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Lab 7Experiment 22 (p.219)
Amino Acid Complexes: Stability constants of Ni(glycinate)n
(2-n)+
Inorganic Chemistry Laboratory
1
Acid-Base Chemistry of GlycineGlycine is an example of a zwitterion.
What is a zwitterion?
A molecule that contains a (+) and (-) electrical charge at different location within the molecule
H2O
2
Glycinate Titration
What is the pH of this solution?
HNO3 is a Strong Acid!
HNO3 H+ + NO3-
[ ] [ ]( ) 3.2005.0log
3
=−==+
pHHNOH
H2N CH C
H
O-
O
How will the pH respond when glycinateis titrated into the solution?
3
Glycinate Titration
What is the pH of this solution?
HNO3 is a Strong Acid!
HNO3 H+ + NO3-
[ ] [ ]( ) 3.2005.0log
3
=−==+
pHHNOH
H2N CH C
H
O-
O
How will the pH respond when glycinateis titrated into the solution?
4
Glycinate Titration with Nickel
What is the pH of this solution?
HNO3 is a still a Strong Acid!
[ ] [ ]( ) 3.2005.0log
3
=−==+
pHHNOH
H2N CH C
H
O-
O
How will the pH response to glycinatetitration differ with Ni2+ in the solution?
5
Ni2+-Glycinate InteractionsHow will glycinate interact with Ni2+?
H2N
O
-O
Ni
H2NO
O
MX+
Ni
H2NO
O
NH2O
O
MX2
Ni
O
H2N
O
NH2
H2N
O
O
OO
MX3
6
Ni2+-Glycinate Interactions
Ni
H2NO
O
MX+
Ni
H2NO
O
NH2O
O
MX2
Ni
O
H2N
O
NH2
H2N
O
O
OO
MX3-
7
Ni2+-Glycinate Interactions
Ni
H2NO
O
MX+
Ni
H2NO
O
NH2O
O
MX2
Ni
O
H2N
O
NH2
H2N
O
O
OO
MX3-
[ ][ ][ ]−+
−
=AM
MA21β
[ ][ ][ ]22
22 −+=
AMMAβ [ ]
[ ][ ]323
3 −+
−
=AM
MAβ
8
The Effect of Ni2+ on pH
HA ⇌ H+ + A-[ ][ ][ ]HA
AHKa
−+
=
Consider the simple glycine (HA) dissociation reaction:
So why does Ni2+ influence this reaction?
Ni2+ preferentially binds to the base form (A-)which alters the apparent Ka according tomass action (LeChatlier’s Principle)
Atot = [A-] + [HA]
Atot = [A-] + [HA] + [MA+] + [MA2] + [MA3-]
9
Equilibrium Theory ApproachWhat we know…..
Mtot, Htot and Atot at any point in the titration
pH at any point in the titration
Glycinate is your titrantM1V1=M2V2
This is what you measure
Atot = [A-] + [HA] + [MA+] + [MA2] + [MA3-]
HA ⇌ H+ + A-
[ ][ ][ ]
10105.2 −−+
== xHA
AHKa
A- + M2+ ⇌ MA+
[ ][ ][ ]−+
−
=AM
MA21β
Equilibrium Expressions that describe these concentrations
2A- + M2+ ⇌ MA2
[ ][ ][ ]22
22 −+=
AMMAβ
3A- + M2+ ⇌ MA3-
[ ][ ][ ]32
33 −+
−
=AM
MAβ
10
Equilibrium Theory Approach
Fractional Saturation (ñ or θ)
The total number of ligandsbound per metal ion
[ ] [ ] [ ][ ] [ ] [ ] [ ]32
232 32
MAMAMAMMAMAMA+++
++= −+
−
θ
Atot = [A-] + [HA] + [MA+] + [MA2] + [MA3-]
[Bound] =
[Metal] =
[ ] [ ] [ ][ ] [ ] [ ]33
221
33
221
132
−−−
−−−
+++
++=
AAAAAA
ββββββθ
11
Equilibrium Theory Approach
Our goal is to cast θ in terms of known values
[ ] [ ] [ ][ ] [ ] [ ]33
221
33
221
132
−−−
−−−
+++
++=
AAAAAA
ββββββθ
[ ] [ ] [ ] [ ]( )+−+
− −+= HOHCHKA H
a
CH [H+] from original HNO3 solution
[ ] [ ] [ ]( )tot
Ha
tot
M
HOHCHKA +−
+ −+
+−
=1
θ
12
Graphical Approximation of Kn
How are pKa values approximated from a pH titration?
pH @ ½ Equivalence Point
[ ][ ]HAApKpH a
−
+= log
[ ][ ]totXHX
=θ
pH
13
Graphical Approximation of Kn
θ
Log[A-]
½
pK1
1 ½
pK2
2 ½
pK3
33
22
11
logloglog
KpKKpKKpK
−=−=−=
14
Graphical Determination of βn
[ ] [ ] [ ][ ] [ ] [ ]33
221
33
221
132
−−−
−−−
+++
++=
AAAAAA
ββββββθ
This expression can be rearranged to generate a less complex polynomial:
( )[ ]( )[ ]( )
( )[ ]( ) 123
2
12
13
1ββ
θθβ
θθ
θθ
+−
−+
−−
=−
−−
−
AAA
What happens at very low [A-]?
( )[ ]( )[ ]( ) 121
21
ββθ
θθθ
+−
−=
−
−
−
AA
( )[ ]−− Aθθ
1
( )[ ]( )θθ−
− −
12 A
15
Graphical Determination of βn
This expression can be further rearranged to generate a less complex polynomial:
( ) [ ]( )[ ]
( )[ ]( ) 232
1
23
21 ββ
θθ
θβθθ
+−
−=
−
−− −
−
− AA
A
( )[ ]( )[ ]( )
( )[ ]( ) 123
2
12
13
1ββ
θθβ
θθ
θθ
+−
−+
−−
=−
−−
−
AAA
( )[ ]( )θθ−
− −
23 A
( ) [ ]( )[ ]2
1
21
−
−
−
−−
AA
θβθθ
16
Experimental Considerations
Prepare 200 mL of this solution
***Nickel is a carcinogen! Ni salt will be massed in the fume hood***
Solid
H2O OH-
pH ~ 7 Glycinate0.4 M
Titrate glycinate into Ni solution in 0.2 mL increments.
Record pH for every aliquot.
……Hope you liked Chemometrics….. 17
How to start your spreadsheet
( )[ ]( )[ ]( )
( )[ ]( ) 123
2
12
13
1ββ
θθβ
θθ
θθ
+−
−+
−−
=−
−−
−
AAA
What do you need to solve for βn?
[ ] [ ] [ ] [ ]( )+−+
− −+= HOHCHKA H
a [ ] [ ] [ ]( )tot
Ha
tot
M
HOHCHKA +−
+ −+
+−
=1
θ
Injection # Volume Atot pH [H+] [OH-] [A-] θ
18
