Lecture 2 Analytical Chemistry (Qualitative Analysis) PDF

Lecture 2 Analytical Chemistry (Qualitative Analysis) BY Mona Alkhateb Lecturer of Analytical Chemistry Faculty of pharmacy – October 6 University  Group III Halides Fluoride (F-) Chloride...

Lecture 2 Analytical Chemistry (Qualitative Analysis) BY Mona Alkhateb Lecturer of Analytical Chemistry Faculty of pharmacy – October 6 University  Group III Halides Fluoride (F-) Chloride (Cl-) Bromide (Br-) Iodide (I-) I. General Charaters F- , Cl- , Br- and I- → known as halides and characterized by their higher electronegativity. As the ionic size increases → the tendency to lose electrons increases → therefore iodide ion is firstly and easily oxidized into free iodine (I2) followed by bromide (Br-) when present in a mixture. Fluoride ions (F- ) are highly stable to held strongly a proton → So it’s difficult to oxidize Fluoride (F- ) into Fluorine (F2), hence. The order of stronger reducing agent is from HI  HBr  HCl  HF. II. General Properties 1- Parent acid: F- Cl- Br- I- Fluoride Chloride Bromide Iodide HF HCl HBr HI Parent acid Hydrofluoric acid Hydrochloric acid Hydrobromic acid Hydroiodic acid They all are irritant and toxic gases Highly soluble in water forming very strongly acidic solutions → Except HF Properties produces the weakest acidic solution in the halogen acid series. HI solution → turns brown on standing due to the liberated iodine (I2). II. General Properties 2- Solubility (Wet Reaction): All the salts of CI-, Br- and I- are soluble → except Ag+, Hg22+, & Cu2+ salts are insoluble. Their lead salts (Pb2+) are slightly soluble in cold water, soluble in hot water. All the salts of F- are insoluble or sparingly soluble → except alkali metal salts, ammonium and silver salts are soluble. II. General Reactions 1- Dry test: A- Action of dilute HCl: No reaction with all halides even on heating → This reaction can differentiate carbonate and sulphur group from halides. B- Action of concentrated H2SO4: The halogen acid gases are evolved according to following equation;. 2X-+ H2SO4 2 HX + SO42- X = may be CI-, I-, Br- and F- The gases can be recognized by; a) The fumes evolved b) Confirmatory chemical test B- Continue action of concentrated H2SO4: F- Cl- Br- I- Fluoride Chloride Bromide Iodide HF  HCl  HBr  + Br2 HI + I2 Irritant, corrosive and itching Irritant colorless Brown fumes gas with pungent Violet fumes colorless gas which fumes with moist air odor ❑ Conc. H2SO4 ❑ Conc. H2SO4 HCl Identification: oxidize HBr to free oxidize HI to free Test for HF: Br2 → in the same I2 → in the same ❑ Exposing a drop of water held by a ❑ Formation of white time H2SO4 is time H2SO4 is glass rod to HF gas → appearance reduced to into SO2 fumes with moist air. reduced to into of oily gelatinous mass due to; ❑ Formation of white SO2, S0 and H2S 2 Br-+ H2SO4 2 HBr + SO42- clouds of NH4CI when ❑ Reaction of HF with glass (SiO2) 2 I-+ H2SO4 2 HI + SO42- a glass rod moistened 2 HBr + H2SO4 Br2 + SO2+ 2 H2O (Silicon dioxide) to give SiF4 (Silicon 2 HI + H2SO4 2 I2 + SO2 + 2 H2O with NH4OH solution is tetrafluoride) 6 HI + H2SO4 3 I2 + S + 4 H2O exposed to the evolved 8 HI + H2SO4 4 I2 + H2S + 4 H2O gas. ❑ Water hydrolyzes SiF4 to give Test for HBr: Test for HI: H2SiO3 (silicic acid) and H2SiF6 NH4OH + HCI NH4CI + H2O (hydrofluorosilicic acid) ❑ Turn starch-iodide ❑ Turn starch 2 F-+ H2SO4 2HF  +SO4 2- SiO2 paper into blue SiF4+ 2H2O paper into blue. H2SiO3 + 2 H2SiF6 H2O 2- Wet reactions: To carry out the wet reactions, a solution of the substance