Chapter 9 Complexation and Precipitation Titrations
Forming Complexes • Most metals ions react with electron-pair donors to form coordination compounds or complexes. • The donor species (ligand) must have at least one pair of unshared electrons available for bond formation. • Cu(H2O)4+2, Cu(NH3)4+2, Cu(NH2CH2COO)2 CuCl4-2
Ligands • A ligands is a neutral molecule or ion having a lone pair that can be used to form a bond to a metal ion. • Chelating agents: unidentate, bidentate, tridentate, tetradentate, pentadentate, hexadentate
Titration of a Single Anion • Calculate the pAg of the solution during the titration of 50ml of 0.05M NaCl with 0.1M AgNO3 after the addition of the following volumes of reagent: (a) 0ml (b) 24.5 ml (c) 25 ml (d) 25.5ml Ksp=1.82×10-10
A: 50 ml of 0.05M NaCl with 0.1 M AgNO3 B: 50 ml of 0.005M NaCl with 0.01 M AgNO3
Signaling the End Point • Mohr method Formation of a colored precipitate • The Mohr method must be carried out at a pH of 7-10 because chromate ion is the conjugate base of the weak chromic acid. white red
Ethylenediaminetetraacetic Acid The EDTA molecule has six potential sites for bonding a metal ion: the four carboxyl groups and two amino groups.
H3Y- K1=1.02×10-2 H2Y-2 K2=2.14×10-3
HY-3 K3=6.92×10-7 Y-4 K4=5.5×10-11
The Nature of EDTA Complexes with Metal Ions • The reagent combines with metal ions in a 1:1 ratio regardless of the charge on the cation.
Equilibrium Calculations Involving EDTA • EDTA titrations are always performed in solutions that are buffered to a known pH to avoid interferences by other cations or to ensure satisfactory indicator behavior.
Calculation of the Cation Concentration in EDTA Solutions • Calculate a4 • Calculate conditional formation constants K’ x x 0.015M-x Calculate equilibrium [Ni+2] at pH=3 and pH=8
Indicator for EDTA TitrationsErtichrome Black T K1=5×10-7 redblue K2=2.8×10-12 blue orange redblue
Compounds changing colour when binding to metal ion. Kf for Metal-In- < Kf for Metal-EDTA. • Before Titration: • Mg2+ + In- MgIn • (colourless) (blue) (red) • During Titration: Before the end point • Mg2+ + EDTA MgEDTA • (free Mg2+ ions) (Solution red due to MgIn complex) At the end point: MgIn + EDTA MgEDTA + In- (red) (colourless) (colourless) (Blue)
Requirements for Indicator • Metal-indicator complex must be less stable than the metal-EDTA complex. • Binding between metal and indicator must not be too weak. It has to avoid EDTA replacing at the beginning of the titration. • In general, the metal-indicator complex should be 10 to 100 times less stable than the metal-titrant complex.
Titration of Ca+2 and Mg+2 • The K’ of EDTA of Ca+2 and Mg+2 are too close to differentiate between them in an EDTA titration. • Generally, they will titrate together. • This titration is used to determine total hardness of water.
Titration of Ca+2 • EB-T cannot be used to indicate the direct titration of Ca+2 in the absence of Mg+2 with EDTA. • The indicator forms too weak a complex with Ca+2 to give a sharp end point.
Resolution • A small measured amount of Mg+2 is added to the Ca+2 solution. • Ca+2 gives more stable K’ than Mg+2. • A correction is made for the amount of EDTA used for titration of the Mg+2 .
Kf for Mg+2-EDTA < Kf for Ca+2-EDTA. • Before Titration: • Ca+2+EDTA+Mg2++In- MgIn+CaEDTA • During Titration: Before the end point • Ca2+ + EDTA CaEDTA At the end point: MgIn + EDTA MgEDTA + In- (red) (colourless) (colourless) (Blue)
EDTA Titration Curve Region 1 Excess Mn+ left after each addition of EDTA. Conc. of free metal equal to conc. of unreacted Mn+. Region 2 Equivalence point:[Mn+] = [EDTA] Some free Mn+ generated by MYn-4 Mn+ + EDTA Region 3 Excess EDTA. Virtually all metal in MYn-4 form.