Qualitative Analysis of Cations Past Paper 2024 PDF

Qualitative Analysis of Cations (Basic Radicals) Cations classified into six groups. Each group has a common reagent which can be used to separate them from the solution for further examination. The group reagents used for the classification of most common cations are: 1.Hydrochlor...

Qualitative Analysis of Cations (Basic Radicals) Cations classified into six groups. Each group has a common reagent which can be used to separate them from the solution for further examination. The group reagents used for the classification of most common cations are: 1.Hydrochloric acid (HCl) 2.Hydrogen sulphide (H2S) (in acid medium) 3.Ammonium chloride (NH4Cl)+ Ammonium hydroxide 4.Hydrogen sulphide (H2S) (in Basic medium) 5.NH4Cl + NH4OH + Ammonium carbonate (NH4)2CO3 6.Has no specific reagent 4- If barium has first been precipitated as chromate, Sr2+ can also be separated from Ca2+ by treatment of the acetate salts of Sr and Ca with excess conc. of (NH4)SO4 in faintly ammoniacal solution, Ca2+ forms the soluble (NH4)2[Ca(SO4)2] or by using saturated solution of Ca SO4. SrSO 4 (NH4)2[Ca(SO4)2] white ppt soluble Applications  ii. Determination of cations 1. Determination of iron as Fe2O3  Iron must be present in the form of Fe3+, i.e. if Fe2+ is present it must be oxidised to Fe3+ by boiling with concentrated HNO3.  Solution of Fe3+ is precipitated as hydrous oxide using slight excess of NH4OH (NH3 + H2O), washed with NH4NO3, dried, ignited apart from filter paper and weighed as Fe2O3. 2 Fe3+ + 6NH3 + (X + 6H2O) FeCl3. XH2O + 6 NH4+ ignition Fe2O3 + XH2O Fe2O3. XH2O 2M.wt.ofFe3 Salt C.F = M.wt.ofFe2 O3 N.B. 1. Avoid washing of Fe2O3. X H2O with NH4Cl otherwise some of Fe2O3 is partially converted to FeCl3 which is lost by volatilization during drying. Fe2O3 + 6 NH4Cl 2 FeCl3 + 6NH3 + H2O 2. Iron can be precipitated using the organic precipitant cupferron and finally ignited and weighed as Fe2O3. Or precipitated by 8-hydroxyquinoline (oxine) as oxinate. 3. Organic hydroxy compounds e.g. citrate or tartarate must be absent as it form soluble complexes with iron. 2. Determination of aluminium as Al2O3 Al3+ is precipitated as Al(OH)3 with ammonia in presence of NH4Cl or M.R. NH4Cl or M.R. AlCl3 + 3 NH4OH Al (OH)3 + 3 NH4Cl (3NH3 + 3 H2O) The precipitated Al(OH)3 is washed with either NH4NO3 or NH4Cl, filtered, dried, ignited with the filter paper and weighed as AL2O3. N.B. 1. The use of NH4Cl or M.R. is to control the pH of the medium to avoid dissolution of the amphoteric Al(OH)3 in excess NH4OH. 2. Al3+ can be precipitated as aluminium oxinate at pH 4-10 using 8- hydroxyquinoline reagent.  3. Determination of Fe3+/AL3+mixture  The mixture is treated with excess NaOH, where non- amphoteric Fe(OH)3 is precipitated and the amphoteric Al(OH)3 is completely dissolved in the form of AlO2-. 2 Fe3+ + 3 NaOH Fe (OH)3 + 3Na+ Al3+ + 3 NaOH Al (OH)3 + 3Na+ Al(OH)3 + OH- AlO2 + 2H2O  The residue Fe(OH)3 is determined as mentioned under determination of iron.  AL3+ is determined in the combined filtrate and washing after acidification where AL3+ is produced and determined as mentioned under determination of AL3+