Complexation Titrimetry L e c t u r e 8 Complexation Titrimetry Associate prof . L.V. Vronska Associate prof . M.M. Mykhalkiv

Outline Complexation Titrimetry and its classification. Complexing reactions, which use in titrimetry, requirements to them. Complexonometric titrimetry (chelatometry) and its methods. Indicators in methods of complexonometric titrimetry. Use of complexonometric titrimetry in pharmaceutical analysis.

In the method complexation titration allocate such methods: Complexation titration is titration in which the reaction between the analyte and titrant is a complexation reaction. In the method complexation titration allocate such methods: Mercurimetry – titrant is solution of Hg(NO3)2 Fluoridometry – titrant is solution of NaF Cyanidometry - titrant is solution of KCN Complexonometry - titrant is solution of sodium edetate

Requirements to reactions in complexation titration reactions between the titrant and analyte must be stoichiometricaly, quantitatively formation constant of complex should be more than   108 reaction of formation of complex compound should proceed quickly there should be a possibility of fixing of a point of equivalence or the end point in the conditions of titration carrying out competing reactions should not proceed

In mercurimetry use formation of halide complexes of Hg (II) In mercurimetry use formation of halide complexes of Hg (II). In the conditions of titration take place following reaction: 2Cl- + Hg2+=HgCl2 Similar reactions take place in case of determination of bromide-, iodide-, thiocyanide- and cyanide-ions; it is possible to define also salts of Hg (II).

Hg(NO3)2 + 2 NaCl = HgCl2 + 2 NaNO3 Mercurimetry Titrant: secondary standard solution of Hg(NO3)2 Standardization: on primary standard solution of sodium chloride NaCl: Hg(NO3)2 + 2 NaCl = HgCl2 + 2 NaNO3 !!!The main lack of mercurimetry – high toxicity of compounds of Mercury.

Na2[Fe(CN)5NO] + Hg(NO3)2 = 2NaNO3 + Hg[Fe(CN)5NO] Mercurimetry Indicators: 1. Solution of sodium pentacianonitrozo-ferrate (ІІІ) (sodium nitroprussidum) Na2[Fe(CN)5NO], which forms with Hg2+-ions insoluble white salt: Na2[Fe(CN)5NO] + Hg(NO3)2 = 2NaNO3 + Hg[Fe(CN)5NO] 2. A solution of diphenylcarbazone which forms with ions Hg2+ precipitate of dark blue colour.

Mercurimetric determination of chloridic with sodium nitroprussidum

Hg2+ + 4I- = [HgI4]2- Mercurimetry Mercurimetric determination of iodide is based on such reaction: Hg2+ + 4I- = [HgI4]2- In the end point we observe appearance of red precipitate: [HgI4]2- + Hg2+ = HgI2 red precipitate

Mercurimetric determination of iodide

Mercurimetry Hg2+ + 2SCN- = Hg(SCN)2 Fe3+ + 3SCN- = Fe(SCN)3 Determination of thiocyanide SCN- - ions is based on reaction: Hg2+ + 2SCN- = Hg(SCN)2 As indicators we use solution of Fe (III) salts To the end point: Fe3+ + 3SCN- = Fe(SCN)3 we observe red colour of solution In the end point: 2Fe(SCN)3 + 3Hg(NO3)2 = 3Hg(SCN)2+2Fe(NO3)3. red colour of solution disappears

Determination of mercury (ІІ) salts Mercurimetry Determination of mercury (ІІ) salts Titrant – solution of potassium thiocyanide KSCN Indicator – ions of Fe3+. To the end point: Hg2+ + 2SCN- = Hg(SCN)2 In the end point appears red colouring of iron (ІІІ) thiocyanide solution: In the end point: Fe3+ + 3SCN- = Fe(SCN)3

Fluoridometry Titrant: primary standard solution of NaF. Defined ions: aluminium Al3+, zirconium ZrIV, thorium ThIV, calcium Ca2+. We use following reactions for determination: Al3+ + 6F- = [AlF6]3- ZrIV + 6F- = [ZrF6]2- ThIV + 6F- = [ThF6]2- Ions of Са2+ we determine by back titration: Ca2+ + 2F- (excess) = CaF2 6F- (rest) + Al3+ = [AlF6]3-

Fluoridometry Al3+ + HOH = AlOH2+ + H+ Indicators: acid-base indicators (Methyl orange), as solutions have acidic reaction of medium: Al3+ + HOH = AlOH2+ + H+ Alizarine sulfate (forms complex with zirconium ZrIV – red-violet colour, with thorium ThIV– violet) Fluoridometry serves for definition of rather high contents of substances (0,2-0,5 mol/L). The relative error can reach 1-3 %.

