Lab 7

Experiment 22 (p.219)

Amino Acid Complexes: Stability constants of Ni(glycinate)n

(2-n)+

Inorganic Chemistry Laboratory

1

Acid-Base Chemistry of Glycine

2

Glycine 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

Ni2+-Glycinate Interactions

3

How will glycinate interact with Ni2+?

MX+ MX2 MX3

Ni2+-Glycinate Interactions

4

MX+ MX2 MX3

-

Ni2+-Glycinate Interactions

5

MX+ MX2 MX3

-

AM

MA21

22

22

AM

MA

32

33

AM

MA

Glycinate Titration

6

What is the pH of this solution?

HNO3 is a Strong Acid!

HNO3 H+ + NO3-

3.2005.0log

3

pH

HNOH

How will the pH respond when glycinate is titrated into the solution?

Glycinate Titration

7

What is the pH of this solution?

HNO3 is a Strong Acid!

HNO3 H+ + NO3-

3.2005.0log

3

pH

HNOH

How will the pH respond when glycinate is titrated into the solution?

Glycinate Titration with Nickel

8

What is the pH of this solution?

HNO3 is a still a Strong Acid!

3.2005.0log

3

pH

HNOH

How will the pH response to glycinate titration differ with Ni2+ in the solution?

The Effect of Ni2+ on pH

9

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 to mass action (LeChatlier’s Principle)

Atot = [A-] + [HA]

Atot = [A-] + [HA] + [MA+] + [MA2] + [MA3-]

Equilibrium Theory Approach

10

What we know…..

Mtot, Htot and Atot at any point in the titration

pH at any point in the titration

Glycinate is your titrant M1V1=M2V2

This is what you measure

Atot = [A-] + [HA] + [MA+] + [MA2] + [MA3-]

HA ⇌ H+ + A-

10105.2

xHA

AHKa

A- + M2+ ⇌ MA+

AM

MA21

Equilibrium Expressions that describe these concentrations

2A- + M2+ ⇌ MA2

22

22

AM

MA

3A- + M2+ ⇌ MA3-

32

33

AM

MA

Equilibrium Theory Approach

11

Fractional Saturation (ñ or q)

The total number of ligands bound per metal ion

32

2

32 32

MAMAMAM

MAMAMA

q

Atot = [A-] + [HA] + [MA+] + [MA2] + [MA3-]

[Bound] = [Metal]total =

33

2

21

3

3

2

21

1

32

AAA

AAA

q

Equilibrium Theory Approach

12

Our goal is to cast q in terms of known values

33

2

21

3

3

2

21

1

32

AAA

AAA

q

HOHCH

KA H

a

CH [H+] from original HNO3 solution

tot

Ha

tot

M

HOHCH

KA

1

q

Graphical Approximation of Kn

13

How are pKa values approximated from a pH titration?

pH @ ½ Equivalence Point

HA

ApKpH a

log

totX

HXq

pH

Graphical Approximation of Kn

14

q

Log[A-]

½

pK1

1 ½

pK2

2 ½

pK3

33

22

11

log

log

log

KpK

KpK

KpK

Graphical Determination of n

15

33

2

21

3

3

2

21

1

32

AAA

AAA

q

This expression can be rearranged to generate a less complex polynomial:

123

2

1

2

1

3

1

q

q

q

q

q

q

AA

A

What happens at very low [A-]?

121

2

1

q

q

q

q

A

A

Aq

q

1

q

q

1

2 A

Graphical Determination of n

16

This expression can be further rearranged to generate a less complex polynomial:

232

1

2

3

2

1

q

q

q

qq

A

A

A

123

2

1

2

1

3

1

q

q

q

q

q

q

AA

A

q

q

2

3 A

2

1

2

1

A

A

q

qq

Experimental Considerations

17

Prepare 200 mL of this solution

***Nickel is a carcinogen! Ni salt will be

massed in the fume hood***

Solid

H2O OH-

pH ~ 7 Glycinate 0.4 M

Titrate glycinate into Ni solution in 0.2 mL increments.

Record pH for every aliquot.

……Hope you liked Chemometrics…..

How to start your spreadsheet

18

123

2

1

2

1

3

1

q

q

q

q

q

q

AA

A

What do you need to solve for n?

HOHCH

KA H

a

tot

Ha

tot

M

HOHCH

KA

1

q

Injection # Volume Atot pH [H+] [OH-] [A-] q