chap 5. coordination chemistry

8/7/2019 chap 5. coordination chemistry

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Chapter 5. Coordination Chemistry

CESE 219 Chemistry for Environmental Engineering S.K. Hong 1

CESE 219

Chemistry for Environmental Engineering

COMPLEX FORMATION

OUTLINE

1. IntroductionDefinitionsComplex Stability

2. Hardness2.1. Hardness Definition2.2. Hardness Determination2.3. Hardness Removal (Water Softening)

Extra Topics

3. COD Analysis

4. Hydrolysis of Metal Ions

5. Complexes with Inorganic Ligands

6. Complexes with Organic Ligands

READING ASSIGNMENT:Water Chemistry , V. L. Snoeyink and D. Jenkins: Chapter 5.Extra Handout for Hardness

PROBLEM SOLVING:Extra Problem for Hardness RemovalHomework #5


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1. Introduction Important in water analysis

(e.g. hardness measurement, COD analysis) Affect the chemistry of metal ions in water

Complexes

; central metal ions + ligands = complex

Ligands Ligands that attach to a central metal

Ion at only one point monodentate

Ligands that attach at two or more sites multidentate

Total possible number of attachments to a central atom

Coordination number

Liqand + central metal

Naming complexes

a) Ligands are named as

Molecules if neutral

~ ium if positive

~ o if negative

b) central metal

anionic ate

neutral

cationic

e.g) Fe(CN) 64- (hexa cyano ferrate)

hexa cyano ferrate

Ligand + Central metal ions complex

K ; (complex formation) stability constant

Large K stable complex

A - metal ions ; Alkali metal (Na +/K+)

Alkaline earth metals (Mg 2+/Ca 2+, Al3+, Si 4+)

Coordinate with ligands containing oxygen as electron donor

anionic complexnegative Ligands# of Ligands

* Note : prefix Number of Ligands

No suffixes


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B ? metal ions; Ag +, Zn 2+, Hg 2+ , Pb 2+ , Sn 2+

Coordinate with ligands containing S, P, or N as donor atoms

2. Hardness2.1. Hardness Definition(1) Introduction

Hardness : concentration of divalent metallic cations

Problems

react with soap to form precipitates

react with certain anions in water to form scale in pipes, heaters, boilers

(Note : no health problem)

Table 1.

Cations causing hardness Anions

Ca 2+ HCO 3-

Mg2+ SO 42-

Sr 2+ Cl-

Fe 2+ NO 3-

Mn2+ SiO 32-

Degree of Hardness (measured in terms of CaCO 3)

Table 2.

mg/L (CaCO 3) Degree of hardness

0 ~ 75 Soft

75 ~ 150 Moderately hard

150 ~ 300 Hard Softening

300> Very hard required

(Note: Generally finished water 50~150, 80~100 hardness is desirable (for aesthertic and

corrosion control))

Source of Hardness : water contacts with soil and rock (leaching cations)

groundwaters are harder than surface waters


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(2) Type of Hardness

Total hardness

= sum of the calcium and magnesium ion concentration(Note: concentrations of other divalent cations are very low compared Ca and Mg)

Two ways divalent cations

anions associated with cations

Based on divalent cations

Calcium Hardness (CaH) : hardness attributable to Ca 2+

Magnesium Hardness (MgH) : hardness attributable to Mg 2+

Total hardness = CaH + MgH

Based on anions associated

Carbonate hardness (CH)

- portion of the total hardness chemically equivalent to the carbonate and bicarbonate

alkalinities

- the amount of Ca 2+ and Mg 2+ ions that can be removed as insoluble carbonates

(eg. CaCO 3) by boiling the water

- temporary hardness

Non carbonate hardness (NCH)

- portion of the total hardness in excess of the carbonate and bicarbonate alkalinities

- the amount of Ca 2+ and Mg 2+ ions remaining in solution after boiling

- permanent hardness

(3) Equivalence diagram (bar diagram)

Cations (Ca 2+, Mg 2+, Na +)CO 2

Anions (HCO 3-, SO 4

2-)

a) Alkalinity < total hardness

Ca 2+ Mg2+

HCO 3- SO 4

2- Cl-

Carbonate hardness = alkalinity (the rest of hardness = noncarbonated hardness)

optional

Alkalinity (at natural pHs)


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b) Alkalinity > hardness

Ca 2+ Mg2+ Na +

HCO 3-

Cl-

Carbonate hardness = total hardness

(no noncarbonated hardness)

