qualitative inorganic analysis

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  • 1. Qualitative Inorganic Analysis

2. Analytical chemistry could be divided into three main parts;qualitative, quantitative and applied. Qualitative analysis deals with detection and identification ofdifferent substances singly or in a mixture.This part deals with the qualitative analysis of anions,which could be defined as the negatively charged fragments of salt or compound. Alternatively anions refer to acid radical. An example isNacl: NaClNa+ +Cl - CationAnionBasic radicalAcid radical 3. Anions are divided into six groups: 1- Carbonates and Bicarbonates group 2- Sulphur-containing anions 3- Halides 4- Cyanogen anions 5- Arsinic and phosphorous containing anions 6- Nitrogen- containing anions 4. Carbonates and Bicarbonates group CO 3 2- HCO 3 - I. General characters 1- Parent acid: Carbonic acid (H 2 CO 3 ) is a very weak volatile acid(stronger than HCN and boric acid)Heating of solution of H 2 CO 3 , CO 2will evolve.H 2 CO 3 CO 2+H 2 O Bicarbonates are considered to be the first step of ionization of carbonic acid, while in the second step carbonates are formedH 2CO 3 H ++ HCO 3 - H ++ CO 3 2- 5. 2-Solubility: All carbonated with the exception of those of the alkali metals(Na +and K + ) and of ammonium are insoluble in water.All bicarbonates are soluble in water.II. General Reactions 1- Dry Reactions a- Action of dilute HCl Decomposition with effervescence due to the evolution of CO 2gas, for both CO 3 -- and HCO 3 - CO 3 --+ 2H + CO 2 + H 2 O This is a type of displacement reaction in which stronger acid liberates the very weak carbonic acid, which spontaneously decomposes to CO 2& H 2 O. NaHCO 3 + H +CO 2 + H 2 O + Na + 6. Test for CO 2gas: The solid substance is placed in a test tube, dilute HCl isadded, which immediately displaced the gas, which is evolved (upon warming) and passed into lime water or baryta water contained in another test tube. The production of a turbidity indicates the presence of carbonates or bicarbonates. CO 2+ Ca(OH) 2 CaCO 3+ H 2 O CO 2+ Ba (OH) 2 BaCO 3+ H 2 O With prolonged passage of CO 2 , the turbidity formed dueto the insoluble carbonates, slowly disappears as a result of the formation of a soluble bicarbonate. CaCO 3+ CO 2+ H 2 OCa (HCO 3 ) 2 Boiling 7. 2- Wet Reactions In order to carry out the wet reactions, a solution of the substancein water must be done. Bicarbonates are mostly decomposed on heating with the liberation of CO 2 . 2HCO 3 - CO 3 -- + H 2 O + CO 2 . a- Reaction with AgNO 3 A white precipitate of silver carbonate is immediately formed. CO 3 --+2Ag + Ag 2 CO 3 The precipitate is soluble in mineral acids (nitric acid) and in ammonia. Ag 2 CO 3+ 2H + 2 Ag ++ CO 2+ H 2 O Ag 2 CO 3 +4NH 3 2[Ag (NH 3 ) 2 ] ++ CO 3 2- The precipitate becomes yellow or brown if the mixture is boiled. Ag 2 CO 3 Ag 2 O +CO 2 boiling 8. b- Reactions with BaCl 2 , CaCl 2and MgSO 4 : White precipitates of BaCO 3 , CaCO 3and MgCO 3will be obtained upon the addition of these reagents to samples of carbonate solution. BaCl 2+ NaCO 3 BaCO 3+ 2 NaClCa +++ CO 3 -- CaCO 3 Mg +++ CO 3 -- MgCO 3 The precipitate is soluble in mineral acids For HCO 3 -; No ppt. on cold since all bicarbonates are soluble in water Ba +++2HCO 3 - Ba(HCO 3 ) 2 Soluble H 2 O + CO 2 + BaCO 3 Boiling 9. III. Mixture of CO 3 2-& HCO - 3 Both anions haves similar reactions, but CO 3 2-form precipitatesimmediately on cold upon the addition of CaCl 2 , BaCl 2or MgSO 4 , while the bicarbonates of these metals are soluble. Separation: Add excess CaCl 2(BaCl 2or MgSO 4 ) to a solution of the mixtureCO 3 