Lecture 6 Pharmaceutical Analytical Chemistry I PDF

Lecture (6) Dr. Ahmed Abdel-Monem Cations of group (II) Name of group: Copper – Arsenic gp. or Hydrogen sulphide gp. Gp. II (A) Gp. II (B) Copper subgroup Arsenic subgroup Cu2+ Contains...

Lecture (6) Dr. Ahmed Abdel-Monem Cations of group (II) Name of group: Copper – Arsenic gp. or Hydrogen sulphide gp. Gp. II (A) Gp. II (B) Copper subgroup Arsenic subgroup Cu2+ Contains As3+ pb2+ Sb3+ Hg2+ Sn2+ Bi3+ Cd2 Group reagent: H2S gas in acidic medium (0.2 – 0.3N HCl ) Cations of group (II) Precipitated as sulphides Black ppt. CuS, HgS and pbS Brown ppt. Bi2S3 CdS Canary yellow ppt. As2S3 Yellow ppt. SnS2 Orange ppt. Sb2S3  Precipitation Adjust the acidity of filtrate remaining after ppt of group I to 0.2 – 0.3 N HCl using crystal violet indicator till the bluish green colour is produced. Warm and pass H2S until complete pptn. Boil and filter. Wash the ppt. with 5% NH4NO3 and pass H2S ppt Filtrate gp. II "A" & "B" as sulphide gps. III – IV  Notes Why adjust the acidity of HCl to 0.2 – 0.3 N If HCl is smaller than 0.2N If HCl is greater than 0.3N decrease the amount of sulphide Increase the amount of sulphide ions ions and incomplete pptn. of lead, and pptn. of cations of gp. IV Cadmium and tin as sulphides  Notes (2) Why heat after addition of H2S ? H2S gas is slightly soluble in water so we must heat to increase the solubility of H2S and increase its concentration in the solution (3) Why the pptd. of sulphides are washed with 5% NH4NO3 solution ? To remove soluble cations of other groups (III – VI). (4) Why HCl not HNO3 or H2SO4? HNO3 oxidize H2S to free S and dissolve the ppted of sulfides. H2SO4 precipitates group V as sulphate along with group II sulfides.  Separation gp. II "A" from gp II "B" Ppt. (gp. II "A" & "B" as sulphide ) Add 2 ml 2M NaOH & few drops of ammonium poly sulphide and heat on boiling water bath for 3 min. with stirring, and filter ppt. Filtrate gp. II "A" gp. II "B“ "Copper sub gp." "Arsenic sub-gp."  Identification and Confirmation of Bi3+ (1). Wash white ppt. of Bi(OH)3 with water, dissolve washed ppt. in the minimum volume of dil HCl and pour a small part of the above solution into a large volume of water  white ppt. of BiOCl. Bi(OH)3 + 3 HCl  Bi3+ + 3 Cl- + 3 H2O Bi3+ + 3 Cl- + H2O  BiOCl  + 2 HCl Bismuth oxychloride  Identification and Confirmation of Cu2+ & Cd2+ The filtrate containing both Cu2+ and Cd2+ in the form of soluble amine complexes. Cu2+ as [Cu(NH3)4]2+ Cd2+ as [Cd(NH3)4]2+ Each cation will be tested in presence of the other cation  Confirmation for Cu 2+ (1)- The blue colour of the amine complex [Cu(NH3)4]2+. (2)- Add acetic acid to decompose the copper amine complex until the blue colour is disappeared. [Cu(NH3)4]2++ 4 CH3COOH  Cu2++ 4 NH4+ + 4 CH3COO- Then the solution is treated with : (A)- add K4[Fe(CN)6] solution  reddish brown ppt. 2Cu2+ + [Fe(CN)6]4-  Cu2[Fe(CN)6]  (B)- add solution of KI  white ppt. in brown soln. 2Cu2+ + 4 I-  Cu2I2 (white ppt.) + I2  Confirmation for Cd2+ Soln. + KCN dropwise until the blue colour of the copper amine complex is disappeared, warm + H2S  Canary yellow ppt. Cd2+ + H2S  CdS  + 2 H+ Role of KCN: Prevent the reaction of Cu with H2S due to the reaction of KCN solution with copper amine complex lead to formation of the more stable cuprocyanide complex [Cu(CN)3]2- which not reacted with H2S.  Cations of group (III) - Name of group: Iron gp. or ammonium hydroxide gp. - Contains : Iron Fe2+, Fe3+ Aluminum Al3+ Chromium Cr3+ Pptd. as :hydroxides - Fe(OH)3  reddish brown - Al(OH)3  white gelatinous - Cr(OH)3  greenish - MnO(OH)2  reddish brown Group reagent : Conc NH4OH with NH4Cl Notes on gp III: (1) Why remove H2S before adding NH4OH to ppt. of gp. III ?  Avoid ppt. of gp IV as sulphides upon addition of alkali.  Avoids oxidation of H2S to sulphate by nitric acid and prevent precipitation of gp V as sulphates. (2)Why add NH4Cl before the pptn. of gp. III with NH4OH ?  NH4Cl lower the ionization of NH4OH by common ion effect, thus, the hydroxide of Zn2+, Co2+, Ni2+ and Mg2+ are not pptd.  Addition of NH4Cl allows coagulation and easy filtration of gp III hydroxides. (3) Why oxidize Fe2+ into Fe3+ before the pptn. of gp. III.? Because : A)- Fe(OH)3 is more insoluble than Fe(OH)2 and precipitates better. (4) Why boil to remove xss. NH4OH ? Cr(OH)3 is slightly soluble in excess NH4OH on cold forming soluble chromium ammine complex which decomposed by boiling and reprecipitated as Cr(OH)3  MnO(OH)2 MnO(OH)2 Identification and Confirmation: Fe(OH)3 & MnO(OH)2 are dissolved in dil. HNO3 and 1 drop of H2O2 and boil (to decompose xss. H2O2) and divided into two parts (A)- For Fe3+ 1- Soln. + KCNS -  blood red colour. The reaction must be on cold because the blood red colour is decomposed on hot. Fe3+ + 6 CNS -  [Fe(CNS)6]3- 2- Soln. + K4[Fe(CN)6] soln.  Prussian blue ppt. Fe3+ + K4[Fe(CN)6]  KFe[Fe(CN)6] + 3 K+ (B)- For Mn2+ - The second portion + 50% HNO3 + one gm of red lead boil for 1-2 min.  pink or purple colour of permanganate. 5 Pb3O4 + 2 Mn2+ + 24 H+  2 MnO4- + 15 Pb2+ + 12 H2O (C)- For Al3+ (1) Soln. + 1 gm of solid NH4Cl & boil until only a faint odour of NH3 persists  white gelatinous ppt. AlO2- + NH4Cl + H2O  Al(OH)3 + Cl - + NH3  (2) Specific test : (Aluminon test) - Al(OH)3 dissolves in HCl + sodium acetate + aqueous solution of aluminon, allow to stand + xss. NH3 soln.  red solution (D)- For Cr3+ : (1) Perchromic acid test : - Soln. + dil H2SO4 + ether + 1 ml H2O2  deep blue colour in etherial layer 2 CrO42+ + 2 H+  Cr2O72- + H2O Cr2O72- + 7 H2O2  2 CrO83- + 4 H+ + 5 H2O Perchromic acid (2)- Lead acetate test : - soln. + acetic acid + lead acetate soln.  yellow ppt. CrO42- + Pb2+  PbCrO7  THANK YOU