in water must be done (Salt Solution). Reactions: 1- Reaction with AgNO3 2- Reaction with BaCl2 3- Chlorine water test  1- Reaction with AgNO3 Steps : S.S + dil.HNO3 + AgNO3 F- Cl- Br- I- Fluoride Chloride Bromide Iodide AgCl AgBr AgI AgF White ppt. Yellowish–white ppt. Yellow ppt. Soluble (Creamy ppt) Soluble in dil. No ppt. since ammonia solution to AgF is soluble give the ammine Sparingly soluble in dil. in water complex. Insoluble in dil. ammonia solution but Ag++ CI- AgCI ammonia solution soluble in Conc. but very slightly AgCI + 2NH3 [Ag(NH3)2]CI ammonia solution. soluble in Conc. Silver amine chloride ammonia solution. Ag++ Br- AgBr AgBr + 2NH3 [Ag(NH3)2]Br Ag++ I- AgI AgCI is reprecipitated upon Silver amine bromide treatment of the ammine complex with acid. [Ag(NH3)2]CI + 2H+ 2 NH4++ AgCI  Note: All Silver halides are insoluble in dil HNO3 ??? ❑ Because their parent acids (HI, HBr and HCl) are stronger than HNO3 Order of acids: H2SO4 > HI > HBr > HCl > HNO3 > …… All Silver halides are soluble in : ❑ Cyanide (CN-) → Due to the formation of soluble argento cyanide complex AgCN + CN- (Ag (CN)2)- Soluble complex ❑ Thiosulphate (S2O32-) → Due to the formation of soluble silver thiosulphate complex AgX + S2O32- (Ag(S2O3-)2)-3 + X-  Q) Why does a precipitate dissolve in form of a complex Ksp Kinstability (Solubility product (Instability complex constant) constant) Ppt Kinst → Ag+ conc Formation ppt Product solubility Kinst → Ag+ conc No ppt Ksp → ppt Ksp → soluble (complex formation) High Ksp 𝐚𝐧𝐝 𝐥𝐨𝐰 Kinstability constant → 𝒕𝒉𝒆 𝒑𝒑𝒕. 𝒅𝒊𝒔𝒔𝒐𝒍𝒗𝒆𝒔 Low Ksp 𝐚𝐧𝐝 𝐡𝐢𝐠𝐡 Kinstability constant → 𝒕𝒉𝒆 𝒑𝒑𝒕. 𝐰𝐢𝐥𝐥 𝐟𝐨𝐫𝐦𝐞𝐝 (𝐝𝐨𝐞𝐬 𝐧𝐨𝐭 𝒅𝒊𝒔𝒔𝒐𝒍𝒗𝒆𝒔) 2- Reaction with BaCl2 F- Cl- Br- I- Fluoride Chloride Bromide Iodide BaF2 White gelatinous ppt. Ba2++ 2F- BaF2 No ppt Partially Soluble in dil. HCl or HNO3 Insoluble in acetic acid  3- Chlorine water test F- Cl- Br- I- Fluoride Chloride Bromide Iodide Chlorine water oxidizes I- and Br- into I2 and Br2 which can be extracted with chloroform or carbon tetrachloride No Rx Iodide react first with chlorine water before bromide as it has more reducing character Orange or yellow or brown color Pink or violet color The color is due to liberation of Br2 in The color is due to chloroform layer. liberation of I2 in chloroform layer The color disappears on excess chlorine Excess chlorine water water due to the formation of bromic acid oxidize I2 to colorless (HBrO3). iodic acid (HIO3) exx. CI2 2Br-+ CI2 Br2+ 2CI- 2 HBrO3 exx. CI2 colorless 2I-+ CI2 I2+ 2CI- 2 HIO3 colorless  Special tests for Halides 1- Fluorides (F-): A- Boron fluoride test : Fluoride + borax + conc. H2SO4 → give HF + boric acid → they react to produce boron fluoride gas (BF3) → the gas turns the Bunsen flam into green Na2B4O7+ H2SO4+ 5H2O 4H3BO3+Na2SO4 Borax Boric acid 2NaF + H2SO4 2HF + Na2SO4 Sodium fluoride Hydrofluoric acid H3BO3+ 3HF BF3  + 3H2O boron fluoride gas  Special tests for Halides 2- Chlorides (Cl-): A- Chromyl chloride test (Characteristic for Cl-): In NaOH This test is a specific test for chloride even in the presence of other halides. It's classified as a dry reaction. In T.T: Solid Cl- + potassium dichromate (Cr2O72-) + Conc. H2SO4 → Brown to red fumes (brownish red fumes) of chromyl chloride (CrO2CI2) are evolved → attach the T.T to another tube by a pent tube dipped into a NaOH solution → formation of yellow ppt. of sodium chromate (CrO42-) 4CI- + Cr2O72- + 6H+ 2CrO2 Cl2  + 3H2O Red fumes CrO2CI2  + 4OH- CrO42- + 2CI- + 2H2O Yellow ppt A- Continue Chromyl chloride test: Confirmation for