2-/HCO 3 -a white ppt. indicates CO 3 --,centrifuge or filter ContrifugateWhite ppt.May be HCO 3 - CaCO 3 2- H + CO 2+ H 2 O Confirmatory test:1) Boil 2) Add ammonia solutionwhite ppt. Ca (HCO 3 ) 2+ 2 NH 3CaCO 3 + (NH 4 ) 2CO 3 10. Sulphur-containing anions This group of anions, are; 1- Sulphide (S 2- )2- Sulphites (SO 3 2- ) 3-Thiosulphate (S 2 O 3 2- ) 4- Sulphates (SO 4 2- )5- Perasulphate (S 2 O 8 2- ).I. General characters 1- Parent Acids: a- Hydrogren sulphide or Hydrosulphuric acid (H 2 S) It is a gas with offensive rotten egg odour and poisonous. In solution it gives a weak acid, which ionizes in two steps;H 2 SH + + HS -(hydrosulphide ion) HS - H + + S --(sulphide ion)Both HS -and S --ions give the same reactions. 11. b- Sulphurous acid:(H 2 SO 3 ) This acid is only known in solution (like H 2 CO 3 ). It has moderate strongacidity. Like H 2 CO 3in water; present in equilibrium as follows:H 2 SO 3 H + + HSO 3 - H + + SO 3 -- H 2 O + SO 2 heat Acid sulphite c- Thiosulphuric acid: (H 2 S 2 O 3 ) It is not known in the free form, and decomposes to give, H 2 O, SO 2and S. It's more stronger than sulphurous acid in solutions. It consists of SO 3 2-solution and S, which upon boiling givesS 2 O 3 2- .d- Sulphuric acid: (H 2 SO 4 ): It's a colourless oily liquiud (B.P. 330 0 C).General properties of H 2 SO 4 1- Acid properties;It is one of the strongest acids, ionize in dilute solutions in two steps,H 2 SO 4 H + + HSO 4 -(hydrogen sulphate) HSO 4 - H + + SO 4 -- (sulphate) 12. Metals can liberate hydrogen from H 2 SO 4solution. H 2 SO 4 + Zn o ZnSO 4 + H 2Being a strong acid can replace weak acids like, boric acids,hydrocyanic acid and volatile acids or their decomposition products due to its high B.P. 2NaCl + H 2 SO 4 Na 2 SO 4 + 2HCl2- Dehydrating properties; Conc. H 2 SO 4has a great tendency to combine with water to from stable hydrates H 2 SO 4 .x H 2 O. So it is used as a dehydrating agent forcertain substance, and used mostly in the dissectors. It causes charring for certain organic substances as sugars due to the vigorous abstracting of water from theses substances.3- Oxidizing properties: It's considered to be as moderately strong oxidizing agent when heated with most reducing agents H 2 SO 4H 2 O + SO 2+ [O] It is reduced to SO 2 , while with active reducing agents it may bereduced to S oor H 2 S. heat 13. 2-Solubility: All Na + , K +and NH 4 +salts of sulphur containing anions are soluble in water. Sulphides :Other sulphides are in-soluble except those of Ca ++ ,Ba ++ , & Sr 2+dissolve due to hydrolysis.Sulphites:Other sulphites are all in-soluble. Thiosulphates:Most S 2 O 3 2- are soluble, Ag + , Pb ++ , Hg 2+& Ba ++saltsare slightly soluble. Sulphates:All sulphates are soluble except Pb ++ , Ba ++and Sr ++ .Ca ++& Mg ++salts are slightly soluble. 14. 3-Complexing agent: Thiosulphate form complex with Fe 3+ Fe 3+ + 2S 2 O 3 -- (Fe(S 2 O 3 ) 2 ) -purple color 4-Reducing agent: Sulphides, sulphites and thiosulphates are reducing agents. They reduce solutions of I 2 , KMnO 4and K 2 Cr 2 O 7with varying activities in acidified solutions. I 2 +S 2- 2I - +S o lodine (brown)ColourlessH + 2KMnO 4 + 5S 2- + 16H + 2Mn ++ + 5SO 4 -- + 8H 2 O +2K + 15. I 2 +SO 3 2- +H 2 OSO 4 2- +2I - +2H + 2 MnO 4 - + 5 SO 3 -- + 6H + 2Mn ++ + 5SO 4 -- + 3H 2 O Cr 2 O 7 -- + 3SO 3 2- + 8H + 2Cr 3+ + 3SO 4 -- +4H 2 O I 2 +2S 2 O 3 --H+S 4 O 6 2- +2I - Tetrathionate Fe 3+ +2S 2 O 3 2- S 4 O 6 2- +Fe 2+ H + 8MnO 4 - + 5 S 2 O 3 -- + 14H + 8Mn ++ +10SO 4 -- +7H 2 O 4Cr 2 O 7 2- + 3S 2 O 3 2- + 26H + 8 Cr 3+ +6SO 4 -- + 13 H 2 O 16. II. General Reactions 1- Dry Reactions a- Action of dilute HCl

