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Announcements• Quizzes They are all graded and may be up on Sapling Your “final” quiz will be on the remainder of
information in the class and tentatively will be given via Sapling over Wed., 6/4 – Sun., 6/8 (Week 10)
• Reading updates Chapter 9, Section B5 is on ISEs Chapter 17, all of Section D is on EDTA Chapter 26, Sections A4 and A5
Course websitehttp://faculty.sites.uci.edu/chem2l/chem-h2lcm3lc/
Syllabus & Sapling Honesty Agreement
The power of turnitin.com; an example
The power of turnitin.com; an example
The power of turnitin.com; an example
Week 7 Objectives
By the end of the week, you will be able to:
• Explain the properties of EDTA and the specific experimental conditions required to use EDTA effectively
• Perform calculations involving EDTA chelation
• Explain the purpose of an auxiliary complexingagent, such as Calmagite and Eriochrome Black T
Flashback:Week 2
Flashback:Week 4
Barium-MTB Complexation Equilibrium:
Kf is the Ba-MTB Formation Constant
Metal Complexation Formation Constants
Kf ≈ 105
Alpha Fractions for free and complexed Ba2+
The total Barium Concentration in solution is conserved.
Alpha Fractions!
Alpha Fractions for free and complexed Ba2+
The ligand concentration ([MTB]) determines the speciation.
Metal Complex Formation Constants
Weak Metal Complex Formation Constants have similar values of K1 to Kn
Therefore, many species co-exist in solution!
Used as a food preservative to sequester Mn+, and attenuate O2 oxidation chemistry with your food
At what pH does this predominantly exist?
Used as a food preservative to sequester Mn+, and attenuate O2 oxidation chemistry with your food
Note: Ki terms are also sometimes written as β values
That is, β1 = K1, and β4 = K1K2K3K4
pH
Most EDTA titrations are performed at pH >= 10.
αY4-
formationconstant
formationconstant
α
α
′ α
formationconstant
α
α
′ α
For experiments, this conditional formation constant (Kf’) is helpful because you do not need to know the free concentration of ligand to calculate Kf or concentration of metal species
pH dependent
formationconstant
Kf’ should be at least 106 to get a sharp end point with 10 mMmetal ion solution (and thus closest to the equivalence point)
Kf’ should be at least 106 to get a sharp end point with 10 mMmetal ion solution (and thus closest to the equivalence point)
50 mL of 10 mM solutions of variouscations at pH 6.0
EDTA titrations for metal ions
pCa or pMg canbe used to determine the titration endpoint.
pCa
What if we have both Ca2+ and Mg2+ present?
Precipitation Calculation for EDTA Titration of Ca2+ and Mg2+
What if we have both Ca2+ and Mg2+ present?
Precipitation Calculation for EDTA Titration of Ca2+ and Mg2+
“only”
EDTA titrations for metal ions
EDTA will displace weaker ligands -- you will use this process with calmagite to determine the endpoint of an EDTA titration for Mg2+
Hydroxyquinoline: a metal chelator that fluoresces upon binding!
Mg
Trivalent Cations
Divalent Cations
8-hydroxyquinoline
Hydroxyquinoline: a metal chelator that fluoresces upon binding!
Fluorometric Detection of Mg2+ in Seawater
8-hydroxyquinoline-5-sulfonic Acid
Mg2+
Kf = 10‐16.5
Kf = 10‐16.5
This is key, because not only does NH3 prevent precipitation but it also complexes some Zn2+. Cool!
We use an ammonia buffer solutionfor a reason...
αY4- = 0.355 at pH =10.
Marcel Pourbaix(1904–1998)
http://corrosion‐doctors.org/Biographies/PourbaixBio.htm
Recall:Pourbaix Diagrams
from Pourbaix, Atlas of electrochemical equilibria in aqueous solutions, 1974
Solubility product Ksp = 3.0×10−17
Zn2+ + 2OH- → Zn(OH)2(s)
Ksp = [Zn2+][OH -]2
At pH=10, [Zn2+] = ??
Zinc Hydroxide
Solubility product Ksp = 3.0×10−17
Zn2+ + 2OH- → Zn(OH)2(s)
Ksp = [Zn2+][OH -]2
At pH=10, [Zn2+] = Ksp /[OH -]2
Zinc Hydroxide
Solubility product Ksp = 3.0×10−17
Zn2+ + 2OH- → Zn(OH)2(s)
Ksp = [Zn2+][OH -]2
At pH=10, [Zn2+] = Ksp /[OH -]2
Zinc Hydroxide
= (3.0×10−17) /(1.0×10−4)2
= 3.0×10−9 M
This is the maximum free Zinc concentration.
Ammonia Buffer Solution
At pH=10, [NH3] = ??
