Complex-Formation Titration

Complex-Formation

• A metal sample in solution is titrated with a standard solution using an indicator until the endpoint is reached.
• Complex ions or coordination compounds form when a cation bonds with an electron donor based on its coordination number.
• Complex formation is useful in assays for substances containing metals and determining water hardness based on Mg and Ca levels (in ppm).
• The equation for complex formation is: M + L ⇌ ML

Ligands

• A ligand is an ion or molecule bonding covalently with a cation/neutral metal by donating electrons.
• A complex is produced when a metal ion coordinates with a single ligand through one donor group.
• A chelate is produced when a metal ion coordinates with a single ligand with two or more donor groups.
• Multidentate ligands, having more than one electron-donating group, react more completely with cations giving sharper endpoints.
• Multidentate ligands react with metal ions in a single-step process.

Classification of Ligands

• Ligands are classified by nature as either inorganic, like EDTA, or organic, like 8-hydroxyquinoline.
• Ligands are classified by the number of electron-pair donors: uni-, bi-, tri-, tetra-, penta-, and hexadentate.

Standard EDTA

• EDTA (Ethylenediaminetetraacetic acid) is the most widely used complexometric titrant.
• EDTA is a hexadentate aminopolycarboxylic acid standard solution stable in dilute solutions.
• Stability in dilute solutions comes with high sensitivity to pH and may be selective in reactivity with metal ions.
• EDTA is commercially availabile in reagent quality as a free acid (H4Y) or as a dihydrate sodium salt (Na₂H₂Y•2H₂O).
• CaCO3 is a primary standard and Eriochrome Black T is a common indicator.
• EDTA forms chelates with all cations in a 1:1 ratio, regardless of the cation's charge.

Standard Solutions

• Standard ZnSO4 is commonly used in residual titrations.
• Dithizone is used as an indicator with ZnSO4.
• Nitrilotriacetic acid (NTA) is a tetradentate aminopolycarboxylic acid standard solution.
• NTA is the second most common complexing agent used in titrimetry.
• Ethyleneglycol bis(B-aminoethylether)-N,N,N’,N’-tetraacetic acid (EGTA) is an aminopolycarboxylic acid standard.
• EGTA is an ether analog of EDTA that selectively titrates Ca in the presence of Mg ions.

Masking

• Masking determines the concentration of metals in the presence of another metal.
• A masking agent selectively reacts with a component in a solution to prevent interference.
• It is an auxiliary agent that preferentially forms a complex with potentially interfering ions.

Auxiliary Complexing Agents

• Auxiliary complexing agents permit multiple metals to be titrated in alkaline solution with EDTA.
  • KCN (cyanide ion): permits titration of Mg and Ca ions in the presence of Cd, Co, Cu, Ni, Zn, and Pd, which form stable CN-1 complexes.
  • Triethanolamine (TEA - C6H15NO3): Provides sufficient alkalinity to prevent complexation of Al-EDTA in the presence of Mg-EDTA for Mg determination.
  • Ammonium fluoride (NH4F): Masks Ca, Mg, and Al to allow titration of Zn.
• Thioglycols allow complexation of Zn at pH 6 by inactivating Hg and Cu, which interact with sulfhydryl groups (-SH).
• Ascorbic acid, Citrates, and Tartrates may be used to mask Sb in an analyte solution.
• Adjusting the pH can allow selective titrations.

EDTA Titrations

• Direct EDTA titrations are used for Calcium, Magnesium, and Zinc.
• Residual EDTA Titrations are used for Aluminum and Bismuth.

Indicators

• Indicators are organic dyes that form colored chelates with metal ions in the sample.
• Titration results in metal-ligand complex formation with the titrant, eventually yielding a free indicator.
• Eriochrome Black T (EBT) is one of the oldest and most widely used indicators, exclusive at pH 7-11.
  • EBT is ideal for Mg titrations, but not for Ca.
  • At the endpoint, wine red color (complexed) turns to blue color (free): M-EBT + HY3- ⇋ H-EBT + MY2- (red)         (blue)
• EBT is oxidized slowly by dissolved oxygen; ascorbic acid retards oxidation or solid EBT is added directly prior to titration.
• Calmagite is structurally similar to EBT with better stability in aqueous solutions.
• Arsenazo I is an excellent indicator for EDTA titrations with rare earth metals and resists small amounts of Fe+3 during Ca and Mg titrations.
• Arsenazo III is Ca-sensitive and is used for Ca determination from biological samples and Uranium/Thorium determination.

Further indicators

• Xylenol Orange is commonly used in tetrasodium salt form in acidic solutions, appearing red (complexed) and turning yellow (free) at the endpoint.
• Hydroxynaphthol blue is a deep blue color at the endpoint.
• Dithizone is a rose pink color at the endpoint.