  • Sulphide; S 2-

H 2 S gas; evolved upon adding dil. HCl to a solid sample.The gas evolved has its characteristic rotten egg odour, and could beidentified by 1- blackening of filter paper moistened with lead acetate sol. S --+ 2H + H 2 S 2- alternatively, a filter paper moistened with cadmium acetate solution,turns yellow H 2 S + Cd ++ CdSYellow H 2 S has reducing character,It reacts with l 2solution, acid KMnO 4 ,acid K 2 Cr 2 O 7 H 2 S+Pb ++ PbSblack 17. It bleaches the brown color of l 2solution, changes the pink color ofacid KMnO 4into colorless and changes the orange color of acidK 2 Cr 2 O 7into green. H 2 S + l 2 2l -+ 2H ++S o 5H 2 S + 6H ++2 MnO 4 - 2Mn +++ 8H 2 O + 5S o 3H 2 S + 8H ++ Cr 2 O 7 -- 2Cr 3++ 7H 2 O + 3S o 2- Sulphite: SO 3 2- Upon treatment of SO 3 -- with dil. HCl, SO 2gas will evolve, due to thedecomposition of the liberated unstable H 2 SO 3 SO -- 3+ 2H + H 2 SO 3 SO 2 +H 2 O The evolved SO 2gas has a characteristic bunt sulphur odor and turbid lime water (like CO 2 ) due to the formation of the insoluble CaSO 3whichis soluble upon prolonged passage of SO 2due to the formation of soluble calcium bisulphite. Ca (OH) 2+SO 2 CaSO 3+ H 2 O 18. CaSO 3+ SO 2+ H 2 OCa(HSO 3 ) 2 . SO 2like H 2 S has reducing character, bleaches the brown color of iodine, reacts with acid KMnO 4and acid K 2 Cr 2 O 7 . l 2+ SO 2+ H 2 OSO 3+ 2H + + 2l - 2 MnO 4 -+ 5 SO 2+ 6H + 2Mn +++ 5SO 3+ 3H 2 O Cr 2 O 7 2-+3 SO 2+ 8H + 2Cr 3+ + 3SO 3+ 4H 2 O3- Thiosulphate; S 2 O 3 2- No immediate change on cold, but on warming with dil. HCl or standing, the solution become turbid due to the liberated yellow colloidal sulphur with evolution of SO 2gas. This is due to the decomposition of theproduced unstable thiosulphuric acid.S 2 O 3 --+ 2H + H 2 S 2 O 3 H 2 O +SO 2+ S o Thiosulphate has the same action of sulphite with HCl in addition toformation of yellow colloidal precipitate. 19. 4- Sulphate: SO 4 2- No reaction with dil. HCl. 2- Wet Reactions a- Reaction with BaCl 2 :Add BaCl 2reagent to neutral sample solution: 1- S 2-: No visible reaction 2- SO 3 2-: White ppt. of BaSO 3is formed which is soluble in dil. HCl. Ba ++ + SO 3 2- BaSO 3 3- S 2 O 3 --: No ppt. in dilute solution, but a ppt. is formed from veryconcentrated solution.4- SO 4 --: A white ppt. of BaSO 4is formed which is insoluble in dil. HCl,even upon boiling. Ba ++ + SO 4 -- BaSO4White 20. b- Reaction with AgNO 3 :Add AgNO 3reagent to the neutral sample solution 1- S 2-: a black ppt. of Ag 2 S is formed which is soluble in hot dil. HNO 3 ,insoluble in ammonia and KCN solution 2 Ag + + S -- Ag 2 S black 2- SO 3 2- : A white crystalline ppt. of Ag 2 SO 3is formed, which on boiling withwater un

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