Cations of Group II PDF

Cations of Group II  Cations Group I brings together eight cations that subdivided into the copper group consisting of (mercuric, bismuth, cadmium, copper and lead) and the arsenic group consisting of (arsenic, antimony and tin)  The group reagent is hydrogen sulphide in acid medium (0.3 M...

Cations of Group II  Cations Group I brings together eight cations that subdivided into the copper group consisting of (mercuric, bismuth, cadmium, copper and lead) and the arsenic group consisting of (arsenic, antimony and tin)  The group reagent is hydrogen sulphide in acid medium (0.3 M HCl).  The subdivision to II-a and II-b subgroups is based on: The solubility in excess of sodium sulphide Na2S or Sodium Hydroxide NaOH to form thioanions and oxyanions. 20 Cations of Group II  The group reagent is hydrogen sulphide in acid medium (0.3 M HCl). The real significance of the [H+] as a control of [S2-] can be seen by examining the ionization of H2S. Due to common ion effect,  Increasing the concentration of H+ ions in solution of H2S in water, shift the equilibrium to the left, that decrease the sulphide ion concentration.  Addition of strong base shifts this equilibrium to the right with a resultant increase the [S2-]. 21 Cations of Group II  The metal will precipitate when the product of [M2+] and [S2-] exceeds the solubility product. This means that the limiting factor in precipitation is the sulphide ion concentration which is controlled by the pH (the hydrogen ion conc.).  Therefore, to precipitate cations of group II (having lower solubility product than those of group IIIB), we need relatively small amount of sulphide ion.  If the [H+ ] is controlled to become 0.3 M, the [S2-] will be 1.5 x 10-19 M. This is sufficient to precipitate the sulphides of the elements of cation group II only. 22 Cations of Group II Adjustment of the acidity An increase in [S2-] by decreasing the acidity (< 0.3 M HCl) results in:  a) Precipitation of sulphides of group IIIB.  b) Dissolution of sulphides of group IIB as thioanions (see group IIB). A decrease in [S2-] by increasing the acidity (> 0.3 M HCl) results in :  a) Prevention of the precipitation of CdS, PbS, SnS2 (have higher solubility products).  b) Formation of stable soluble complexes as [CdCl4]2-, [SnCl62-], [SbCl4]-, so they cannot precipitate by addition of H2S. 23 Cations of Group II 24 Cations of Group II  N.B. :  (1) Unless Hg2+ is an element of group IIA, but it appears in both IIA and IIB subgroups because its precipitate HgS in group IIA is partially soluble in alkali sulphide (Na2S) and pass with group IIB  (2) Stannous sulphide SnS is incompletely soluble in either NaOH or Na2S. Therefore, to separate stannous with the subgroup IIB cation sulphides, we have to oxidize stannous (Sn2+) to stannic (Sn4+) by boiling with hydrogen peroxide.  The excess H2O2 must be decomposed by boiling, otherwise it will oxidize the H2S reagent to colloidal sulphur which is difficult to separate. 25 Cations of Group II Group II A  Mercuric Hg2+  Bismuth Bi3+  Copper Cu2+  Cadmium Cd2+  Lead Pb2+ 26 Cations of Group II (A) 1- Mercuric (II) ion (Hg2+)  Group Precipitation 0.3 M H+ Hg2+ + H2S ↔ HgS ↓ Black Confirmatory Test With ammonium hydroxide HgCl2 + 2 NH4OH ↔ Hg (NH2)Cl↓ white mercuric amino chloride With Stannous chloride (SnCl2) HgCl2 + SnCl2 → Hg2Cl2 ↓ white + [SnCl6]2- Hg2Cl2 + xss SnCl2 → Hgo ↓ black + [SnCl6]2- With potassium iodide (KI) Hg2+ + KI ↔ HgI2 ↓ Scarlet Red HgI2 ↓ + xss KI ↔ [HgI4]2- soluble complex 27 Cations of Group II (A) 