Complexometric Titrations 1920220http:\\asadipour.kmu.ac.ir 41 slides.

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Complexometric

Titrations

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Complex-Formation TitrationsGeneral Principles

• Most metal ions form coordination compounds with electron-pair donors (ligands)

• Mn+ + qLm- MLqn-mq Kf = [MLq

n-mq]/[Mn+][Lm-]q

• The number of covalent bonds formed is called the “coordination number” .

• e.g., Cu2+ has coordination number of 4• Cu2+ + 4 NH3 Cu(NH3)4

2+

• Cu2+ + 4 Cl- Cu(Cl)42-

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• Ligands are classified regarding the number of donor groups available:

• e.g., NH3 = “unidentate” :NH3

• Glycine = “bidentate” : NH2CH2COO -

• (also, there are tridentate, tetradentate, pentadentate, and hexadentate chelating agents)

• Multidentate ligands (especially with 4 and 6 donors) are preferred for titrimetry.– react more completely with metal ion– usually react in a single step– provide sharper end-points

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Nitrilotriacetic acid (NTA)

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• The most useful complex-formation reactions for titrimetry involve chelate formation

• A chelate is formed when a metal ion coordinates with two of more donor groups of a single ligand (forming a 5- or 6- membered heterocyclic ring)

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• Aminopolycarboxylic acid ligands

• e.g., ethylenediaminetetraacetic acid (EDTA)

• EDTA is a hexadentate ligand• EDTA forms stable chelates with most metal ions

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Complex-Formation TitrationsSolution Chemistry of EDTA(H4Y)

• 5 forms of EDTA, (H4Y, H3Y-, H2Y2-, HY3-, Y4-)

• EDTA combines with all metal ions in 1:1 ratio• Ag+ + Y4- AgY3-

• Fe2+ + Y4- FeY2-

• Al3+ + Y4- AlY-

• KMY = [MYn-4]/[Mn+][Y4-]

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11

pKa1= 1.99

pKa2= 2.67

pKa3= 6,16

pKa4= 10.26

920220http:\\asadipour.kmu.ac.ir 41 slides


• 5 forms of EDTA, (H4Y, H3Y-, H2Y2-, HY3-, Y4-)

Y4- complexes with metal ions, and so the complexation equilibria are very pH dependent.

Y4- complexes with metal ions, and so the complexation equilibria are very pH dependent.

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Complex-Formation TitrationsEquilibrium Calculations with EDTA

• Mn+ + Y4- MYn-4 Ks = [MYn-4]/[Mn+][[Y4-]

• Need to know [Y4-], which is pH-dependent• pH dependence of Y4-:• CT = [Y4-] + [HY3-] + [H2Y2-] + [H3Y-] + [H4Y]

• Define: a4 = [Y4-]/CT …………… [Y4-] =a4 ×CT

• a4 = (K1K2K3K4) / ([H+]4 + K1[H+]3 + K1K2[H+]2 + K1K2K3[H+] + K1K2K3K4)

• Conditional Formation Constant, K’MY

• Ks = [MYn-4]/[Mn+][[Y4-]

• Ks =[MYn-4]/[Mn+][[a4CT]

• K’s = a4 KMY = [MYn-4]/[Mn+][[CT] 920220


Fig. 9.2. Effect of pH on Kf’ values for EDTA chelates.

Kf’ = conditional formation constant = Kfa4.

It is used at a fixed pH for equilibrium calculations (but varies with pH since a4 does).

Kf’ = conditional formation constant = Kfa4.

It is used at a fixed pH for equilibrium calculations (but varies with pH since a4 does).

©Gary Christian, Analytical Chemistry, 6th Ed. (Wiley)

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pH=6

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Theoretical titration curves for the reaction of 50.0mL of

0.040 0M metal ion with 0.080 0 M EDTA at pH 10.00

Complex-Formation Titration curve

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Titration curve• 50.0mL 0.020M Ca2+ with 0.050M EDTA, pH 10.0

• K’s for specific pH at pH 10.0,• K’s = (a4)(KCaY) = (0.35)(5.0 x 1010) = 1.75 x 1010

• (a) pCa values before the equivalence point (10.0ml)• Ca2+ + Y4- CaY2-

• [Ca2+] =((50.0 x 0.020) –(10.0 x 0.050))/(60.0) = 0.0083M• pCa = 2.08 at 10.0ml EDTA