2.2. Hardness Determination; determined by titration against EDTA

(ethylene ? diamine - tetra acetic acid) complex formation

(1) complex formation

central metal ions + ligands complex

K ; stability constant or complex formation constant

Large K ; more stable complex formation

Ligands

Monodentate : Ligands that attach to a central metal ion at only one point

Multidentate : Ligands that attach at two or more site (chelating agent)

(2) complex formation between Ca 2+ , Mg 2+ and EDTA

chelating agent EDTA has six bonding sites

4 acetate groups

2 nitrogen atom


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Ca 2+, (Mg-EBT) -

(red)

(EDTA-Ca) 2-, (EDTA-Mg) 2-,

(H-EBT) -, (bule)

Ca 2+ + EDTA 4- (EDTA-Ca) 2- K = 10 +10.7

Mg2+ + EDTA 4- (EDTA-Mg) 2- K = 10 +8.7

; EDTA combines first with the Ca2+

, then Mg2+

(3) Hardness determination

difficult to detect the end point of EDTA titration (no color change ~ colorless)

; indicators are needed

total hardness determination

- Eriochrome Black T (EBT) is used as an indicator

[H-EBT]2- + Mg 2+ [Mg-EBT] - + H+ K = 10 7

Bule Red

- Stability constant

[Ca-EDTA] 2- > [Mg-EDTA] 2- > [Mg-EBT] -

- The stronger complexing agent decomposes the existing complex and with draws the metal

ion into a complex with itself

- EDTA (at pH=10) EDTA

EDTA first reacts with Ca 2+ to form (EDTA-Ca) 2- complex

Then breaks up Mg-EBT complex and forms (EDTA-Mg) 2- complex

Color change (red blue)

Ca 2+ hardness determination

- murexide is used as an indicator (ammonium purpurate)

[Murexide] + Ca 2+ [Ca-Murexide] 2+

Purple Pink- color change from pink to purple when Ca-murexide is broken

; end point of EDTA titration (at pH =12)

Mg2+ hardness determination

TH ? CaH = MgH

Note : Ca(OH) 2, Mg(OH) 2 formation during hardness measurement


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2.3. Water Softening

(1) Introduction Removal of hardness

Chemical precipitation most popular

Ion exchange

Reverse osmosis

Precipitation softening

- most commonly used large-scale hardness removal process

- dependent on low solubility of CaCO 3 and Mg(OH) 2

Mg2+ + 2OH - Mg(OH) 2 (Magnesium Hydroxide)

Ca 2+ + CO 32- CaCO 3 (Calcium Carbonate)

- Lime : Ca(OH) 2 lime softening

Soda Ash : Na 2CO 3 (+ Ca(OH) 2) lime ?soda ash softening

(2) Softening reactions

lime ? soda Ash precipitation softening

a) CO 2 removal

Ca(OH) 2 + CO 2 CaCO 3(s) + H 2O

- necessary to raise pH sufficiently to precipitate the hardness

- does not remove hardness but increase a lime demand

b) Removal of carbonate calcium hardness with lime

Ca(OH) 2 + Ca(HCO 3)2 2CaCO 3(s) + 2H 2O

- 1 mole Ca(OH) 2

c) Removal of carbonate magnesium hardness with lime

2Ca(OH) 2 + Mg(HCO 3)2 2CaCO 3(s) + Mg(OH) 2(s) + 2H 2O

- 2 mole Ca(OH) 2 d) Removal of noncarbonated calcium hardness with soda ash

Na 2CO 3 + CaSO 4 CaCO 3(s) + Na 2SO 4

or Na 2CO 3 + CaCl 2 CaCO 3(s) + 2NaCl

e) Removal of noncarbonated magnesium hardness with lime and soda ash

Ca(OH) 2 + Na 2CO 3 + MgSO 4 CaCO 3(s) + Mg(OH) 2(s) + Na 2SO 4

MgSO 4 + Ca(OH) 2 Mg(OH) 2(s) + CaSO 4

CaSO 4 + Na 2CO 3 CaCO 3(s) + Na 2SO 4


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Softening reactions

* The negative alkalinity is the alkalinity in excess needed to be neutralized before precipitation