chromate: 1- Acidify the solution with acid + add lead acetate → yellow ppt. of lead chromate (PbCrO4). CrO42- + Pb2+ PbCrO4 Yellow ppt 2- Perchromic acid test (CrO83-): Acidify the solution with dil. H2SO4 + add 1-2 ml alcohol or ether + a little H2O2 solution → The organic layer is colored blue 2 CrO42- + 2H+ Cr2O72-+ H2O Cr2O72- + 7H2O2 2 CrO83-+ 5H2O + 4H+ Perchromic acid Blue color in organic layer  No Interference 1- Bromides and iodides → do not interfere → give rise to the free halogens (Br2 and I2), → which yield a colorless solution with NaOH 6 Br-+ Cr2O72-+ 14H+ 2 Cr3+ + 3Br2 + 7H2O 6 I-+ Cr2O72- + 14H+ 2Cr3+ + 3I2 + 7H2O Br2+ 2OH- OBr- + Br- + H2O (hypo bromite) I2+ 2OH- OI- + I- + H2O (hypo iodite) 2- Cyanides (CN-) do not interfere with the test 3- Iodides (I-): 1- lodide is oxidized in acid solution (dil. H2SO4) with nitrite solution or H2O2 or FeCl3 into free l2. 2I- + 2NO2- + 4H+ I2+ 2NO + 2H2O 2I-+ H2O2+ 2H+ I2+ 2H2O 2- I- reacts with cupper sulphate (Cu2+) → forming a white ppt. of cupper iodide (Cu2I2) + the I- is oxidized to free I2 → Thus a white ppt. in brown solution is formed on treating I- with CuSO4 solution. 2Cu2+ + 4I- Cu2I2  + I2 White ppt. in brown solution 3- I- reacts with mercuric chloride solution (HgCl2) → a yellow-scarlet red ppt. of mercuric iodide (HgI2) will be formed → dissolves in excess iodide forming soluble colorless complex. Excess I- HgCI2+ 2I- HgI2 + 2CI- (HgI4)2- Yellow-Scarlet red ppt. Soluble complex Nessler reagent  Group IV Cyanogen OR Cyanide containing anions Thiocyanate Cyanide (CN-) (SCN-) Ferrocyanide Ferricyanide [Fe(CN)6]4- [Fe(CN)6]3- Has mild reducing effect Has oxidizing effect All cyanogen anions are highly poisonous → All experiments should be carried out cautiously in the fume cupboard.  CN- SCN- [Fe(CN)6]4- [Fe(CN)6]3- Cyanide Thiocyanate Ferrocyanide Ferricyanide HCN HSCN H4[FeCN)6] H3[FeCN)6] Parent acid Hydrocyanic acid Thiocyanic acid Ferrocyanic acid Ferricyanic acid It is a colorless volatile It is a colorless toxic liquid with White crystalline Brownish poisonous gas with unpleasant odor. solid. crystalline solid. bitter almonds odor. Imp. They soluble in water to give strongly acids solution. Characters On passing CO2 to CN- solution → HCN is produced with HCO3-. CN-+ CO2+ H2O HCN + HCO3- I. General characters II. General Properties 1- Solubility: CN- → All cyanides are insoluble in water except alkali metals (Na+, K+), ammonium salt, alkaline earth metals ( Ba2+, Sr2+ and Ca2+) and mercuric cyanide SCN- → All thiocyanates are soluble in water except Ag+, Hg2+2 & Cu2+2. Pb (SCN)2 (same as PbCI2) is sparingly soluble in cold water, but soluble in hot water Ferro and Ferricyanide → All are insoluble in water except those of alkali metals, ammonium salt and alkaline earth metals.  Analysis of Cyanide-containing anions III. General Reactions 1- Dry reaction: A- Action of dilute HCl: 1- For CN-: Evolution of hydrogen cyanide gas (HCN) → with characteristic bitter almond odor. 2- For SCN- , Ferrocyanide and Ferricyanide: No reaction.  Confirmatory Tests for HCN gas 1- Convert HCN into SCN- (Iron thiocyanate test) : HCN gas + ammonium polysulphide → formation of SCN- → add dil. HCI + a drop of FeCI3 solution → blood red color is produced 2- Using AgNO3 solution: Pass HCN gas into AgNO3 solution → a white ppt. of silver cyanide (AgCN) is formed → soluble in ammonia solution. 2NH3 HCN + AgNO3 AgCN (Ag(NH3)2)CN White ppt. Soluble 3- Prussian blue test: Pass HCN gas into NaOH solution + add drops of FeSO4 → heat to boiling → the HCN is converted into ferrocyanide → add FeCl3 solution → Prussian blue ppt is produced. 