ANIONS 1

• Anions are negatively charged ions produced by the ionization of acids and salts.
• Ions are separated into groups based upon the solubilities of the calcium, barium, cadmium, and silver salts
• GROUP 1 includes ions whose calcium salts are insoluble in slightly basic solution.
• GROUP 2 includes ions whose calcium salts are soluble but whose barium salts are insoluble in slightly basic solution.
• GROUP 3 is composed of anions whose calcium and barium salts are soluble but whose cadmium salts are insoluble in slightly basic solution.
• GROUP 4 is composed of anions whose calcium, barium, and cadmium salts are soluble but whose silver salts are insoluble in solution slightly acid with nitric acid.
• GROUP 5 contains anions whose calcium, barium, cadmium, and silver salts are soluble in water and acids.
• GROUP 1 ANIONS

  Carbonate ion

  Sulfite ion

  Arsenite ion

  Arsenate ion

  Phosphate ion

  Oxalate ion

  Fluoride ion
• This group of anions forms insoluble salts of calcium in slightly basic solution. The precipitating agent is calcium acetate ion.
• Reagent for detecting carbonate ions- dilute hydrochloric acid and potassium chlorate
• Carbonate ions should be analyzed first because carbonate reagent is always used in the whole analysis of anions thus creating false positive result.
• Dilute hydrochloric acid decomposes carbonate ion and forms carbon dioxide gas thus, the effervescence.
• Effervescence is the escape of gas from an aqueous solution and the foaming or fizzing that results from that release.
• The addition of potassium chlorate will oxidize the sulfite, sulfide, and thiosulfate ions, an acid solution, to the sulfate ion or free sulfur
• Carbonate ions remove the presence of cations (sodium, potassium, ammonium)
• Second procedure involves removal of heavy metals (common: Mercury, Lead, Chromium, Zinc and Copper
• Reagent for removal of heavy metals: Sodium carbonate solution
• Sodium carbonate precipitates them in their carbonate and hydroxide salts with Ksp ranging from 10-14 to 10-20. This can be separated from calcium salts since calcium salts have solubility of 10-9 or less.
• Separation of Group I anions into subgroups by the reagent acetic acid
• Addition of Acetic acid dissolves all calcium salts except Calcium fluoride and calcium oxalate (residue in third day analysis).
• confirmatory reagent for sulfite ions: hydrogen peroxide
• Confirmatory reagent for arsenite ions- thioacetamide
• positive indicator for arsenite: yellow brown precipitate
• Addition of hydrogen sulphide (from thioacetamide) produces yellow precipitate
• If a yellow precipitate does not form immediately, the arsenite ion is absent.
• The arsenite ion will give a yellow precipitate of arsenic pentasulfide after a period of 60 seconds or more.
• If arsenite ion is present must be destroyed by heating an acid solution. Otherwise, this ion will react with free iodine liberated in the arsenate test.
• confirmatory reagent for arsenate ion: potassium iodide
• positive indicator for arsenate: brown yellow solution
• Same as arsenite, addition of hydrogen sulphide (from thioacetamide) produces yellow precipitate but action is very slow. Instead, addition of KI is used which reduces arsenate to arsenite and liberates iodine compound which is color brown.
• confirmatory reagent for potassium ion: ammonium molybdate solution
• positive indicator for phosphate ion: yellow precipitate
• Phosphate ions react with ammonium molybdate in a nitric acid forming yellow precipitate of ammonium phosphomolybdate. Tartaric acid should be added to make solution strongly acidic so as not to precipitate arsenate ions with molybdate.
• The excess tartaric acid, in strongly acid solution (nitric acid), forms a complex with arsenic. Otherwise the arsenate ion would precipitate with ammonium molybdate in the confirmatory test for the phosphate.
• Do not boil the soluble complex between tartaric acid and arsenic may be broken down with heat.
• Absence of phosphate ion turns to white precipitate.
• confirmatory reagent for oxalate ion: potassium permanganate
• positive indicator of oxalate ions: disappearance of pink color
• Addition of sulfuric acid dissolves calcium oxalate producing oxalic acid. Oxalic acid is a reducing agent reducing permanganate ions to manganous ions, thus color changes from purple to colorless.
• confirmatory reagent for fluoride ion: sulfuric acid