Experiment # 9 : Cation Analysis: Group ( I )

Transcription

1 Experiment # 9 : Cation Analysis: Group ( I ) I. PURPOSE OF THE EXPERIMENT The purpose of inorganic qualitative analysis is to enable the acquisition of powers of observation and deduction the learning of reactions of inorganic chemistry the development of practical chemical skills II. INTRODUCTION In many real situation, a chemist must gather observation to decide the chemical identity of an unknown sample. Given a totally unknown sample, how does one go about determining what is actually present? This process is called qualitative analysis. Cations are classified into five groups on the basis of their behavior against some reagents by using group reagents; we can decide presence or absence of groups of cations and can also separate these groups for further examination. The group reagents used for the classification of most common cations are hydrochloric acid, hydrogen sulphide, ammonium sulphide and ammonium carbonate. Classification is based on whether a cation reacts with these reagents by the formation of precipitates or not. Therefore, it can be said that classification of most common cations is based on the differences of solubilities of their chlorides, sulphides, and carbonates. The five groups are as follows: - Group I Cations form precipitates with dilute hydrochloric acid. Ions of this group are Lead, Mercury (I), and Silver Group II.Cations do not react with hydrochloric acid, but form precipitates with hydrogen sulphide in dilute mineral acid medium. Ions of this group are: - II A. Mercury (II), Copper, Bismuth, Cadmium Sulphides of II A are insoluble in (NH 4 ) 2 S 69

2 IIB.Arsenic (III), Arsenic (V), Antimony (III), Antimony (V), Tin (II) and Tin (IV). Sulphides of II B are soluble in (NH 4 ) 2 S. GROUP III. Cations do not react either with dilute hydrochloric acid, or with hydrogen sulphide in dilute mineral acid medium. They form precipitates with ammonium sulphide in neutral or ammonical media. They are Cobalt (II), Nickel (II), Iron (II), Iron (III), Chromium (III), Aluminium, Zinc, and Manganese (II). GROUP IV. Cations do not react with the reagents of group I, II and III. They form precipitates with Ammonium carbonate in the presence of Ammonium chloride in neutral or slightly acidic media. GROUP V. Cations do not react with reagents of the previous groups. They are Magnesium, Sodium, Potassium, Ammonium, Lithium, and Hydrogen ions. III. EXPERIMENTAL 3.1 Chemicals 3.2 Equipment 3.3 Procedure Take 5ml known solution sample in the test tube and follow the instructions accordingly, and report your results in the table given below: Group I No. Reagent Pb 2+ Hg 2+ 2 Ag + 1. Add 3M HCl (a) + NH 3 sol n (or) NH 4 OH sol n. (b) + boil in hot water 2. NaOH sol n. 3. KIsol n (Potassium.Iodide) 4. K 2 CrO 4 sol n. (Potassium Chromate) 70

3 Flowchart for Cation Group I Salt solution mixture 1. Add Dil.HCL and stil well 2. Filter Filterate I (may contain ions of Group II to V) Residue I AgCl (white ppt.) PbCl 2 (white ppt.) Hg 2 Cl 2 (whte ppt.) 1. Wash with a small amount of D. H Boil with water 3. Filter while hot Residue II (AgCl & Hg 2 Cl 2 ) Filterate II (PbCl 2 ) Divide into two portions Add NH 4 OH soln. and filter Residue III HgNH 2 Cl + Hg(black) Filtrate III Ag (NH 3 ) 2 Cl Add K 2 CrO 4 soln. and then add NaOH soln. in excess Add KI soln., boil and cooled down 1. Dissolve in Aqua Regia 2. Dilute the soln. with H 2 O 3. Add SnCl 2 in excess Acidify with Dil. HCl Greyish black ppt. "Hg" white ppt. AgCl Yello ppt. PbCrO 4 dissolve in excess NaOH to give yellowish green soln. Golden yellow PbI 2 crystals confirmed presence of Pb 2+ 71

4 Equations for Cation Analysis : Group ( I ) 1. Ag + ion (Silver Nitrate salt solution) AgNO 3 (aq) + HCl(aq) AgCl (s) + HNO 3 (aq) AgCl(s) + 2NH 4 OH(aq) Ag (NH 3 ) 2 Cl(aq) + 2H 2 O (l) Ag(NH 3 ) 2 Cl + 2HCl(aq) AgCl(s) + 2NH 4 Cl(aq) Ag (NH 3 ) 2 Cl. Silver Ammonium Chloride 2. Hg 2 2+ ion [Mercury (I) Nitrate salt solution] Hg 2 (NO 3 ) 2 (aq) + 2HCl(aq) Hg 2 Cl 2 (s) + 2HNO 3 (aq) Hg 2 Cl 2 (s) + 2NH 4 OH(aq) HgNH 2 Cl(aq) + Hg(s) + NH 4 Cl(aq) + 2H 2 O(l) HgNH 2 Cl(aq) + Aqua Regia HgCl 2 (aq) 2HgCl 2 (aq) + SnCl 2 (aq) Hg 2 Cl 2 (s) + SnCl 4 (aq) Hg 2 Cl 2 (aq) + SnCl 2 (aq) 2Hg(s) + SnCl 4 (aq) (excess) Hg 2 (NO 3 ) 2 Mercury (I) Nitrate HgNH 2 Cl.Mercury (II) amido Chloride Hg 2 Cl 2 Mercury (I) Chloride **Aqua Regia: 3 volumes of concentrated HCl added 1 volume of concentrated HNO Pb 2+ ion [Lead (II) nitrate salt solution] Pb(NO 3 ) 2 (aq) + 2HCl(aq) PbCl 2 (s) + 2HNO 3 (aq) PbCl 2 (s) + K 2 CrO 4 (aq) PbCrO 4 (s) + 2KCl(aq) PbCrO 4 (s) + 4NaOH(aq) Na 2 CrO 4 (aq) + Na 2 Pb(OH) 4 (aq) PbCl 2 (s) + 2KI (aq) PbI 2 (s) + 2KCl(aq) Golden yellow crystal K 2 CrO 4..Potassium Chromate PbCrO 4.Lead Chromate Na 2 CrO 4.Sodium Chromate Na 2 Pb(OH) 4..Tetrahydroxy Sodium Plumbate (Disodium Lead (II) tetrahydroxide) 72

5 IV. RESULTS and DISCUSSIONS The given unknown salt mixture solution is found to be Cation Group (I) (i) Silver ion is confirmed (ii) Mercury (I) ion is confirmed (iii) Lead (II) ion is confirmed Discuss: (a) What is the group reagent for Cation Group (I)? (b) How can you separate Lead ion from Mercury(I) ion and Silver ion? ( c) What is an Aqua Regia? V. CONCLUSION The given unknown salt mixture solution is found to be Silver ion, Mercury (I) ion and Lead (II) ion from Cation Group (I). 73