 4+
Ammonium sulfide group ((NH 4 ) 2 S) Ni 2+ , Co 2+ , Fe 2 + / 3 + , Mn 2+ , Cr 3+ , Al 3+ , Zn 2+ , Ti 2 + / 4 + , V 2+ , W 3+
Ammonium carbonate group ((NH 4 ) 2 CO 3 ) Ca 2+ , Ba 2+ , Sr 2+
soluble group NH 4 + , Mg 2+ , K + , Na + , Li + , Cs +

The cation separation process is therefore not only of historical importance in education and training, but also of propaedeutic and didactic importance, since practical-methodical knowledge (experimental laboratory methods) as well as basic and material knowledge of inorganic chemistry can be clearly conveyed and practiced.

literature

  • Gerhart Jander: Introduction to the inorganic-chemical internship . S. Hirzel Verlag, Stuttgart 1990 (in 13th edition), ISBN 3-7776-0477-1 (suitable for student teachers)
  • Gerhart Jander: Textbook of analytical and preparative inorganic chemistry . S. Hirzel Verlag, Stuttgart 2002 (in 15th edition), ISBN 3-7776-1146-8 [suitable for graduate chemists]
  • Michael Wächter: chemistry laboratory . Verlag Wiley-VCH, Weinheim 2011, pp. 215-241, ISBN 978-3-527-32996-0
  • Udo R. Kunze, Georg Schwedt: Fundamentals of the qualitative and quantitative analysis , 5th revised edition, Wiley-VCH, Weinheim 2002, ISBN 3-527-30858-X
  • Gerdes, Eberhard: Qualitative Inorganic Analysis - A Companion for Theory and Practice , 2. Corr. u. revised 1st edition. 2001, Springer Verlag Berlin, ISBN 3-540-67875-1
  • Bertram Schmidkonz: Practical course in inorganic analysis . Verlag Harri Deutsch, Frankfurt 2002, ISBN 3-8171-1671-3
  • Michael Wächter: Substances, particles, reactions . Verlag Handwerk und Technik, Hamburg 2000, pp. 154–169 ISBN 3-582-01235-2

Web links