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(10)Complexometry

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allelipraise's version from 2018-06-06 19:40

Section 1

Question Answer
Complexometry/Chelometry for the analysis of divalent and polyvalent cations - many cations will form complexes in solution with a variety of substances that have a pair of free electrons (e.g., N, O, S atoms in the molecule) capable of satisfying the coordination number of the metal (Lewis acid)
metallochromic indicatorso dithizone o eriochrome black T o hydroxynaphthol blue
Formation constant (Kf) equilibrium constant for the reaction of metal ion with a ligand
Ligandmolecule which provides group of attachment to metal ion; a Lewis base
Monodentate ligandbinds to a metal ion through only 1 atom; e.g., ammine, aquo, halides, hydroxo, acetato, sulfato
Bidentate a ligand with two donor groups; e.g., glycine, ethylenediamine
Tetradentate ligand e.g., trien (triethylenediamine), NTA (nitrilotriacetic acid)
HexadentateEDTA (ethylenediaminetetraacetic acid), CDTA (cyclohexanediaminetetraacetic acid), EGTA (ethyleneglycol bis-(β-aminoethyl ether)-N,N’-tetraacetic acid
OctadentateDTPA (diethylenetriaminepentaacetic acid)
Chelatea chemical compound in the form of a heterocyclic ring, containing a metal ion attached by coordinate bonds to at least two nonmetal ions
coordination numberThe number of molecules of the ligand will depend on___of the metal and on the number of complexing groups on the ligand molecule.
multidentate ligands (4-6 donor groups):react more completely with cations and thus provide sharper endpoints
Sequestering Agentschelating agents which form water-soluble complexes with divalent or polyvalent metal ions
Metal Ion Indicatorsorganic dyes which form colored chelates with many metal ions - often discernible to the eye at concentrations of 10-6 to 10-7 and can function as visual indicators
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Section 2

Question Answer
DithizoneBi, Al
Eriochrome Black T Bi, Mg, Zn
Xylenol OrangeBi, Ti, Th
Hydroxynaphthol BlueCa
Arsenazo Irare earth metals (lanthanides)
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Section 3

Question Answer
Dithizoneused in residual titration of Bi3+, Al3+, where excess 0.05 M Na2EDTA VS is backtitrated with 0.05 M ZnSO4 VS
Dithizoneendpoint: purplish-green (free) to bright rose pink (metal- indicator complex)
Eriochrome Black Tpolymerize in acidic solutions (red-violet product) - pH in the range of 8 to 10 conveniently maximizes the indicator form HIn2- (blue) enabling the associated complexing reaction to go to effective completion by facilitating the removal of H+
Eriochrome Black TBuffer: ammonia-ammonium chloride buffer TS
Calmagite- structurally similar to eriochrome black T - advantage: more stable in solution
Xylenol Orange direct titration with EDTA VS: Th(IV), Bi(III), endpoint is red to yellow  residual titration where excess EDTA is backtitrated with Th(IV) or Bi(III) VS: Zr(IV), Fe(III)
Hydroxynaphthol Blue Used in the titration of Ca2+ with EDTA at pH 12 using NaOH as pH adjusting agent - Pink when in complex with metal; free indicator is blue
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Section 4

Question Answer
0.05 M Disodium Edetate (Na2EDTA) VSstandardized against chelometric standard calcium carbonate using hydroxynaphthol blue indicator (red to blue, free)
0.05 M Zinc Sulfate VS- commonly used backtitrant in residual complexometric titrations such as in the assay of Al3+ and Bi3+ compounds - standardized against 0.05 M Disodium EDTA VS, endpoint indicated by dithizone TS (purplish green to rose pink)
0.037 N Ferric Chloride VSstandardized by indirect iodometric titration using Sodium Thiosulfate VS as titrant, and starch TS indicator
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Section 5

Question Answer
Residual Assayuseful for the determination of cations that form stable EDTA complexes and for which a satisfactory indicator is unavailable
Displacement Methodsused when no indicator for an analyte is available
Maskingto determine a metal in the presence of another metal-employed to increase selectivity of EDTA since it is capable of complexing with divalent or polyvalent cations
Masking Agent protects come component of the analyte from reaction with EDTA
Demasking Agent releases the metal ion from the masking agent
Auxiliary Complexing Agentsbind the metal strong enough to prevent the hydroxide from precipitating, but weakly enough to give up the metal ion when EDTA is added
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