2- Bismuth (III) ion (Bi3+)  Group Precipitation 0.3 M H+ Bi3+ + H2S ↔ Bi2S3 ↓ dark brown Confirmatory Test With excess water (Oxysalt formation) BiCl3 + H2O ↔ BiOCl ↓ + 2H+ large xss white turbidity bismuth oxychloride With Sodium Stannite ([HSnO2]-) Bi3+ reacts with Sodium Stannite fresh solution (prepared by reaction of stannous chloride with xss NaOH till dissolution of the formed white ppt) Bi3+ + [HSnO2]- ↔ Bio ↓ black + [SnCl6]2- 28 Cations of Group II (A) 3- Copper (II) ion (Cu2+)  Group Precipitation 0.3 M H+ Cu2+ + H2S ↔ CuS ↓ black Confirmatory Test With ammonium hydroxide Cu2+ + 4 NH4OH ↔ [Cu(NH3)4]2+ deep blue colour With potassium iodide (KI) Cu2+ + KI ↔ Cu2I2 ↓ + I2 White Brown Cuprous iodide With potassium ferrocyanide [Fe(CN)6]4- HAC Cu2+ + [Fe(CN)6]4- ↔ Cu2[Fe(CN)6]↓ Chocolate brown 29 Cations of Group II (A) 4- Cadmium (II) ion (Cd2+)  Group Precipitation 0.3 M H+ Cd2+ + H2S ↔ CdS ↓ yellow Confirmatory Test With ammonium hydroxide CdS is insoluble in excess S2- and in NaOH 30 Cations of Group II Group II B  Arsenious As3+  Arsenic As5+  Antimonous Sb3+  Antimonic Sb5+  Stannic Sn4+ 31 Cations of Group II Group II B Group II is boiled with H2O2, the acidity is adjusted at 0.3 M HCl and H2S or thioacetamide is added, the following sulphide precipitates of subgroup IIB are formed.  Arsenious As3+ → As2S3 yellow  Arsenic As5+ → As2S5 yellow  Antimonous Sb3+ → Sb2S3 orange  Antimonic Sb5+ → Sb2S5 orange  Stannic Sn4+ → SnS2 brown These precipitates are soluble in NaOH or Na2S (separation of subgroups IIA & IIB) 32 Cations of Group II (B) 1- Arsenious (III) ion and Arsenic (V) ion, As3+, As5+  Group Precipitation 0.3 M H+ 2As3+ + 3H2S ↔ As2S3 ↓ yellow 2As5+ + 5H2S ↔ As2S5 ↓ yellow  Confirmatory Test Soluble in xss Na2S or in NaOH 33 Cations of Group II (B) 1- Arsenious (III) ion and Arsenic (V) ion, As3+, As5+ Discard The arsenious sulphide and arsenic sulphide are insoluble in 12 M HCl (differ from sulphides of antimony and tin), but by boiling they are partially soluble. Then arsenious sulphide and arsenic sulphide are dissolved by conc HNO3 34 Cations of Group II (B) 1- Arsenious (III) ion and Arsenic (V) ion, As3+, As5+ Confirmatory Test After dissolution with conc. HNO3 on H3AsO4 Ammonium molybdate Test 35 Cations of Group II (B) 2- Antimonous (III) ion and Antimonic (V) ion , Sb3+, Sb5+  Group Precipitation 0.3 M H+ 2Sb3+ + 3H2S ↔ Sb2S3 ↓ orange 2Sb5+ + 5H2S ↔ Sb2S5 ↓ orange  Confirmatory Test Soluble in xss Na2S or in NaOH to form thio and oxyanions 36 Cations of Group II (B) 2- Antimonous (III) ion and Antimonic (V) ion , Sb3+, Sb5+ Confirmatory Test After dissolution with 12M HCl Oxysalt formation Sodium thiosulphate Test 37 Cations of Group II (B) 2- Antimonous (III) ion and Antimonic (V) ion , Sb3+, Sb5+  Confirmatory Test After dissolution with 12M HCl Test for Sb3+ in presence of Sn4+ Addition of oxalic acid which forms two complexes with Sb3+ and Sn4+. The complex with Sn4+ is stable, while that with Sb3+ is unstable, so on passing H2S, orange ppt of Sb2S5 is formed. 38 Cations of Group II (B) 3- Stannic (IV) Ion Sn4+  Group Precipitation 0.3 M H+ Sn4+ + 2H2S ↔ SnS2 ↓ brown  Confirmatory Test Soluble in xss Na2S or in NaOH to form thio and oxyanions Test for Sn4+ in presence of Sb3+ We have to boil with iron wire (Feo) in acid medium for two reasons: Then Sn2+ react with HgCl2 39 Cations of Group II (B) 3- Stannic (IV) Ion Sn4+ Then Sn2+ react with HgCl2 The test may give white ppt (Hg2Cl2) or grey ppt (Hg2Cl2 + Hgo) or finally black ppt (Hgo) (oxidation-reduction reaction). 40