chapter 9 complexation and precipitation titrations
TRANSCRIPT
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 methodFormation 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.
2Ag+(aq)+X-(aq)
2Ag+(aq)+CrO4-2(aq) Ag2CrO4(s)
AgX(s) white
red
Ethylenediaminetetraacetic Acid
C CN N
CH2-COOH
CH2-COOH
HOOC-H2C
HOOC-H2C
H H
HH
The EDTA molecule has six potential sites for bonding a metal ion: the four carboxyl groups and two amino groups.
Acidic Properties of EDTA
C CN N
CH2-COOH
CH2-COOH
HOOC-H2C
HOOC-H2C
H H
HH
C CN N
CH2-COOH
CH2-COO-
-OOC-H2C
HOOC-H2C
H H
HH
H++H
H4Y
C CN N
CH2-COOH
CH2-COO-
-OOC-H2C
-OOC-H2C
H H
HH
H++H
C CN N
CH2-COO-
CH2-COO-
-OOC-H2C
-OOC-H2C
H H
HH
H++H
H3Y-
K1=1.02×10-2
H2Y-2
K2=2.14×10-3
C CN N
CH2-COO-
CH2-COO-
-OOC-H2C
-OOC-H2C
H H
HH
H+
C CN N
CH2-COO-
CH2-COO-
-OOC-H2C
-OOC-H2C
H H
HH
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.
M+n+Y-4 MY+(n-4)
]][[
][4
)4(
YM
MYK
n
n
MY
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.
Conditional Formation Constant
.applicable is αfor which pH at theonly constant a is K
][
][
][
][
]][[
][
][][][][][
][][][][
4'MY
)4(
4'
4
)4(
4
)4(
432
234
43213212
213
14
43214
Tn
n
MYMY
Tn
n
MYn
n
MY
T
CM
MYKK
CM
MYK
YM
MYK
YHYHYHHYYC
KKKKHKKKHKKHKH
KKKK
D
HKKK
D
HKK
D
HK
D
H
KKKKHKKKHKKHKHD
][
][
][
][
][][][][
3213
221
2
31
1
4
0
43213212
213
14
Calculation of the Cation Concentration in EDTA Solutions
Calculate 4 Calculate conditional formation constants K’
Ni+2+Y-4 NiY-2
184
)4(
102.4]][[
][
YNi
NiYK
n
n
MY
0.015M-xx x
Calculate equilibrium [Ni+2] at pH=3 and pH=8
pH 4 pH 4
2 3.7×10-14 8 5.4×10-3
3 2.5×10-11 9 5.2×10-2
4 3.6×10-9 10 3.5×10-1
5 3.5×10-7 11 8.5×10-1
6 2.2×10-5 12 9.8×10-1
7 4.8×10-4
Value for 4 for EDTA at selected pH value
)(101.8][Ni1027.2015.0
1027.2102.4104.5K 8pH
)(102.1][Ni1005.1015.0
1005.1102.4105.2K 3pH
KαK
CY][H]Y[H]Y[H][HY][Y][Ni
102162
16183'NiY
5282
81811'NiY
NiY4'NiY
T432
2342
Mxx
x
Mxx
x
Indicator for EDTA TitrationsErtichrome Black T
H2O+H2In-HIn-2+H3O+
H2O+HIn-2In-3+H3O+
O2N
SO3-
OH
N
N
OH
K1=5×10-7
K2=2.8×10-12
red blue
blue orange
MIn-+HY-3 HIn-2+MY-2
red blue
At the end point: MgIn + EDTA MgEDTA + In-
(red) (colourless) (colourless) (Blue)
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)
Compounds changing colour when binding to metal ion.Kf for Metal-In- < Kf for Metal-EDTA.
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 .
At the end point: MgIn + EDTA MgEDTA + In-
(red) (colourless) (colourless) (Blue)
Before Titration:• Ca+2+EDTA+Mg2++In- MgIn+CaEDTA
During Titration: Before the end point• Ca2+ + EDTA CaEDTA
Kf for Mg+2-EDTA < Kf for Ca+2-EDTA.
EDTA Titration CurveEDTA Titration CurveRegion 1
Excess Mn+ left after each additionof 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 3Excess EDTA. Virtually all metalin MYn-4 form.