Decoding Anions: Methods and Confirmatory Tests in Analytical Chemistry

ANALYTICAL CHEMISTRY LABORATORY chromium (III), Cr3+ gallium (III), Ga3+ iron (III), Fe3+ ANION ANALYSIS ANIONS • • • • Anions are negatively charged species, which may be monoatomic or polyatomic. They are usually prepared from dissolving acids or salts in water. The procedures for detecting ions in aqueous solution is called Anion Analysis. o the concentrations of the ions to be identified are approximately 0.01 M in an aqueous solution. o The "semimicro" level of qualitative analysis employs methods used to detect 1-2 mg of an ion in 5 mL of solution. Anion analysis makes use of many different processes which involves various important principles and laws of chemistry. o Anion Analysis is made up of series of separations of anions into groups and subgroups and eventually into individual ion followed by identification of each ion. SEPARATION OF ANION • • • CHARGES OF COMMON POLYATOMIC IONS 1+ ammonium, NH411acetate, C2H3O2chlorate, ClO3cyanide, CNhydroxide, OHhypochlorite ClOiodate, IO3nitrate, NO3nitrite, NO2perchlorate, ClO4- Separations of anions rely on different characteristics of the individual ions. These may involve redox reactions to change oxidation state, differential solubility in an acid, base, or water, or precipitating certain ions. There are 21 commonly occurring anions. These are 1. Arsenate AsO4-3 12. Iodide I2. Arsenite AsO3-3 13. Nitrate NO3- 1 -3 3. Borate BO3 14. Nitrite NO2 -1 4. Bromide Br15. Oxalate C2O45. Carbonate CO3-2 16. Phosphate PO4-3 6. Chlorate ClO3 17. Sulfate SO4-2 7. Chloride Cl18. Sulfite SO3 -2 -2 8. Chromate CrO4 19. Sulfide S-2 9. Ferricyanide Fe(CN)6-2 20. Thiocyanide CNS10. Ferrocyanide Fe(CN)6-4 21. Thiosulfate S2O3-2 11. Fluoride FCLASSIFICATION OF ANION 2carbonate, CO32hexafluorosilicate, SiF62oxalate, C2O42selenate, SeO42silicate, SiO32sulfate, SO42tartrate, C4H4O62- 3arsenate, AsO43phosphate, PO43- OXIDATION NUMBERS OF COMMON MONOATOMIC IONS 1bromide, Brchloride, Clfluoride, Fhydride, Hiodide, I- 2oxide, O2selenide, Se2sulfide, S2telluride, Te2- 3nitride, N3phosphide, P31+ cesium, Cs+ copper (I), Cu+ hydrogen, H+ lithium, Li+ potassium, K+ rubidium, Rb+ silver, Ag+ sodium, Na+ thallium (I), Tl+ 4carbide, C4- 3+ aluminum, Al3+ bismuth (III), Bi3+ cerium (III), Ce3+ thorium (IV), Th4+ tin (IV), Sn4+ zirconium (IV), Zr4+ 2+ barium, Ba2+ cadmium, Cd2+ calcium, Ca2+ cobalt (II), Co2+ copper (II), Cu2+ iron (II), Fe2+ lead (II), Pb2+ magnesium, Mg2+ manganese (II), Mn2+ mercury (II), Hg2+ nickel (II), Ni2+ tin (II), Sn2+ zinc, Zn2+ 4+ germanium (IV), Ge4+ lead (IV), Pb4+ silicon (IV), Si4+ GROUP 1 ANIONS • • • • Sulfite, oxalate, arsenate (carbonate, arsenite, phosphate, fluoride) Distinguished from other groups because their calcium salts are insoluble in neutral or basic solution ex. CaSO3 , CaC2O4 ,Ca3 (ASO4)2 Precipitating agent is calcium acetate, Ca(Ac)2 Divided into 2 subgroups by action of diluted HAc: o Group IA – soluble in HAc (sulfite, arsenate) o Group IB – insoluble in dil HAc (oxalate) Group Precipitate Agent: Calcium Acetate - Ca(AC)2 GROUP 2 ANION • • • Chromate and sulfate ions Include ions whose Ca salts are soluble but whose Ba salts are insoluble in slightly basic solution Are precipitated by Ba(Ac)2 , the precipitating agent, as BaCrO4 and BaSO4 in a slightly basic solution GROUP 3 ANION • Acetate • • • Chlorate • Fluoride • Sulfide, ferrocyanide (and ferricyanide) Are ions whose Ca and Ba salts are soluble but whose Cd salts are insoluble in neutral or slightly basic solution Precipitated by Cd(Ac)2 as CdS and Cd2Fe(CN)6 GROUP 4 ANION • • • Thiocyanate, iodide (and chloride, bromide, thiosulfate) Are ions whose Ca, Ba and Cd salts are soluble but Ag salts are insoluble in slightly acid with nitric acid Precipitated by AgAc as AgCNS and AgI GROUP 5 ANION • • • • Chlorate, borate (and nitrite, nitrate) Are not precipitated by any of the precipitating agent for groups I to IV Designated as water-soluble group since most common salts of theses anions are soluble in water and acid Anions whose Ca, Ba, Cd and Ag salts are soluble in water and acid CONFIRMATORY TEST • • • • • • • • ANIONS Chloride Cyanide Thiocyanate solution Ferricyanide CONFIRMATORY TEST NaCl + HNO3 + Hg2 (NO3 )2 → white ppt NaCN + HNO3 + Hg2 (NO3 )2 → NVR NaSCN + FeCl3 → deep (bloody) red Nitrate NaNO2 + conc. H2SO4 + FeSO4 → (+) NaNO3 + diluted CH3COOH + FeSO4 → (-) NaNO3 + conc. H2SO4 + FeSO4 → (+) K3 Fe(CN)6 + FeSO4 → Turnbull’s blue K3 Fe(CN)6 + CuSO4 → muddy yellow Carbonate Na2CO3 + conc. H2SO4 → effervescence Arsenate Na3AsO4 + (NH4 )2MoO4 → white ppt Dihydrogen NaH2PO4 + (NH4 )2MoO4 → NVR phosphate Nitrite & NaNO2 + diluted CH3COOH + FeSO4 → (+) (+) for brown ring junction between two liquids ( becomes brown solution if disturbed) NaCH3COO + conc. H2SO4 → vinegar-like odor NaClO3 + AgCH3COO + HNO3 + NaNO2 → white ppt NaF + Ca(OH)2 → white slimy ppt