COMPLEXOMETRIC TITRATION

Complexometry : is the type of volumetric analysis involving the formation of complexes which are slightly ionized in solution, like weak electrolyte and sparingly soluble salt. Complex is formed by the reaction of metal ion (Mn+) with a legand which may be either an anion e.g. [Ag(CN)2]- or neutral molecule e.g. [Ag(NH3)2]+ The metal ion is known as Central metal atom. The anion or neutral molecule is known as Ligand (L)

M+ + L ML Ag+ + 2 CN- [Ag(CN)2]- Cu2+ + 4 CN- [Cu(CN)4]2- Ag+ + 2 NH3 [Ag(NH3)2]+ Cu2+ + 4 NH3 [Cu(NH3)4]2+ Central metal atom = acts as Lewis acid (electron acceptor) Ligand = acts as Lewis base (electron donor) Coordinate bond (dative) = The bond formed between central metal atom (ion) (acceptor) and the Ligand (donor)

Dative bond is similar to covalent bond (formed of two electrons) But in dative bond the electrons pair are donated from one atom to the other. The atom gives electron pair is known as donor, while the atom accept electron pair is known as acceptor. The bond is represented by an arrow () from donor to acceptor. NH3  NH3  Cu  NH3 

Coordination number = The no * Coordination number = The no. of coordinate bonds formed to a metal ion by its ligands. * Characters of coordination number * 1- It is even number: 2 e.g. Ag+ , 4 e.g. Ni2+ , Cu2+ , 6 e.g. Fe3+ , Cr3+ 2- It is usually double the charge of the metal. The charge of a complex is the algebraic sum of the charges of the central ion and ligand .. e.g.    [Ag(CN)2] -  Ag+ + 2 CN - 1 (+ve) + 2 (-ve) = 1 (-ve) e.g. [Fe(CN)6]3-  Fe3+ + 6 CN - 3 (+ve) + 6 (-ve) = 3 (-ve) The higher the valence of metal ion the more stable the complex e.g.Ferricyanide is more stable than Ferrocyanide

Unidentate (Monodentate) Ligand or "Simple Ligand" Types of complexing agents (( Classification of ligands according to the no. of sites of attachment to the metal ion )) Unidentate (Monodentate) Ligand or "Simple Ligand" The ligand attached to metal at one site e.g. H2O , NH3 , CN - , Cl - , I - , Br - , (i.e. forming one coordinate bond, or capable of donating one unshared pair of electrons)

The ligand attached to metal at two sites. Bidentate Ligand The ligand attached to metal at two sites. Ethylene diamine

The Ligand attached to metal at 3 sites Tridentate Ligand: The Ligand attached to metal at 3 sites Tetradentate Ligand: The Ligand attached to metal at 4 sites Diethylene triamine Triethylene tetramine

Chelation Chelate : It is a complex formed between the ligand containing two or more donor groups and metal to form ring structure. (heterocyclic rings or chelate rings) Chelating agents: organic molecules containing two or more donor groups which combine with metal to form complex having ring structure. Chelates are usually insoluble in water but soluble in organic solvent. Sequestering agent : Ligands which form water soluble chelates e.g. EDTA.

Factors affecting stability of complex [A]- Effect of central metal ion : (1)- Ionic size (metal radius): - Smaller an ion (small radius of metal)  greater its electrical field  more stable complex (2)- Ionic charge (metal charge): - Metal of higher charge give more stable complexes. e.g. Ferricyanide [hexacyanoferrate III] is more stable than Ferrocyanide [hexocyanoferrate II]. (3)- Electronegativity : The higher acidity (electronegativity) of metal (Mn+)  the higher stability of complex. (4)- Metal which has incomplete outer shell (has high acidity) have more tendency to accept electrons  more stable complex. e.g. Ca2+ , Ni2+ , Zn2+ , Mn2+ , Cu2+

[B]- Effect of Ligand: [1]- Basic character: - The higher the basicity (strong base is good electron donor)  the higher the ability of ligand to form complex. e.g. ligand contain electron donating atom. e.g. N > O > S > I- > Br- > Cl- > F- [2]- The extent of chelation: - Multidentate ligands form more stable complexes than monodentate.   [3]- Steric effect: - Large, bulky ligand form less stable complexes than smaller ones due to steric effect. e.g. ethylene diamine complexes are more stable than those of the corresponding tetramethyl ethylene diamine.

Complexones Amino polycarboxylic acid compounds used as complexing agents for many metal ions. Complexone I: H3Y Complexone II: H4Y

Complexone III: Na2H2Y.2H2O Titration involving EDTA known as complexometric titration. EDTA is a hexadentate ligand, containing 4 oxygen and 2 nitrogen donor.