2- Wet reactions: Reactions: 1- Reaction with AgNO3 2- Reaction with BaCl2 3- Reaction with FeCl3 4- Reaction with Co(NO3)2 No observed results for all the Cyanogens with BaCl2.  1- Reaction with AgNO3 CN- SCN- [Fe(CN)6]4- [Fe(CN)6]3- Cyanide Thiocyanate Ferrocyanide Ferricyanide AgCN AgSCN Ag4[Fe(CN)6] Ag3[Fe(CN)6] white ppt white ppt white ppt Orange red ppt Soluble in excess CN-, Ag++ SCN-  AgSCN Insoluble in dil. HNO3 Insoluble in dil. HNO3 ammonia solution, but and dil. ammonia. but soluble in dil. Silver thiocyanate insoluble in dil. HNO3. ammonia 4 Ag++ [Fe(CN)6]4-  Ag4[Fe(CN)6] 3 Ag++ [Fe(CN)6]3-  Ag3[Fe(CN)6] Ag++ CN-  AgCN Silver Silver cyanide Silver Ferrocyanide Ferricyanide Exx. CN- (Ag(CN)2)- Soluble complex H+ HCN + AgCN  2- Reaction with FeCl3 V. important; differentiating test. CN- SCN- [Fe(CN)6]4- [Fe(CN)6]3- Cyanide Thiocyanate Ferrocyanide Ferricyanide Fe (CN)3 Fe(SCN)3 Fe4[Fe(CN)6]3 Fe [Fe(CN)6] white ppt Blood red color Prussian blue ppt Brown color Soluble in excess CN- forming ferricyanide. 3[Fe(CN)6)4-+ 4Fe3+ Fe4[Fe(CN)6]3 Fe3++ [Fe(CN)6]3- Fe[Fe(CN)6] Prussian blue ppt Brown color Fe3++ 3 CN- Fe (CN)3 Iron (III) cyanid Exx. CN- [Fe(CN)6]3- Ferricyanid  Precautions for the blood red color of SCN- with FeCl3 1- To increase the sensitivity of the test Ensure the presence of iron in the Fe3+ state Acidification of the medium with dil. HCI Cooling of the solution before testing Removal of interfering ions by precipitation or complexation 2- F-, PO43-, oxalate and tartrate bleach the color as they form complex with ferric → Must be absent For example; F- reacts with iron to form stable complex 6 F-+ Fe3+ (FeF6)3- 3- Hg2+ reacts with SCN- → forming unionized Hg (SCN)2 which is colorless. 4- Iodides interferes by being oxidized by Fe3+ into the brown color I2. 2I- + 2Fe3+ H+ I2 + 2Fe2+ 3- Reaction with Co(NO3)2 CN- SCN- [Fe(CN)6]4- [Fe(CN)6]3- Cyanide Thiocyanate Ferrocyanide Ferricyanide Co(CN)2. 2H2O [Co(SCN)4]2- Co2[Fe(CN)6] Co3[Fe(CN)6]2 Buff ppt Blue color in ether layer Greyish green ppt Red ppt Cobaltous cyanide Vogel's Reaction: dihydrate ❑ The reaction of Co2+ with SCN- to produce a characteristic blue color extractable with ether or amyl alcohol; [Fe(CN)6]4- + 2Co2+ 2 [Fe(CN)6]3- + 3Co2+ known as vogel's reaction. Co2++ 4SCN- [Co (SCN)4]2- Extractable with ether (blue) Co3[Fe(CN)6]2 ❑ Steps: Soln. + dil. H2SO4 + SnCl2 + Co2[Fe(CN)6] Red ppt ether + NH4SCN → Blue color in Greyish green ppt Cobalt ferricyanide ether layer Cobalt ferrocyanide  Special tests for Cyanide containing anions CN- SCN- [Fe(CN)6]4- [Fe(CN)6]3- Cyanide Thiocyanate Ferrocyanide Ferricyanide 1- Oxidation test : 1- Reduction test: 1- Prussian blue test: 1- Reduction test: → Convert CN- into ❑ Oxidizes I- into a I2 Depends on the reduction of SCN- ❑ Oxidized to ferrocyanide → add FeCl3 (brown colored) → with metallic zinc and dil. acid → ferricyanide by identified by starch giving H2S and HCN which can be oxidizing agents, or CHCI3. tested such as, MnO4-, NO3, 2- Iron thiocyanate test: H2O2 and CI2 Zno + 2H+ 2 (H) + Zn2+ ❑ CN- + ammonium polysulphide → add FeCl3 → blood red color is 2SCN- +4(H) 2  HCN +  H2S+ S2- 2[Fe(CN)6]4- + CI2 2[Fe(CN)6]3- + 2 I- extractable with ether Ferrocyanide Ferricyanide 2- Vogel’s test ❑ N.B; → if SCN- is present → the CN- must be isolated first by Previously mentioned. 2[Fe(CN)6]3- + 2CI- precipitation e.g. as zinc cyanide 2[Fe(CN)6]4- + 2 I2 Ferricyanide Ferrocyanide 30