(3) Lime and soda ash requirement

based stoichiometry of softening (type of hardness) reactions

(4) Caustic soda softening

- Advantage : less sludge production

- Disadvantage : expensive, hazard

CO 2 + 2NaOH Na 2CO 3 + H2O

Ca(HCO 3)2 + 2 NaOH CaCO 3(s) + Na 2CO 3 + 2H 2O

Mg(HCO 3)2 + 4 NaOH Mg(OH) 2(s) + 2Na 2CO 3 + 2H 2O

CaSO 4 + 2NaOH Ca(OH) 2 + Na 2SO 4

MgSO 4 + 2NaOH Mg(OH) 2 + Na 2SO 4

; solubility diagram of Ca(OH) 2 and Mg(OH) 2

(5) Recarbonation

stabilization of softened water

pH (=8.5) control and to avoid CaCO 3 ppt

CO 32- + CO 2 + H2O 2HCO 3

-

(CaCO 3 + CO 2 + H 2O Ca(HCO 3)2)

meq dose meq sludge

Demand reaction Ca(OH) 2 Na 2CO 3 CaCO 3 Mg(OH) 2

CO 2 Ca(OH) 2 + CO 2 CaCO 3(s) + H 2O 1 0 1 0

CaCH Ca(OH) 2 + Ca(HCO 3)2 2CaCO 3(s) + 2H 2O 1 0 2 0

MgCH 2Ca(OH) 2 + Mg(HCO 3)2

2CaCO 3(s) + Mg(OH) 2(s) + 2H 2O

2 0 2 1

CaNCH Na 2CO 3 + CaSO 4 CaCO 3(s) + Na 2SO 4 0 1 1 0

MgNCH Ca(OH) 2 + Na 2CO 3 + MgSO 4

CaCO 3(s) + Mg(OH) 2(s) + Na 2SO 4

1 1 1 1

Neg Alk * Na(HCO)3 + Ca(OH)2 CaCO 3(s) + NaOH 1 0 1 0


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3. COD Analysis

(1) Introduction: Chemical Oxygen Demand test

means to measure organic strength of domestic and industrial wastes

measure the total quantity of oxygen required for oxidation to carbon dioxide and water

organic matter is converted to carbon dioxide and water regardless of biological

assimilability

generally greater than BOD values

inability to differentiate between biologically oxidizable and inert organic matter

advantage : short time required for evaluation

reliable correlation between COD and BOD

more detailed test methods ,

dichromate (Cr 2O72-) are used as oxidizing agent

(2) Complex formation between Hg 2+ and Cl -

: chloromercury( ) complex

: inorganic interference during COD test

inorganic ions are also oxidized by Cr 2O72-(K2Cr2O7)

error in measurement

chloride (Cl -) ions are the most serious problem

Cr2O72+ + 14H + + 6Cl - 2Cr 3+ + 3Cl 2(aq) + 7H 2O

prevented by complexation with Hg 2+ (HgSO 4)

reduce free chloride ions

Stepwise formation

Hg2+ + Cl - HgCl + logK1=7.15

HgCl-+ Cl

- HgCl 2 logK2=6.9

HgCl 2 + Cl- HgCl 3

- logK3=2.0

HgCl 3- + Cl - HgCl 4

2- logK4=0.7

Overall formation

Hg2+ + Cl - HgCl + log 1=7.15

Hg2+ + 2Cl - HgCl 2 log 2=14.05

Hg2+ + 3Cl - HgCl 3- log 3=16.05

Hg2+

+ 4Cl-

HgCl 42-

log 4=16.75


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Example 5- 1 Complex formation during COD test

Step #1 [Hg2+

], [Cl-

], [HgCl+

], [HgCl 20

], [HgCl 3-

], [HgCl 42-

]six unknowns six equation required

(Note : not acid- base reaction No H +, OH -

But general frame work should be followed)

Step #2

Mass balance (0.4g HgSO 4 in 60mL total sample)

CT,Hg = 2.24 10-2M = [Hg 2+]+[HgCl +]+[HgCl 2

0]+[HgCl3-]+[HgCl 4

2-]

CT,Cl = 9.39 10-3M = [Cl -]+[HgCl +]+2[HgCl 2

0]+3[HgCl 3-]+4[HgCl 4

2-]

Complex formation reactions

]][[

][10

215.7

1ClHg

HgCl

22

0)(205.14

2]][[

][10

ClHg

HgCl aq

32305.16

3

]][[

][10

ClHg

HgCl

42

2475.16

4]][[

][10

ClHg

HgCl

total of six equations

Step #3 Solve for Cl - and other complexes

Exact solution eliminate [Hg 2+] from mass balance

solve for [Cl -] using equation

Approximate solution three trial/error method

CT,Hg =[Hg2+]

=[Hg 2+]+[HgCl 20]

=[Hg 2+]+[HgCl 20]+ [HgCl +]

Answer

[Cl -]=3 10 -8M

[HgCl 20]=1.6 10 -3

majority of Cl-

ions are complexed with Hg2+

chap 5. coordination chemistry

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