Pharmaceutical Analytical Chemistry I Lecture 10 PDF

Lecture (10) Dr. Ahmed Abdel-Monem Halides  Halides Members: 1- Fluoride ( F- ) 2- Chloride ( Cl- ) 3- Bromide ( Br- ) 4- Iodide ( I- ) Halogens Halides...

Lecture (10) Dr. Ahmed Abdel-Monem Halides  Halides Members: 1- Fluoride ( F- ) 2- Chloride ( Cl- ) 3- Bromide ( Br- ) 4- Iodide ( I- ) Halogens Halides Fluoride (F-) Electronegativity Fluorine (F) Chlorine (Cl) +e Chloride (Cl-) Bromine (Br) Bromide (Br-) Iodine (I) Iodide (I-) - Halides are the anionic forms of halogens. - They are monovalent ions - They are highly electronegative, since their tendency for gaining electrons is very great. - The electronegativity decreases from F- to I-. - The iodide ion is easily oxidized into free I2 by loosing readily an electron. - However, there is no chemical oxidant which is powerful enough to oxidize F- to F2. Hence, the negative fluoride ion is highly too stable to hold strongly a proton. That is why HF is the weakest acid, while HI is the strongest halogen acid ? Anion Fluoride (F-) Chloride (Cl-) Bromide (Br-) Iodide (I-) Parent Acid HF HCL HBr HI Name Hydrofluoric Hydrochloric Hydrobromic Hydroiodic acid acid acid acid Strength Weak Strong Very strong Strongest Color Colorless gas Odour Irritating odour In moist air Fumes in moist air Stability in air Stable Stable Oxidation and Oxidation and formation of liberation of free bromine. iodine. Redox F- is not reducing Cl- is weak Br- is moderate I- is strong reducing properties agent reducing agent reducing agent agent Dry Reaction 1- Action with dil. HCl No reaction even on heating 2- Action with dil. H2SO4 No reaction even on heating 3-Action with Conc. H2SO4 Evolution of halogen acids - 2X + H2SO4 ⎯→ 2HX + SO42- halogen ion halogen acid  3-Action with Conc. H2SO4 Evolution of colorless hydrofluoric acid "HF" gas which is For F- fumes in moist air. Hydrofluoric acid gas is characterized by, - The test tube acquires oily appearance due to corrosive action of the gas on the glass The gas can be tested by, holding a glass rod with a drop of water at its tip in the evolving gas, the tip of the rod will be coated with a white gelatinous coat of silicic acid. - The reaction is explained by the corrosive action on glass. - Silicon dioxide "SiO2" of the glass will react with "HF" producing silicon tetrafluoride "SiF4" which in contact with "H2O" is hydrolysed with the formation of gelatinous silicic acid & hexafluorosilicic acid. 2- 2F- + H2SO4 ⎯→ 2HF  + SO 4 4HF + SiO2 ⎯→ SiF4 + 2 H2O 3 SiF4 + 3H2O ⎯→ H2SiO3 + 2 H2SiF6 Silicic acid hexafluorosilicic acid  Evolution of colorless hydrochloric acid "HCl" gas For Cl- hydrochloric acid "HCl" gas can be tested by i) Its pungent irritating odour ii) Turning blue Litmus paper moistened with water into red iii) It forms white fumes with moist air due to the formation of droplets of HCl. iv) Formation of white clouds of NH4Cl when a glass rod moistened with NH4OH soln. is held near the mouth of the test tube. NH4OH + HCl ⎯→ NH4Cl + H2O  Warm For Br- Evolution of reddish-brown fumes of Br2 gas accompanied with HBr For I- Evolution of violet fumes of I2 gas accompanied with HI N.B.: - HBr & HI are more active reducing agents than HCl. (HI is the most active). Hence, that they are readily oxidized as soon as they are formed, by H𝟐SO4 into free Br2 & I2 respectively, and the sulphuric acid reduced to different products (SO2 ; H2S ; or S) according to the degree of reduction. 4-Action with Conc. H2SO4 + MnO2 Mix the solid of halide with equal amount of MnO2 then add conc. H2SO4 and warm For F- Evolution of hydrofluoric acid "HF" gas - HF has no reducing properties. - It is not affected by MnO2 or other oxidizing agent. For Cl-, Br- & I- Evolution of Cl2 , Br2 & I2 gas respectively Halides react with conc. H2SO4 producing acid of halides that react with MnO2 to produce free halogen. HCl, HBr & HI are reducing agents. (HI is the most powerful and HCl is the least powerful). For this, strong oxidizing agents such as MnO4- & MnO2- and higher oxides (e.g., PbO2)of heavy metals oxidize halides in acid media yielding free halogen The free halogen (Cl2 & Br2 & I2) could be detected by: Characteristic color of Br2 (brown), I2 (violet) and Cl2 gas (greenish tint). I2 changes starch paper into blue , while Br2 turn it orange. Cl2 and Br2 change a starch –KI paper into blue due to oxidation of I- to I2 and I2produce a blue adsorption complex with starch Wet Reaction I. Reaction with AgNo3 For F- No ppt. is formed due to silver fluoride is soluble For Cl- white ppt. of AgCl Silver chloride is soluble in dil. NH3 soln. to give the silveramine