H ₂ S gear

The H ₂ S passage is a chemical separation passage for cations in qualitative analysis . From a mixture of substances, the original substance or original sample , chemically similar cations are precipitated in groups using specific reagents . The precipitate is then separated and analyzed, with the supernatant, i. H. the remaining cations in solution, work continues and the next group is brought to precipitation. The cations in the H ₂ S gang are classified in the order in which they are precipitated as follows:

• Silver group: Ag ⁺ , Hg ₂ ²⁺ , Pb ²⁺ ; Precipitation reagent: HCl
• Calcium group: Ca ²⁺ , Sr ²⁺ , Ba ²⁺ and residues of Pb ²⁺ ; Precipitation reagent: H ₂ SO ₄
• Copper / tin group: Cu ²⁺ , Hg ²⁺ , Bi ³⁺ ; Sb ³⁺ , Sn ²⁺ ; Precipitation reagent: H ₂ S in acid ( pH <3)
• Iron group: Fe ³⁺ , Al ³⁺ , Cr ³⁺ ; Precipitation reagent: NH ₃
• Zinc group: Co ²⁺ , Ni ²⁺ , Mn ²⁺ , Zn ²⁺ ; Precipitation reagent: H ₂ S in basic ( pH > 8)
• Soluble group: Mg ²⁺

Other cations of the soluble group such as K ⁺ , NH ₄ ⁺ and Na ⁺ are detected from the original sample (e.g. by coloration of the flame, etc.).

Within a group, very small differences in the chemical properties of the forms of precipitation are sometimes used for separation. These differences can consist , for example, in different solubilities or different redox behavior . After the cations have been separated within a group, specific individual ion detections are carried out.

Since many proofs are sensitive to foreign ions, a quantitative precipitation must be ensured in order to avoid being carried over to the next analysis group. Quantitative precipitation (if possible, no residual concentration of the precipitated species should remain in the solution) is only possible if the reaction conditions ( pH value , etc.) are strictly observed . At many points in the separation process, the completeness of the precipitation must be checked before further work can be carried out.

The individual groups

The silver group

Ag ⁺ , Hg ₂ ²⁺ and Pb ²⁺ are precipitated as chloride salts with dilute HCl ( AgCl , Hg ₂ Cl ₂ , PbCl ₂ ).

PbCl ₂ has the greatest solubility product of the three salts; it dissolves simply by heating. The precipitate is therefore taken up in H ₂ O , heated and filtered off. Pb ²⁺ is detected in the filtrate as PbCl ₂ by cooling down again (characteristic: fine white needles). The yellow precipitates with CrO ₄ ²⁻ and I ^- serve as further evidence for Pb ²⁺ .

The white precipitate of AgCl and Hg ₂ Cl ₂ remaining in the filter is mixed with NH ₃ . Ag ⁺ goes into solution as a silver diammine complex [Ag (NH ₃ ) ₂ ] ⁺ . Reprecipitation of AgCl after acidification with HCl indicates silver. Also yellowish AgI with I ^- solution is considered silver proof.

The Hg ₂ Cl ₂ remaining in the filter is disproportionated when the NH _{3 is} added . A black mixture of Hg (NH ₂ ) Cl and elemental Hg forms . This counts as Hg proof.

The calcium group

After the silver group has been precipitated, the supernatant is used for the precipitation of the calcium group ( Ca ²⁺ , Sr ²⁺ , Ba ²⁺ ). By adding dilute H ₂ SO ₄ , the cations are precipitated as white sulfates: BaSO ₄ , SrSO ₄ , CaSO ₄ . A part of the precipitate can be examined under the microscope for characteristic CaSO ₄ needles (plaster of paris). Since the sulfates are sparingly soluble, they have to be converted into the corresponding carbonates for further separation by an excess of Na ₂ CO ₃ (soda digestion): BaCO ₃ , SrCO ₃ , CaCO ₃ . The carbonates are then dissolved in acetic acid CH ₃ COOH.