It reacts with most cations (divalent, trivalent, tetravalent) (except glycoprotein VI(gp. VI) "Alkali gp6.") forming freely sol. stable complexes. The formed complexes contain the metal and EDTA in the ratio of 1:1 irrespective to the charge of the metal ion. The general reactions for the formation of metal – EDTA complexes as follows M2+ + H2Y2- MY2- + 2 H+ M3+ + H2Y2- MY- + 2 H+ M4+ + H2Y2- MY o + 2 H+ Mn+ + H2Y2- (MY)n-4 + 2 H+

2 moles of hydrogen ions are formed in each case. EDTA is not selective chelating agent. Formation or dissociation of complexes is affected by pH. i)- In acidic medium: i.e.  [H+]  ionization of EDTA  stability of metal – EDTA complex  shift the reaction backward. ii)- In slightly alkaline solution  the reaction is forward   stability of complexes (chelates). iii)- In strong alkali  pptn. of metal as hydroxide.

Detection of End Point Metal indicator ((Metallochromic Ind.)). Acid-base Indicator. Specific Indicator. Turbidity end point (appearance of turbidity). Instrumental method.

Metal Indicators (Metallochromic Ind) They are organic dyes which form colored complexes with metals (i.e. act as legand), it exhibits a color in the free form and a different color in the complex form. Mn+ + Ind.  M – Ind. M – Ind. + EDTA  M – EDTA + free Ind. Act as ligand to form complex with metal (act as Lewis base and the metal acts as Lewis acid). The reaction between metal and ind. must be reversible. The metal-ind. complex should be less stable than the metal-EDTA complex. The color of free form different than color of complex one. Changes its color according to the pH of the medium.

Murexide: Ammonium salt of Purpuric acid or ammonium purpurate It can be represented by H4 Ind. - OH - OH - H4 Ind.- H3 Ind.2- H2 Ind.3- H + H + Reddish violet Violet Blue pH : < 9 9-11 > 11

It is used for the determination of Ca 2+ , Co 2+ , Ni 2+ , & Cu 2+ salts at pH 9-11 Metal Colour of complex Colour of indicator Ca 2+ Pink violet Cu 2+ Orange Violet Co 2+ Yellow Ni 2+ yellow

Eriochrome Black T (EBT) It can be represented by H2Ind - The color of Ind. change with the change of pH. EBT contains 2 replaceable phenolic hydrogen. OH - OH - H2 Ind.- H Ind.2- Ind.3- H + H + Wine red Blue Yellow pH : < 7 7-11 > 11

M. Ind.- + H2Y 2-  MY 2- + H Ind.2- + H + Wine red Blue It is used for the determination of Mg 2+ , Zn 2+ , Cd 2+ , pb 2+ , Hg 2+ & Mn 2+ salt at pH 7 – 11 using ammonia buffer (pH = 10) M 2+ + H Ind.2-  M. Ind.- + H +  M. Ind.- + H2Y 2-  MY 2- + H Ind.2- + H + Wine red Blue  Mg 2+ + H Ind.2-  Mg Ind.- + H+ Wine red Mg Ind.- + H2Y 2-  MgY 2- + H Ind.2- + H + Wine red Blue ETB cannot be used for the determination of Cu 2+ , Fe 3+ , Al 3+ , Co 2+ and Ni 2+

Specific Indicator Examples: (1)- Thiocyanate (CNS -) a)- It is specific ind. for Fe 3+ b)- When sample of Fe 3+ is treated with CNS – a blood red complex is formed. c)- Upon titration against EDTA, the end point is detected by decolorization of the blood red color.   (2)- Salicylic acid a)- It is specific for Fe 3+ , gives violet color. b)- The end point is detected by decolorization of violet color

Applications of Complexometric Titrations EDTA Titration Requirements for direct EDTA titrations: M-EDTA complex must be more stable than M-Ind. complex in buffered medium. The compound to be determined is water soluble. The reaction between EDTA and metal must be rapid. If the reaction is slow it must be catalyzed. Mn+ should not be ppt. at the pH of titration. If Mn+ is ppt. as MOH, auxiliary reagent must be added to prevent pptn. of M n+.

1- pb2+ salt is ppt. as pb(OH)2 at the pH suitable for titration. add tartaric acid (auxiliary reagent) which converts pb(OH)2 to soluble lead tartarate complex. 2-Sometimes buffer acts as auxiliary reagent. during titration of Cu2+ salt in alkaline medium, Cu(OH)2 is ppt. and the reaction with EDTA becomes slow. upon using ammonia instead of alkali hydroxides, the soluble [Cu(NH3)4]2+ is formed which is less stable than Cu-EDTA and hence the reaction forward rapidly.