[NH3]total = 0.100M
NH3 + H2O <=> NH4+ + OH - pKb = 4.74
Ammonia Buffer Solution
At pH=10:
[NH3]total = 0.100M
NH3 + H2O <=> NH4+ + OH -
[NH3] = 0.0846 M
pKb = 4.74
And recall, we think we have:[Y4–] = 3.55 x 10‐5 M[Zn2+]max = 3.0 x 10‐9 M
Zinc - Ammonia Complexation
At [NH3] = 0.0846M, αZn2+ = 1.61 x 10-5
Zinc - Ammonia Complexation
At a TOTAL Zinc concentration of 1.00 x 10-4 M:
[Zn2+] = αZn2+ [Zn2+]tot
[Zn2+] = (1.61 x 10-5)(1.00 x 10-4)
[Zn2+] = 1.61 x 10-9 M
Therefore: no precipitation!
EDTA Titration
You would like to perform a titration of 50.00 mL of a 1.00 x 10-4 M Zn2+ solution with a 1.00 x 10-4 M EDTA solution.
What is pZn at the equivalence point?
Log Kf for the ZnY2- complex is 16.5. Both solutions are buffered to a pH of 10.0 using a 0.100M ammonia buffer.
The alpha fraction for Y4- is 0.355 at a pH of 10.0.The alpha fraction for Zn2+ is 1.61 x 10-5.
Once more, with feeling:
EDTA Titration
You would like to perform a titration of 50.00 mL of a 1.00 x 10-4 M Zn2+ solution with a 1.00 x 10-4 M EDTA solution.
Total moles of Zinc = ??
EDTA Titration
You would like to perform a titration of 50.00 mL of a 1.00 x 10-4 M Zn2+ solution with a 1.00 x 10-4 M EDTA solution.
Total moles of Zinc = (1.00 x 10-4 M)(0.050L)
= 5.0 x 10-6 moles
EDTA Titration
You would like to perform a titration of 50.00 mL of a 1.00 x 10-4 M Zn2+ solution with a 1.00 x 10-4 M EDTA solution.
Total moles of Zinc = 5.0 x 10-6 moles
Equivalence point volume = ??
EDTA Titration
You would like to perform a titration of 50.00 mL of a 1.00 x 10-4 M Zn2+ solution with a 1.00 x 10-4 M EDTA solution.
Total moles of Zinc = 5.0 x 10-6 moles
Equivalence point volume = 0.100L
EDTA Titration
You would like to perform a titration of 50.00 mL of a 1.00 x 10-4 M Zn2+ solution with a 1.00 x 10-4 M EDTA solution.
Total moles of Zinc = 5.0 x 10-6 moles
Equivalence point volume = 0.100L
[ ZnY2- ] = ??
EDTA Titration
You would like to perform a titration of 50.00 mL of a 1.00 x 10-4 M Zn2+ solution with a 1.00 x 10-4 M EDTA solution.
Total moles of Zinc = 5.0 x 10-6 moles
Equivalence point volume = 0.100L
[ ZnY2- ] = 5.0 x 10-5 M
We assume a stoichiometric reaction.
EDTA Titration
You would like to perform a titration of 50.00 mL of a 1.00 x 10-4 M Zn2+ solution with a 1.00 x 10-4 M EDTA solution.
Total moles of Zinc = 5.0 x 10-6 moles
Equivalence point volume = 0.100L
[ ZnY2- ] = 5.0 x 10-5 M
[ Zn2+ ] = ??
[ ZnY2- ] = 5.0 x 10-5 M
We assumed a stoichiometric reaction. But actually, there is a little bit of free (uncomplexed) EDTA and free (uncomplexed) Zinc in solution.
αZn2+ = 1.61 x 10-5
Log Kf = 16.5. αY4- = 0.355
[ ZnY2- ] = 5.0 x 10-5 M
We assumed a stoichiometric reaction. But actually, there is a little bit of free (uncomplexed) EDTA and free (uncomplexed) Zinc in solution.
1.66 x 10-8 M
EDTA Titration
You would like to perform a titration of 50.00 mL of a 1.00 x 10-4 M Zn2+ solution with a 1.00 x 10-4 M EDTA solution.
Total moles of zinc = 5.0 x 10-6 moles
Equivalence point volume = 0.100L
[ ZnY2- ] = 5.0 x 10-5 M 1.66 x 10-8 M
Feature 17.5 in Skoog, et al., goes over a similar problem, but via a slightly different method.
from Pourbaix, Atlas of electrochemical equilibria in aqueous solutions, 1974
Marcel Pourbaix(1904–1998)
http://corrosion‐doctors.org/Biographies/PourbaixBio.htm
Recall:Pourbaix Diagrams
Because Zn(OH)2 can (twice) deprotonate too!
? ?
Because Ksp = 3.0 x 10‐17
[Zn2+][ZnO2
2–]
Week 7 Objectives
By the end of the week, you will be able to:
• Explain the properties of EDTA and the specific experimental conditions required to use EDTA effectively
• Perform calculations involving EDTA chelation
• Explain the purpose of an auxiliary complexingagent, such as Calmagite and Eriochrome Black T