AUDIO -CHM 102--MODULE 1 --QUALITATIVE ANALYSIS.pptx

a presentation on quantitative analysis

1. CHM 102 INTRODUCTORY CHEMISTRY II MODULE 1 INSTRUCTOR 1 DR. MRS L.M.DUROSINMI

2. COURSE OUTLINE BRIEF INTRODUCTION TO CHM 102 TOPICS

3. THEORY AND TECHNIQUES OF QUALITATIVE INORGANIC ANALYSIS   Qualitative Inorganic Analysis: A method of  analytical chemistry  which seeks to find  elemental  composition of inorganic compounds through various reagents. Deals with careful detection of   ions  in an  aqueous   solution . The solution is treated with various  reagents to test for  reactions  characteristic of certain ions, which may cause color change, formation of ppt or other visible changes.

4. PHYSICAL APPEARANCE OF INORGANIC SALTS . Black ----- MnO , MnO 2 , FeO , CuO , Co 3 O 4 , Ni 2 O 3 ;  Also sulfides   of Ag + , Cu + , Cu 2+ ,Ni 2+ , Fe 2+ , Co 2+ , Pb 2+ , Hg 2+ , Bi 3+ Blue- ------ Hydrated Cu 2+  salts Red ------- HgO , HgI 2 , Pb 3 O 4 Green ------Cr 3+ , Ni 2+ , hydrated Fe 2+  salts

5. PHYSICAL APPEARANCE OF INORGANIC SALTS , contd . Light Pink ------Hydrated Mn 2+  salts Orange --- KO 2 , K 2 Cr 2 O 7 , Sb 2 S 3 ,  Ferrocyanide Reddish Pink ----- Hydrated Co 2+  salts. Yellow ------- Chromates , AgBr , As 2 S 3 , AgI , PbI 2 , CdS ----- CdO , Fe 2 O 3 , PbO 2 , CuCrO 4

6. DETECTING CATIONS Cations  are usually classified into six groups. Each group has a common reagent which can be used to separate them from the solution. To obtain meaningful results, the separation is done in sequence.

7. 1st Group of Cations Most important cations in 1st group are  Ag + ,  Hg 2+ and Pb 2+ . The ions form insoluble  chlorides . Thus, the group reagent to separate them is  hydrochloric acid . 1–2  M . Concentrated HCl must not be used, because it forms a soluble complex ion ([PbCl 4 ] 2− ) with  Pb 2+ . Consequently, the  Pb 2+  ion would go undetected. The chlorides of these elements cannot be distinguished from each other by their colour - they are all white solid compounds. But PbCl 2  is soluble in hot water, and can therefore be differentiated easily. Ammonia is used as a reagent to distinguish between the other two. While AgCl dissolves in ammonia (due to the formation of the complex ion [Ag(NH 3 ) 2 ] + ), Hg 2 Cl 2  gives a black precipitate.

8. 2nd Group of Cations These consists of ions that form acid-insoluble  sulfides . The Cations include: Cd 2+ ,  Bi 3+ ,  Cu 2+ ,  As 3+ , As 5+ ,  Sb 3+ , Sb 5+ , Sn 2+ , Sn 4+  and Hg 2+ . Pb 2+. The reagent used is any substance that gives S 2−  ions in such solutions; most commonly used are  hydrogen sulfide  (at 0.2-0.3 M) in the presence of dilute HCl .     CdS , is yellow. All the precipitates, except for  HgS , are soluble in dilute nitric acid. HgS is soluble only in  aqua regia , which can be used to separate it from the rest.  

9. 2nd Group of Cations The action of ammonia is also useful in differentiating the cations . CuS dissolves in ammonia forming an intense blue solution, whereas CdS dissolves forming a colourless solution. The sulfides of As 3+ , As 5+ , Sb 3+ , Sb 5+ , Sn 2+ , Sn 4+  are soluble in yellow  ammonium sulfide , where they form  polysulphide  complexes.   If it forms a yellow precipitate, then Cd 2+  or Sn 4+  is present; or if it forms a brown precipitate, then Sn 2+  must be present; or if a red orange precipitate is formed, then Sb 3+  is present.

10. 3rd Group of Cations This group of cations  includes ions that form hydroxides which are insoluble even at low concentrations. Cations :  Fe 2+ , Fe 3+ ,  Al 3+ , and  Cr 3+ . The group is determined by making a solution of the salt in water and adding  ammonium chloride  and ammonium hydroxide. Ammonium chloride is added to ensure low concentration of hydroxide ions.  

