Complexometric Titration PDF

SIC2004: Analytical Chemistry I | SIC2022 & SID 2003: Basic Analytical Chemistry COMPLEXOMETRIC TITRATION  Complexometry Formation of complex which are slightly ionized in solution Complex formed by reaction of metal ion (electron acceptor) with ligand (electron donor) Ch...

SIC2004: Analytical Chemistry I | SIC2022 & SID 2003: Basic Analytical Chemistry COMPLEXOMETRIC TITRATION  Complexometry Formation of complex which are slightly ionized in solution Complex formed by reaction of metal ion (electron acceptor) with ligand (electron donor) Chelating agents form strong 1:1 complexes with metal ions Chelating agent used to titrate metal ions in solution: complexometric titration  Complexometric titration Titration type Electron donor Electron acceptor Acid-base Base Acid Complexometric Ligand Metal ion Ligands: Unidentate: 1 lone pair (eg NH3, Cl-, Br-) Bidentate: 2 lone pair (eg ethylene diamine) Multidentate: more than 2 lone pair (eg EDTA) Multidentate ligands more stable  Multidentate ligands Stability increases with multidentate ligands Ethylenediaminetetraacetic acid (EDTA) Forms very stable complexes with many metal ions. Forms “cage” around metal ions. Hexadentate: donate electron pair from 2 amine group and 4 carboxylate groups  EDTA The fully protonated form of EDTA, H6Y2+ is a hexaprotic weak acid. Formation constant, Kf  Conditional formation constant Most EDTA is not Y4- below pH 10.37 Kf‘ treats all free EDTA in one form for complex formation  Important features of EDTA Complexes form are stable & water soluble Instantaneous formation with metals in 1:1 ratio Equivalence point readily read Formation or dissociation of complexes affected by pH  Metal indicators Organic dyes which from chelates Free form different colour from complex Reversible reaction between metal indicator and metal ion MIn less stable than M-EDTA complex Colour change dependent on pH M-In + EDTA(colorless) M-EDTA(colorless) + In Common indicators  Eriochrome black T (EBT) Free EBT is blue; metal-EBT complex is wine red (at neutral pH) Best used for Mg2+, Pb2+ and Zn2+. Can’t be used for determination of Cu2+, Fe2+, Al3+, Co2+, Ni2+. Example: Mg2+ + HIn2- MgIn + H+ MgIn + H2Y2- MgY2- + HIn2- + H+ Wine red blue  CaIn + H2Y2- CaY2- + HIn2- Ca2+ + HIn2- Transition metal indicator Metal EDTA complex; Metal Indicator complex complex & metal EDTA complex indicator free form  Direct titration Similar to acid-base titration M-EDTA complex must be more stable than M-In complex in buffered medium Reaction between metal and EDTA must be rapid. Slow reaction must be catalyzed. If metal precipitate to metal hydroxide, must add auxiliary reagent (ammonia, citric acid, tartaric acid) to prevent precipitation.  Back titration Addition of known excess of EDTA. Excess EDTA then titrated with standard solution of a second “weak” metal ion. 3 conditions back titration is necessary: (a) Analyte precipitates in the absence of EDTA (b) Analyte reacts too slowly with EDTA (c) Analyte blocks the indicator  Zn Zn EDTA In EDTA-Hg EDTA-Hg EDTA-Hg EDTA-Zn In EDTA-Zn EDTA-Hg In-Zn EDTA-Hg In EDTA-Hg EDTA EDTA-Zn In EDTA-Zn EDTA-Hg In-Zn EDTA-Hg EDTA-Hg Titration of second metal Once all EDTA complexed, EDTA in excess + Hg from burette complexes metal then starts forming + indicator (free form) with excess EDTA complex with indicator  Example of back titration Ni2+ can be analyzed by back titration using standard Zn2+ with xylenol orange indicator. A solution containing 25 mL of Ni2+ is treated with 25 mL of 0.05283 M EDTA. Titration with 0.02299 M Zn2+ requires 17.61 mL to reach endpoint. Calculate the molarity of Ni2+ in the unknown? SOLUTION: mol EDTA = 25.00 mL x 0.05283 M = 1.3208 mmol EDTA mol Zn required = 17.61 mL x 0.02299 M = 0.4049 mmol Zn 1 mol EDTA : 1 mol Zn 1.3208 mmol EDTA – 0.4049 mmol Zn = 0.9159 mmol Ni Concentration of Ni = 0.9159 mmol / 25 mL = 0.03664 M.  EDTA titration of mixtures EDTA not selective as it chelates with most metal ions Increase EDTA selectivity by: – Controlling pH of medium – Adjustment of oxidation number of metal ion – Masking and demasking agent  pH effect Kf ’ = Kf x αY4- > 106  Control pH of medium Trivalent and tetravalent cations (Fe3+, Th4+, Bi3+) and Hg2+ form stable complexes at pH 1-3 using concentrated HNO3. Divalent metals (Co2+, Ni2+, Cu2+, Zn2+, Pb2+, Cd2+) form stable complexes at pH 4-6 using acetate buffer. Alkaline earth metals (Ba2+, Ca2+, Sr2+) and Mg2+ form stable complexes at pH 10 using ammonia buffer  Adjust oxidation number of metal ion Solves interference between Mn+ of same pH group. Example, Ascorbic acid is reducing agent used in: (i) Removal of interference of Fe3+ in group 1 – reduces to Fe2+ (ii) Removal of interference of Hg2+ in group 1 – reduce to Hg0 (precipitate) (iii)Removal of interference of Cu2+ in group 2 – reduce to cuprous Ø Fe2+, Hg0 and cuprous don’t react with EDTA  Masking and demasking agent Can estimate 2 or more metals in the same solution Masking agent: – Forms complexes with interfering ions more strongly with metal than with indicator or EDTA Demasking agent: – Releases the masked metal ion – Cyanide complexes of Cd, Cu, Fe, Zn can be demasked by formaldehyde in acetic acid Masking agent