The acetic acid solution is now mixed with CrO ₄ ²⁻ solution. Due to the pH-dependent chromate / dichromate equilibrium, the concentration of CrO ₄ ²⁻ in the acidic solution is so low that only BaCrO ₄ precipitates. Sr ²⁺ and Ca ²⁺ remain in solution. The BaCrO ₄ precipitate is already considered evidence of Ba ²⁺ . In addition, after dissolving in HCl, you can test for green flame color and reprecipitate as sulfate with H ₂ SO ₄ .

The Sr ²⁺ remaining in the solution is then separated off as chromate by increasing the pH value (achieved by adding NH ₃ ), which increases the CrO ₄ ²⁻ concentration. Ca ²⁺ remains in solution. Sr ²⁺ can then be detected by the carmine-red color of the flame.

Ca ²⁺ is finally precipitated as CaC ₂ O ₄ in perchloroacetic acid solution with C ₂ O ₄ ²⁻ . The brick-red color of the flame and the characteristic tuft-like plaster of paris needles of CaSO ₄ serve as evidence .

After the carbonates have been dissolved in acetic acid, a combined test for Sr ²⁺ and Ba ²⁺ can be carried out. With sodium rhodizonate, both cations give a brown-red precipitate on a filter. If the precipitate is caused by Sr ²⁺ , it disappears with a drop of HCl; the barium precipitate turns pink during the acid treatment.

The copper / tin group

H ₂ S is now introduced into the residual sulfuric acid solution of the calcium group . Due to the pH-dependent S ²⁻ concentration, only the sparingly soluble sulfides of Cu ²⁺ , Hg ²⁺ , Bi ³⁺ , Sb ³⁺ and Sn ^{2+ precipitate} . The zinc group, which is also to be precipitated later with H ₂ S , remains in solution.

Due to the color of the sulfides, initial conclusions can be drawn about the cations present.

• CuS, HgS and Bi ₂ S ₃ are black,
• CdS is yellow,
• SnS is brown and
• Sb ₂ S ₃ is orange.

The next step is the separation of the tin group (with Sb ³⁺ and Sn ²⁺ ) from the copper group ( Cu ²⁺ , Hg ²⁺ , Bi ³⁺ ). The separation takes place by utilizing the amphoteric character of the ions of the tin group. When treated with KOH, they go into solution as oxo and thio complexes. The solution is separated from the precipitate of the copper group, by acidifying again Sb ₂ S ₃ and SnS precipitate again.

In an excess of HCl, Sb ₂ S ₃ and SnS dissolve through formation of the chloro complexes [SbCl ₄ ] ^- and [SnCl ₆ ] ²⁻ . This solution is divided into two parts. The first part is treated with C ₂ O ₄ ²⁻ solution in order to keep tin in solution as a stable Sn (C ₂ O ₄ ) ₃ ²⁻ . The orange Sb ₂ S _{3 is} precipitated with H ₂ S. The orange color of the precipitate is evidence of Sb ³⁺ . Metallic iron (e.g. a nail) is added to the second part of the solution. Sb ³⁺ is reduced to metallic Sb by Fe and can be seen as a black coating. Sn (IV) is reduced to Sn (II). As evidence of Sn ²⁺ , the solution is mixed with Hg ²⁺ , this is reduced by Sn ²⁺ to Hg ₂ ²⁺ and forms the white Hg ₂ Cl ₂ (calomel) with the Cl ^- ions of the solution . Tin can also be detected with the luminous sample.

The sulphides HgS, CuS and Bi ₂ S ₃ remaining after the tin group has been dissolved are treated with HNO ₃ . Cu ²⁺ and Bi ³⁺ go into solution by oxidation. HgS remains behind. The Cu ²⁺ / Bi ³⁺ solution is treated with NH ₃ . Cu ²⁺ forms the intensely blue copper tetrammine complex [Cu (NH ₃ ) ₄ ] ²⁺ (= proof!). Bi ³⁺ precipitates as white Bi (OH) ₃ . The precipitate is centrifuged off and checked for bismuth. When dissolved in HCl, it forms a yellow precipitate with diacetyldioxime. The remaining HgS is oxidatively dissolved with 30% H ₂ O ₂ in hydrochloric acid solution and the amalgam detection is carried out. Evidence for tin from calomel precipitation is also evidence of mercury.