Fluoridometric determination of aluminium with Methyl orange

Fluoridometric determination of aluminium with Alizarine (ammoniac medium)

Cyanidometry Ag+ + 2CN- = [Ag(CN)2]- [Ag(CN)2]- + Ag+ = 2AgCN Titrant: secondary standard solution of potassium cyanide KCN Standardization: on standard solution of AgNO3: Ag+ + 2CN- = [Ag(CN)2]- [Ag(CN)2]- + Ag+ = 2AgCN in the end point appears turbidity Defined substances: heavy metals, which form stable cyanidic complexes of specific structure, for example, [Ni(CN)4]2- , [Co(CN)4]3-, [Zn(CN)4]2-.

[Ni(NH3)4]2+ + 4CN- = [Ni(CN)4]2- + 4NH3 AgI + 2CN- = [Ag(CN)2]- + I- Cyanidometry The reaction of methods: [Ni(NH3)4]2+ + 4CN- = [Ni(CN)4]2- + 4NH3 Medium: ammoniac Indicator: suspension of silver iodide AgI. In the end point: dissolves the precipitate of silver iodide AgI AgI + 2CN- = [Ag(CN)2]- + I- turbidity transparent solution Potassium cyanide is strong toxine!

Chelatometry Complexon І: nitrilotriacetic acid (tetradentate)

Complexon ІІ: (EDТА) ethylenediaminetetraacetic acid Complexon ІІІ: sodium ethylenediaminetetraacetate (Na-EDТА, trylon B, chelaton) - Na2H2Y

Complexon ІV: cyclohexyldiaminetetraacetic acid All metal-EDTA complexes have a 1:1 stoichiometry. These complexes are dissolved in water. Metal-EDTA complexes are named – metal complexonate.

Preference of use of complexons Well soluble in water and some other organic solvents The heightened stability of complexes Reactions passed stoichiometricaly (isn’t present stepwise complexing) Some complexons are specific reagents on specific ions of metals (selectively) Use as masking reagents Reaction is fast

Chelatometry Са2+ + H2Y2- → CaY2- + 2H+ In3+ + H2Y2- → InY- + 2H+ Titrant: 0,05 mol/L solution of sodium edetate (Na2-EDTA) Standardization of titrant on such substances: Zn + H2SO4  ZnSO4 + H2 (ions of Zn2+) MgSO4 (ions of Mg2+) CaCO3 + HCl Ca2+ + 2Cl- + H2O + CO2 (ions of Ca2+) ZnO + HCl  Zn2+ + 2Cl- + H2O (ions of Zn2+) Reactions of methods: Са2+ + H2Y2- → CaY2- + 2H+ In3+ + H2Y2- → InY- + 2H+ Th4+ + H2Y2- → ThY + 2H+

Chelatometry Men+ + H2Y2-  MeYn-4 + 2H+ Men+ + Y4-  MeY(n-4)+ Indicators: metallochrome indicators Common reaction of method: Men+ + H2Y2-  MeYn-4 + 2H+ Men+ + Y4-  MeY(n-4)+ рН of medium influences on the concentration of metal-ions (hydroxycomplexes of metal form) рН of medium influences on the concentration of Y4-

Conditional formation constant and her relationship with thermodynamic and real formation constant

Dependence of molar part edetate-ions from рН of medium pH=1,0 α=2,110-18 pH= 7,0 α= 4,810-4 2,0 3,710-14 8,0 5,410-3 3,0 2,510-11 9,0 5,210-2 4,0 3,610-9 10,0 3,510-1 6,0 2,210-5 11,0 9,810-1

Conditions of chelatometric titration high stability of metal complexonate^ formation constant βC>108. ageing of fixed value of рН of investigate solution use of buffer solutions (for linkage of Н+ ions, which form) only some cations (Fе3+, In3+, Sc3+, ZnIV, ТhIV), which form stability metal-edetate complexes, are titrated in acidic medium

Direct chelatometry Conditions of use: Reaction involving the titrant and analyte must be of known stoichiometricaly, quantitatively The titration reaction must occur rapidly We have indicator, which it allows effectively define the end point Is possible titrate mixture of cations, if Defined ions: metal-cations

