Pharmaceutical Analytical Chemistry-I Qualitative Analysis of Cations PDF

Pharmaceutical Analytical Chemistry-I Qualitative Analysis of Cations Prof. Dr. Maha Mohamed Abdelrahman Pharmaceutical Analytical Chemistry Department-BSU Qualitative Analysis of Cations The scheme for qualitative analysis of a mixture containing all the common cations involves f...

Pharmaceutical Analytical Chemistry-I Qualitative Analysis of Cations Prof. Dr. Maha Mohamed Abdelrahman Pharmaceutical Analytical Chemistry Department-BSU Qualitative Analysis of Cations The scheme for qualitative analysis of a mixture containing all the common cations involves first separating them into six major groups based on their solubilities. Each group is then treated further to separate and identify the individual ions. The schemes shown below give us a brief idea about the separation of these six groups. 2 3 Cations are divided into a number of groups according to the difference in solubility of their chlorides, sulphides, hydroxides & carbonates. o Group reagents are added systematically where the group reagent of any group is capable not only to precipitate the cations of its own group, but also the cations of preceding groups. o So, a test for complete precipitation must be carried out for each group before starting to separate subsequent groups ) group IV reagent ( NH4Cl + NH4OH + H2S ) is capable of precipitating the sulfides of group I , II , III and IV. 4 1- Precipitation 2- Washing 3- Separation 4- Confirmation 1- After separating the precipitate of each group, it must be washed well before analysis ?? → to remove any adsorbed impurities from further groups 2- this washing must be carried out with an electrolyte solution (not by water) ?? → to avoid peptisation peptisation of a precipitate means that water favors its transformation into the colloidal state. 5  (silver group) Lead Pb2+ Silver Ag+ Mercurous Hg22+ 6  1- Precipitation a- Group reagent: cold dil. HCl (precipitate as chlorides) b- Formula of precipitate: AgCl, PbCl2, Hg2Cl2 (white ppt) Q. Why HCl is preferred than NH4Cl?? → it prevents the formation of insoluble salt of bismuth (bismuth oxychloride ppt) BiOCl + 2H+ Bi3+ + Cl- + H2O Q. Account : a slight excess of HCl should be added ?? √ to obtain complete precipitation √ to prevent peptisation of the insoluble Chlorides PbCl2 ↔ Pb2+ + 2Cl- + H2O 7  -Account: a very large excess of HCl should be avoided ? → not to increase the solubility of chlorides by complex formation PbCl2 + Cl- [PbCl3]- + Cl- [PbCl4]2- (VIP: so, we don't use conc. HCl, WHY? ) - Account: the dilute HCl used must be cold ? → because PbCl2 is soluble in hot water, thus it may escape to subsequent groups. - Account: after addition of the cold dilute HCl, the solution must be vigorously shaken for 2 min.? → because PbCl2 can form a supersaturated solution, hence escape to subsequent groups, so we shake to ensure complete precipitation - Account: Pb2+ may escape from gp I to gp II ? 1. Due to its high Ksp (Ksp of PbCI2 = 1.6 X 10-5) 2. Tendency to form supersaturated solution 3. Formation of soluble complex [PbCl4]2- 8  2- washing after complete precipitation, the precipitate is centrifuged, then washed with cold dilute HCl ? why? → to remove surface adsorbed cations of the next groups. Q- Account: HCl (an electrolyte) is preferred than water in washing? (1) as it renders PbCl2 less soluble ( by common ion effect ) PbCl2 ↔ Pb2+ + 2 Cl- HCl → H+ + Cl- (2) to avoid peptisation of the precipitate 9  3- Separation The group precipitate (AgCl + PbCl2 + Hg2Cl2 ) is dissolved in boiling water (where PbCl2 dissolve in hot water, while AgCl and Hg2Cl2 don’t), then centrifuge. Hot centrifugate precipitate (may be PbCl2) (may be AgCl, Hg2Cl2) (2) The precipitate (AgCl, Hg2Cl2) is dissolved in dilute NH4OH (where AgCl dissolves & forms the water- soluble silver ammine complex [Ag(NH3)2]+, while Hg2Cl2 is converted into insoluble black mixture of aminomercuric chloride + finely divided mercury. 10 (2) The precipitate (AgCl, Hg2Cl2) is dissolved in dilute NH4OH Centrifugate Precipitate Hg2Cl2 AgCl dissolves Hg2Cl2 + 2NH3 Hg(NH2)Cl ↓ + Hgo ↓ + NH4Cl White ppt black ppt [Ag(NH3)2]+ Confirm Confirm 11  4- Confirmation 1- For lead (1) the hot centrifugate is cooled → needle crystals of PbCl2 (2) hot solution + dil. acetic acid + K2CrO4 solution → PbCrO4 ↓ Yellow lead chromate ppt. - The precipitate is insoluble in acetic acid but soluble in dilute mineral acids (e.g HNO3) , why ??? + as CrO4-- 2H will transform into Cr2O7-- where lead dichromate is water soluble. 2CrO4-- 2HCrO4- Cr2O7-- + H2O Yellow solution orange solution 12 (3) hot solution (of Pb2+) + KI solution→ PbI2 ↓ Yellow ppt. - The yellow precipitate is soluble in hot water (4) hot solution (of Pb2+) + dilute H2SO4 → PbSO4 ↓ white precipitate √ the white precipitate is soluble in NaOH and ammonium acetate solution ( …mentioned before in anions ) 13  2- For Silver (1) ammoniacal Centrifugate (contains Ag+ as [Ag(NH3)2]+ + dilute HNO3 → white ppt ? Cl- from solution [ Ag (NH3)2 ]+ Ag+ + 2NH3 HNO3 [ Ag (NH3)2 ]+ NH4 + + AgCl White ppt (2) another portion + KI solution → yellow ppt of AgI [ Ag(NH3)2]+ Ag+ + 2NH3 AgI ↓ I- yellow ppt Q- addition of KI solution to [Ag(NH3)2]Cl gives a yellow precipitate?? because ionization of the silver ammine chloride complex provide amount of Ag+ which is sufficient to exceed the Ksp of AgI thus favour its precipitation while not sufficient to exceed Ksp of AgCl 14 3- For Mercurous -the black precipitate is dissolved in aquaregia (3 HCl + 1 HNO3) : Hgo + 6HCl + 2HNO3 3HgCl2 + 2NO + 4H2O HgNH2Cl + 3HCl + HNO3 HgCl2 + NH4NO3 i.e. the black Hgo & the white HgNH2Cl both converted to HgCl2 which is water soluble  destroy excess aquaregia (by boiling) then divide the solution into 2 parts: 1 part + SnCl2 solution → white Hg2Cl2 , which turns to grey then to black (with excess SnCl2 ) 2HgCl2 + Sn2+ Hg2Cl2 ↓ + Sn4+ + 2Cl- white ppt. Hg2Cl2 + Sn2+ Hgo ↓ + Sn4+ + 2Cl- 15 excess black ppt. 2 part + KI solution→ red ppt of HgI2 HgCl2 + 2I- → HgI2 ↓ + 2Cl- red ppt HgI2 + 2I- → [HgI4]-- excess Neesler reagent (soluble Complex) Notes : -if aquaregia was not destroyed well, it will oxidize Sn2+ to Sn4+ thus Sn2+ will not be available to react with HgCl2. - why HgCl2 is written as such in equations (not Hg2+) ??? because HgCl2 is weakly ionised. - while destroying aquaregia, if evaporation is continued to dryness , mercury will be completely lost ??? because HgCl2 is volatile 16 17