11. 3rd Group of Cations , contd Formation of a reddish-brown precipitate indicates Fe 3+ ; a gelatinous white precipitate indicates  Al 3+ ; and a green precipitate indicates Cr 3+  or Fe 2+ . These two are distinguished by adding sodium hydroxide in excess to the green precipitate. If the precipitate dissolves, Cr 3+  is indicated; otherwise, Fe 2+  is present.  

12. 4th Group of Cations These include  Zn 2+ ,  Ni 2+ ,  Co 2+ , and  Mn 2+ . Zinc salts are colourless, Manganese salts are faint pink or colourless, and Nickel and cobalt salts may be brightly coloured, often blue-green. The precipitate, washed in water is reacted with dilute hydrochloric acid. This precipitates nickel salts, if any. The supernatant liquid is filtered and reacted with excess of NaOH. This precipitates any Manganese salts. Hydrogen sulphide is passed through the liquid. If a white pptcipitate forms, Zinc is present.

13. 5th Group of Cations ions in the 5th group are  Ba 2+ ,  Ca 2+ , and  Sr 2+ . Ions in 5th analytical group of cations form  carbonates  that are insoluble in water. The reagent used is  (NH 4 ) 2 CO 3   with a neutral or slightly basic pH. All the cations in the previous groups are separated beforehand, since many of them also form insoluble carbonates. To distinguish between these ions, flame test colour s used. : barium gives a yellow-green flame, calcium gives brick red, and strontium, crimson red.

14. 6th Group of Cations Cations left after separating groups 1-5 are considered to be in the sixth group. These are   Mg 2+ ,  Li + ,  Na +  and  K + . All the ions are distinguished by flame color : lithium gives a red flame, sodium gives bright yellow (even in trace amounts), potassium gives violet, and magnesium, colorless (although magnesium metal burns with a bright white flame).  

15. DETECTING ANIONS 1st Group of Anions consists of CO 3 2− , H CO 3 − , CH 3 COO − ,  S 2− , and  NO− The reagent for Group 1 anions is dilute hydrochloric acid ( HCl ) or dilute sulfuric acid (H 2 SO 4 ).   Carbonates give effervescence with dilute H 2 SO 4  due to the release of CO 2 , a colorless gas which turns  limewater  milky due to formation of CaCO 3  ( carbonatation ).   The milkiness disappears on passing an excess of the gas through the lime water, due to formation of Ca (HCO 3 ) 2 .  

16. 1st Group of Anions, contd Sulfides give the rotten egg smell of H 2 S when treated with dilute H 2 SO 4 . The presence of sulfide is confirmed by adding  lead(II) acetate  paper, which turns black due to the formation of PbS . Sulfides also turn solutions of red  sodium nitroprusside  purple. Sulfites produce SO 2  gas, which smells of burning sulfur , when treated with dilute acid. They turn acidified K 2 Cr 2 O 7  from orange to green Nitrites give reddish-brown fumes of NO 2  when treated with dilute H 2 SO 4 . These fumes cause a solution of  potassium iodide  (KI) and  starch  to turn blue.

17. 2nd Group of Anions The 2nd group of anions consist of  Cl − ,  Br − ,  I − , NO− The group reagent for Group 2 anion is concentrated sulphuric acid (H 2 SO 4 ). After addition of the acid, chlorides, bromides and iodides form precipitates with  silver nitrate . The precipitates are white, pale yellow, and yellow, respectively. The silver halides formed are completely soluble, partially soluble, or not soluble at all, respectively, in aqueous ammonia solution.

18. 2nd Group of Anions, contd Chlorides are confirmed by the  chromyl chloride  test . When the salt is heated with K 2 Cr 2 O 7  and concentrated H 2 SO 4 , red vapours of chromyl chloride (CrO 2 Cl 2 ) are produced. Passing this gas through a solution of NaOH produces a yellow solution of  Na 2 CrO 4 . The acidified solution of Na 2 CrO 4  gives a yellow precipitate with the addition of  (CH 3 COO) 2 Pb . Bromides and iodides are confirmed by the  layer test . A sodium carbonate extract is made from the solution containing bromide or iodide, and  CHCl 3  or  CS 2  is added to the solution, which separates into two layers: an orange colour in the  CHCl 3 or CS 2 layer indicates the presence of Br − , and a violet colour indicates the presence of I − . Nitrates give brown fumes with concentrated H 2 SO 4  due to formation of NO 2 .  

19. 3rd Group of Anions The anions consist of SO 4 2− , Phosphate and borate anions; They react neither with concentrated nor diluted H 2 SO 4 .   Sulfates give a white precipitate of BaSO 4  with BaCl 2  which is insoluble in any acid or base. Phosphates give a yellow crystalline precipitate upon addition of HNO 3  and  ammonium molybdate .   Borates give a green flame characteristic of  ethyl borate  when ignited with concentrated H 2 SO 4  and ethanol.