The iron group

The ions of the iron group ( Fe ³⁺ , Al ³⁺ , Cr ³⁺ ) are precipitated from the supernatant of the copper / tin group with NH ₃ as hydroxides. Here, too, the color of the precipitation can serve as an indication. Fe (OH) ₃ is red-brown, Cr (OH) ₃ is gray-green, Al (OH) ₃ is white.

By treatment with NaOH (often from the slightly acid solution by a sudden increase in pH as alkaline fall performed) is the amphoteric Al (OH) ₃ as Aluminatkomplex [Al (OH) ₄ ] ^- in solution. The hydroxide precipitates again through re-acidification. Now the individual ion detection for aluminum can be carried out: Al ³⁺ gives a red precipitate in acetic acid solution with alizarin S; with eriochrome cyanine it turns violet.

The precipitated hydroxide is washed for a luminous test and then heated with solid KOH and a little water until the KOH is completely dissolved. The contents of the test tube are centrifuged and the filtrate is acidified with glacial acetic acid. Addition of Morin dissolved in ethanol triggers a strong green fluorescence, which can be seen very well under UV light. Addition of NaF or HCl conc. Quenches the fluorescence. Also quite reliable from the analysis substance, slight interference from zinc possible.

Chromium is separated from the remaining precipitate by oxidation to CrO ₄ ²⁻ with H ₂ O ₂ . Ba ²⁺ or Pb ^{2+ is} added to the yellow solution . A yellow precipitate serves as evidence. The remaining iron is now dissolved in acid and detected as a blood-red inner complex salt [Fe (SCN) ₃ ] using a rhodanide solution . In addition, there is the detection with K ₄ [Fe (CN) ₆ ], with Fe ^{3+ an} intense Berlin blue is produced .

The zinc group

The present after precipitation of the iron group ions of the zinc group Co ²⁺ , Ni ²⁺ , Mn ²⁺ , Zn ²⁺ are in an alkaline medium with H ₂ S as a CoS (black), NiS (black), MnS (flesh-colored) and ZnS (white) like. If the precipitate is left to stand for a few minutes, CoS and NiS are converted into a configuration with lower solubility. Therefore, unlike MnS and ZnS, they do not go into solution when treated with acid .

The remaining precipitate of cobalt and nickel is now dissolved oxidatively with 30% H ₂ O ₂ (oxidation of the S ²⁻ to SO ₄ ²⁻ ). Co ²⁺ , Ni ²⁺ are now in solution again. Cobalt is detected with rhodanide solution as a blue complex Co (SCN) _{2 that} can be extracted in butanol . With diacetyldioxime, nickel gives a red, voluminous precipitate.

The Mn ²⁺ and Zn ²⁺ ions in solution are oxidized to MnO (OH) ₂ ( manganese dioxide) with NaOH and H ₂ O ₂ , or kept in solution as Zn (OH) ₄ ²⁻ . MnO (OH) ₂ is oxidized with PbO ₂ to MnO ₄ ^- ( permanganate ). The strong violet color of MnO ₄ ^- proves Mn ²⁺ . White ZnS is precipitated again from the zincate solution with H ₂ S and this is dissolved in H ₂ SO ₄ . A dirty white precipitate is formed with K ₄ [Fe (CN) ₆ ]. Zn ²⁺ can also be detected with Co ²⁺ or Cu ²⁺ and Hg (SCN) ₄ ²⁻ as a blue or violet mixed crystal.

Soluble group

Mg ^{2+ is} the only cation from the soluble group that is detected at the end of the separation process. It is precipitated as Mg (OH) ₂ with NaOH (pH> 10) and then detected as Mg (NH ₄ ) PO ₄ . Mg ²⁺ can also be detected with titanium yellow and NaOH as a red lake or with quinalizarin and NaOH by blue coloration.

Hints

H ₂ S is a poisonous gas that must be handled with care. Nowadays, the S ²⁻ ions required for the separation process are also generated in situ , for example with the help of expensive thioacetamide .

literature

• 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

See also

• Separation passage
• "Classic" cation separation gear