chloride complex which will decompose by addition of dil. acid with the reprecipitation of AgCl. For Br- Yellowish-white ppt. of AgBr Silver bromide is partially soluble in dil. NH3 soln. but completely soluble in Conc. NH3 soln. to give the silveramine bromide complex. For I- Yellow ppt. of AgI Silver iodide is insoluble in either dilute or concentrated ammonia solution p ❑ All silver halides (AgX) are insoluble in dil. HNO3 acid. ❑ All silver halides (AgX) are soluble in KCN & Na2S2O3 due to the formation of soluble complex. II. Reaction with BaCl2 For F- white gelatinous ppt. of BaF2 - - For Cl & Br & I - - ve III. Reaction with Chlorine water Sample soln. + dil. H2SO4 + 1 ml of CHCl3 + drops of chlorine water, shake after each addition. - For F & Cl - - ve - For Br yellow to reddish brown colour in the organic layer Chlorine water oxidizes Br- into Br2, this appear brown colour to the aqueous solution and freely soluble in the organic layer after shaking which will be colored yellow to reddish brown. - For I Violet colour in the organic layer Chlorine water oxidizes I- into I2, this appear brown colour to the aqueous solution and freely soluble in the organic layer after shaking which will be colored violet. Why add drops of Cl2 water ? When add excess chlorine water, the Br2 is converted into yellow bromine monochloride "BrCl" or into colorless hypobromous acid "HOBr" or bromic acid "HBrO3", and the organic layer turns pale yellow or colorless. When add excess Cl2 water, the I2 is also oxidized to colorless Iodic acid. Special Tests For F- Boron fluoride test Principle : When fluoride is mixed with borax and moisten with conc. H2SO4. The formed HF and boric acid react to produce boronfluoride gas (BF3). If the mixture introduced into the edge of a Bunsen flame, the flame is tinged green by BF3 gas. For Cl- Chromyl chloride test Principle : It depends upon the formation of a dark red colour fumes of chromyl chloride, when a dry chloride is heated with potassium dichromate and conc. H2SO4. Procedure:- 1) Mix one part of the solid salt with three parts of powdered K2Cr2O7. 2) Treat in a dry test tube with an equal volume of conc. H2SO4. 3) Then heat, evolution of deep red fumes of chromy1 chloride CrO2Cl2. 4) Pass the fumes into NaOH soln. in another test tube, the NaOH soln. turns yellow and give positive test for chromate. Chromyl Chloride Test Mechanism On reacting potassium dichromate with sulphuric acid, chromate trioxide (oxidation state = +6) is formed. The color of chromate trioxide (CrO3) varies from dark red to brown colour. K2Cr2O7 + H2SO4 → CrO3 + H2O Salt-containing chloride (NaCl) is reacted with sulphuric acid that gives sodium bisulphate (NaHSO4) and hydrochloric acid (HCl) is also formed. NaCl +H2SO4 → NaHSO4 +HCl Chromate trioxide is reacted with hydrochloric acid, which produces chromyl chloride (CrO2Cl2 ) which gives out red fumes. CrO3 + HCl → CrO2Cl2 Confirmation for Chromyl Chloride Test When the red vapor passed into a solution of sodium hydroxide (NaOH), The solution turns yellow (due to formation of sodium chromate Na2CrO4). CrO2Cl2 + NaOH → Na2CrO4 + NaCl +H2O Reacting this solution further with the lead acetate and diluted acetic acid (CH3COOH) produces the yellow precipitate. CrO42- + Pb(CH3COO)2 → PbCrO4 +CH3COONa PbCrO4 is the yellow precipitate of chloride and hence the test is accurate. Br- & I- don’t give positive with chromyl chloride test Due to free halogen (Br2 & I2) fumes will produced which on passing in NaOH they dissolve producing colorless or faint yellow liquid (due to formation of hypobromite or Hypoiodite) that give negative test for CrO42- For Br- Eosin test Principle : when Br- soln. is mixed with fluorescein soln and NH4OH followed by addition of sodium hypochlorite 5% soln and acetic acid, the yellow dye fluorescein is converted to red eosin (tetra-bromo fluorescein). The bromide is oxidized at first to Br2 which brominate the dye For I- Nitrite test / Hydrogen peroxide test Iodide is readily oxidized in acid solution (dil H2SO4) with nitrite soln. or H2O2 soln. into free I2. The free I2 is identified by: (a)- Shaking with CHCl3 or CCl4 violet colour will appear in the organic layer. (b)- Adding starch soln. deep blue colour (due to adsorption product between I2 & starch). Cuprous Iodide test Principle: Iodides are active reducing agents, react with cupric ions e.g., CuSO4 forming a white ppt. of cuprous iodide Cu2I2 and the iodide oxidized into free I2. Hence the white ppt. is formed in brown soln. brown White ppt Mercuric chloride test Principle: when iodide soln. react with HgCl2 , scarlet red ppt.