Analytical Chemistry PHCMp 101 Anions Identification Lab 7

Carbonates/Bicarbonates Sulpher anions Anions Part I Carbonates/Bicarbonates Sulpher anions

CO32-/HCO3- Carbonates/ Bicarbonates   CO32- Carbonates HCO3- Bicarbonates Dry reaction Effervescence occurs due to the evolution of CO2 gas Na2CO3 +HCl  NaCl+H2CO3 H2CO3  CO2+H2O CO2 is colorless & odorless, it can be identified by turning lime water turbid. The turbidity disappears upon prolonged passage of CO2.  Solid + dil. HCl

CO32- Carbonates HCO3- Bicarbonates   CO32- Carbonates HCO3- Bicarbonates Wet reactions White ppt. of BaCO3 soluble in mineral acids No ppt on cold due to the formation of soluble Ba(HCO3)2. By heating or adding NH4OH it gives white ppt. of BaCO3. Ba(HCO3)2+2NH3  BaCO3 + (NH4)2CO3 Ba(HCO3)2 ∆ BaCO3 + CO2+H2O Salt Solution (S.S) +BaCl2 (or CaCl2) S.S + AgNO3 White ppt. of Ag2CO3 which turns black by heating due to the formation of Ag2O Ag2CO3  Ag2O +CO2 No ppt. on cold. Theo: How to diff bet. Carbonate and bicarbonate??

Ex: HCl + Na2CO3  2NaCl + H2CO3 Notes: All carbonate salts are insoluble in water; Exceptions: Na2CO3, K2CO3, (NH4)2CO3 soluble in water. All bicarbonate salts are soluble in water. Displacement reaction: Stronger acid displaces the weaker acid in its salt Ex: HCl + Na2CO3  2NaCl + H2CO3 N.B: The decomposition of carbonates and bicarbonates by dil HCl is a displacement reaction. Any acid stronger than H2CO3 will displace it, thus acetic acid will decompose carbonates. Acid Ka Acetic acid CH3COOH Ka = 1.76x10-5 Stronger Carbonic acid H2CO3 Ka = 4.3x10-7 Hydrocyanic acid HCN Ka = 4.79x10-9 Weaker Boric acid H3BO3 Ka = 5.8x10-10 Weaker No efferves.

Sulpher anions S2- Sulphide SO32- Sulphite S2O32- Thiosulphate   S2- Sulphide SO32- Sulphite S2O32- Thiosulphate SO42- Sulphate Dry reaction H2S(g) S2-+2H+  H2S SO2(g) SO32-+2H+  H2SO3 H2SO3  SO2+H2O SO2(g) S2O32- + 2H+  H2S2O3 H2S2O3  SO2 + H2O +S -ve Solid + dil. HCl It has offensive rotten egg odor. NB: It has reducing property: Changes a paper moistened with acidified dichromate from orange to green It has burnt sulfur odor. NB: It has a reducing property: Changes a paper moistened with acidified dichromate from orange to green. On standing or heating SO2 evolves and soln becomes turbid  Burnt sulfur odor + yellow ppt. of colloidal sulphur HCL is weaker than H2SO4 HCl is stronger than H2S,H2SO3 & H2S2O3 This test can differentiate all sulpher anions

Turns yellow brown  black ppt of Ag2S is formed   S2- SO32- S2O32- SO42- Wet reactions Black ppt. of Ag2S White ppt. of Ag2SO3 which on boiling undergoes self oxidation and reduction producing grey ppt. of metallic Ag 2Ag++ SO32- Ag2SO3 2Ag2SO3 ∆ 2Ag +Ag2SO4+SO2 White ppt. of Ag2S2O3 Turns yellow brown  black ppt of Ag2S is formed 2Ag++ S2O32-Ag2S2O3 Ag2S2O3+H2O  Ag2S +2H++SO42- White ppt of Ag2SO4 (from concentrated solutions) S.S + AgNO3

S.S + acidified K2Cr2O7 soln. S.S + acidified KMnO4 soln.   S2- SO32- S2O32- SO42- S.S + BaCl2 No ppt. White ppt. of BaSO3 soluble in dil HCl No ppt. (except from concentrated solutions) White ppt. of BaSO4 insoluble in dil HCl S.S + FeCl3 Black ppt. of Fe2S3 A dark red color of Fe2(SO3)3 which turns reddish brown on boiling Purple color of complex ferric thiosulphate Fe(S2O3)2- fades rapidly and disappears by boiling S.S + acidified I2 soln. Decolorzation of I2 and soln turns turbid Decolorization of I2 -ve S.S + acidified K2Cr2O7 soln. Color of K2Cr2O7 is changed from orange to green S.S + acidified KMnO4 soln. Decolorization of KMnO4 Theo Q.!