Direct determination of water hardness Water hardness is due to the presence of Ca2+ & Mg2+ salts. EDTA forms complex with Ca2+ & Mg2+, Ca-EDTA complex is more stable than Mg-EDTA complex. At pH 12 EDTA forms complex with Ca2+ only.   Total Ca2+ & Mg2+: Total Ca2+ and Mg2+ determined by titration with EDTA at pH 10 using ammonia buffer and EBT as ind. Upon titration with EDTA, Ca2+ will be chelated first, then Mg2+. For Ca2+ only: Direct titration with EDTA at pH 12 using 8% NaOH and Murexide. Mg2+ is pptd. as Mg(OH)2 leaving Ca2+ which is titrated with EDTA For Mg2+ : Total – Ca2+ = Mg2+

Direct determination of Cu2+ with EDTA The complex of Cu2+ with EDTA is more stable than its complex with murexide ind. Cu2+ + H3Ind.2-  CuH2Ind.- + H+ CuH2Ind.- + H2Y2-  CuY2- + H3Ind.2- + H+ yellow violet   Direct determination of Zn2+ by EDTA - The complex of Zn2+ with EDTA is more stable than its complex with EBT ind. Zn2+ + H Ind.2-  Zn Ind.- + H+ Zn Ind.- + H2Y2-  ZnY2- + H Ind.2- + H+ wine red Blue

Back Titration Addition of known xss. of st. EDTA to the sample The medium is buffered. xss. EDTA is titrated with standard soln. of another metal ion e.g. Mg2+ or Zn2+ It is used in the following cases: Insoluble substances e.g. BaSO4 , Ca(C2O4)2 , PbSO4 , Mg3(PO4)2 … etc. Usually soluble in hot EDTA. The reaction between Mn+ & EDTA is slow (incomplete) e.g. Fe3+ , Al3+ , Cr3+ , Th4 , … etc. The Mn+ is pptd. at the pH suitable for titration e.g. Al(OH)3. The colour change at the end point : From free ind. colour  to M-Ind. complex (opposite that direct titration)

Det. of Aluminium salts: Sample of Al3+ is heated with known xss. of st. EDTA at pH 7-8. The soln. is then adjusted to pH=10 using ammonia buffer. The residual EDTA is titrated against st. Zn2+ using EBT indicator. The colour change from blue to wine red.

Titration of Mixtures EDTA is not a selective reagent (it chelates with most metal ions) Selectivity of EDTA can be increased by one of the following procedures: Control of pH of the medium Adjustment of oxidation number of metal ion Masking and demasking agent

Control of pH of the medium First group: Trivalent & tetravalent cations e.g. (Bi3+ , Fe3+ , Th4+) and Hg2+ titrated (form stable complex) at pH 1-3 using conc. HNO3. Second group: Divalent metals e.g. (Co2+ , Ni2+ , Cu2+ , Zn2+ , pb2+ and Cd2+) titrated (form stable complex) at pH 4-6 using acetate buffer. Third group: Alkaline earth metal e.g. (Ba2+ , Sr2+ , Ca2+) and Mg2+ titrated (form stable complex) at pH=10 using ammonia buffer or 8% NaOH. From the mentioned above, we can titrate Mn+ of the first group at pH 1-3 without interference of the second and third groups or at pH 4-6 we can titrate Mn+ of the second group without interference of the third group. e.g. Mixture of Bi3+ & pb2+: First titrating Bi3+ at pH = 2 using xylenol orange as ind., then increased pH to 5 by adding hexamine and titrating pb2+.

Adjustment of oxidation number of metal ion - This solves the interference between Mn+ of the same group of pH. Examples: Ascorbic acid (vit. C) is reducing agent used in: Removal of interference of Fe3+ in first group (pH 1-3)  reduced to Fe2+ Removal of interference of Hg2+ in first group (pH 1-3)  reduced to Hgo (pptd.). Removal of interference of Cu2+ in second group (pH 4-6)  reduced to cuprous. alkaline Oxidation of Cr3+  to CrO42+ H2O2 Fe2+ , Hgo, Cuprous , CrO42- do not react with EDTA

Masking and demasking agent Masking agents: are reagents which prevent interfering ion from reaction without physical separation. These reagents form complexes with interfering ions which are more stable than complexes formed with ind. & EDTA. Examples of masking agent: (A)- KCN It is used as masking agent for Ag+ , Cu2+ , Cd2+ , Co2+ , Ni2+ , Zn2+ , … etc. M+ + 2 CN -  [M(CN)2] - M+ + 4 CN -  [M(CN)4]2- (B)- Triethanolamine : CH2CH2OH N CH2CH2OH CH2CH2OH - It is used as masking agent for Fe3+ , Al3+ and Sn2+ (C) Fluoride (e.g. NH4F): - It is used as masking agent for Fe3+ and Al3+ to give hexafluoro complex [FeF6]3- and [AlF6]3- (D)- Iodide (KI): - It is used as masking agent for Hg2+ to give tetraiodo complex (HgI4)

Demasking agent : are reagents which regain the ability of masked ion to enter the reaction with ind. and EDTA. Example: - The masking by CN– can be removed by: - mixture of formaldehyde – acetic acid - on addition of demasking agent to [Zn(CN)4]2- , Zn is liberated and titrated. [Zn(CN)4]2- + 4 HCHO + 4 CH3COOH (less stable) CN  Zn2+ + 4 CH2 + 4 CH3COO- OH Cyanohydrin (more stable)