Methods of increase of selectivity of direct titration: Regulation of pH of investigate solutions Fе3+, Ga3+, In3+, Tl3+, Ві3+, ZrIV, ТhIV ions (if lg βС > 20) at рН~2 at рН = 5 determine ions of Аl3+, Co3+, Ni2+, Cu2+, Zn2+, Cd2+ at рН = 9 determine ions of alkaline-earth metal ions Mg2+, Ca2+ and atc. 2. Masking of interfere ions by reagents: F-, P2O72-, CN- - anions and Н2С4Н4О6 Со(II), Ni(II), Zn(II), Cd(II) and Hg(II)-ions may be masked by cyanide-ions 3. Change of oxidation state Fe3+-cations are masked by reduction to Fe2+-ions.

Back chelatometry Conditions of use: Is impossible to choose of indicator The titration reaction is too slow If at optimal value of pH of solution, defined metal-ions form precipitates of hydroxides or basic salts For determination of anions

Ме1х+ + Н2Y2- ↔ Ме1Yх-4 + 2Н+ Н2Y2- + Мg2+ ↔ МgY2- + 2Н+ Back chelatometry Additional titrants: Zn2+ or Мg2+ salts Ме1х+ + Н2Y2- ↔ Ме1Yх-4 + 2Н+ Defined Excess ion of titrant 1 Н2Y2- + Мg2+ ↔ МgY2- + 2Н+ Rest of Additional titrant 1 titrant 2

Back chelatometry and back determination of anions SO42- + Ba2+ ↔ BaSO4↓ defined ions excess of titrant 1 Ba2+ + Н2Y2- ↔ BaY2- + 2Н+ rest of additional titrant 1 titrant 2

Indirect chelatometry or displacement chelatometry Conditions of use: The analytes are unstable substance It is impossible to indicate the equivalent (end) point in direct reaction Analyte doesn’t react with titrant Reaction involving the titrant and analyte mustn’t be of known stoichiometry, quantitatively Formation of very steady complexes metal-indicator For define of anions Defined ions: ThIV, Нg2+, РО43-, СО32- and other.

Indirect chelatometry or displacement chelatometry Меn+ + МgY2- ↔ МеYn-4 + Мg2+ Defined substituent metal Мg2+ + Н2Y2- ↔ МgY2- + 2Н+ substituent Titrant

Indirect chelatometry or displacement chelatometry РО43- + Мg2+ + NH4+↔ МgNH4РО4↓ defined precipitate of defined ions ions МgNH4РО4↓+Н+↔HРО42-+Мg2++NH4+ precipitate of defined ions substituent Мg2+ + Н2Y2- ↔ МgY2- + 2Н+ substituent titrant

Requirement to indicators in chelatometry Metal-ions with metallochromic indicator form complexes 1:1 stoichiometry Coloring of indicator and coloring of MeInd- complexes should be different MeInd-complex should be sufficiently stability Stability of MeInd-complex should be less than stability of MeY-complex, which it is product of titration (difference in 10-100 time) MeІnd-complex should be labile change of coloring at titration should be contrast

Specific Metallochromic Metalloindicators Specific Metallochromic Haven’t color Have color, but Fe3+ - salicyl-sulphonic acid (red) with metal-cations Ві3+ - thiourea (yellow) form complexes, which have other color Metallochromic indicators: –N=N- (Eriochrome Black T, arsenazo III) Triphenylmethane dyes Other indicators (purpuric acid ammonium salt (murexide), diphenylthiocarbasol (dithizon), alizarine)

Mechanism of change of coloring of metallochromic indicators Me2+ + Іnd  MeІnd- blue red MeІnd + H2Y2-  MeY2- + H2Іnd red colorless blue

Colour of MeInd complex Defined substances Indicator pH Colour of indicator Colour of MeInd complex Са2+ Calconecarboxylic acid pH  12 blue Red-violet Mg2+ Eriochrome Black T рН6,3 6,311,6 рН11,6 red yellow

Colour of MeInd complex Defined substances Indicator pH Colour of indicator Colour of MeInd complex Pb2+, Zn2+, Bi3+ Xylenol orange 2  рН  6,4 рН  6,4 yellow Red-violet red Al3+, Zn2+ Dithizon рН6,3 Green-blue

Determination of calcium by chelatometry with Calconecarboxylic acid (BP)

Determination of bismut by chelatometry with Xylenol orange

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