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N1V1=N2V2 V2=20 ml


Titration curve• (b) pCa value at the equivalence point (20.0ml)• Ca2+ + Y4- CaY2-

• [CaY2-] = ((20.0ml x 0.050M)/(70.0ml)) = 0.0142M

• K’MY = [CaY2-] / [Ca2+] [CT] = (0.0142)/ [Ca2+]2

• [Ca2+] = ((0.0142)/(1.75 x 1010))1/2 = 9.0 x 10-7M; • pCa = 6.05 at 20.0ml EDTA

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Titration curve

• (c) pCa value after the equivalence point (25.0ml)

• CT = ((25.0 x 0.050)-(50.0 x 0.020))/(75.0) = 0.0033M

• [CaY2-] = ((50.0ml x 0.020M)/(75.0ml))= 0.0133M

• K’MY = [CaY2-] / [Ca2+] [CT]; [Ca2+] = (0.0133)/(0.0033) K’MY

• [Ca2+] = 2.30 x 10-10 • pCa = 9.64 at 25.0ml EDTA

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Effect of complexing buffer : auxiliary complexing agent

At pH 10.00 Zn2+ + 2 OH– Zn(OH)2 Ksp = 3.0×10–16

Auxiliary complexing agent : ammonia (0.10~0.02M), NH3

tartrate, citrate, triethanolamineZn2+ + NH3 [Zn(NH3)]

2+ [Zn(NH3)2] 2+ [Zn(NH3)3]

2+ [Zn(NH3)4] 2+

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Complexation with auxiliary complexing agent

The equivalent constants, βi, are called overall or cumulative formation constants.

= K1

= K1K2

M + nL MLn βn = [MLn] / [M][L]n = K1K2 ··· Kn

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Effect of complexing buffer : auxiliary complexing agent

Zn2+ + Y4 – ZnY2–

complexing agent : ammonia (0.10~0.02M), tartrate, citrate,triethanolamine

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16-42

–2

102.3Y . Zn

ZnY fK

Zn2+ + NH3 Zn(NH3) 2+ Zn(NH3)2

2+ Zn(NH3)3 2+ Zn(NH3)4

2+

C

MMM

n ][ [Zn2+] =αM .C Zn

YZnYKKCZnMf

4

2

..' Y

ZnYKM

f CZn

4

2

..

At pH 10.00 Zn2+ + 2 OH– Zn(OH)2 Ksp = 3.0×10–16


EDTA Titration with an Auxiliary Complexing Agent

K’’f is the effective formation constant at a fixed pH and fixed

concentration of auxiliary complexing agent.

Now consider a titration of Zn2+ by EDTA in the presence of NH3. • The EDTA is in the form Y4-

• The Zinc bound to EDTA is in the form Zn2+

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YZnYKKCZnM Mf

4

2

..'

]][C[M

[MYn] .K K'

Tn

-4

4fl

MK ..K'' 4f


auxiliary complexing agent

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4

2

24

4 10 5.0 [NH3] Zn

Zn(NH3)

4

K

??? Y . Zn(NH3)

[NH3] ZnY-42

4

4–2

K

16-42

–2

102.3Y . Zn

ZnY fK

11

-424

4–2

2

24

-42

–2

4

16

4

104.6 Y . Zn(NH3)

[NH3] ZnY

[NH3] Zn

Zn(NH3)

Y . Zn ZnY

10 5.0

102.3

4

fK

K

Zn(NH3)42+ + Y4 – ZnY2– + 4 NH3

Zn2+ + 4 NH3 Zn(NH3)42+

Zn2+ + Y4- ZnY2-


Influence of ammonia concentration on the end point for the titration of 50.0ml of 0.00500M Zn2+.

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http:\\asadipour.kmu.ac.ir 41 slides

Metal Ion Indicators

Metal ion indicators are compounds whose color changes when they bind to a

metal ion. Useful indicators must bind metal less strongly than EDTA does.

A typical titration is illustrated by the reaction of Mg2+ with EDTA, using

Eriochrome black T as the indicator.

H2I- HI2- I3-

pka=7 pKa=12 MgIn + EDTA MgEDTA + HI2-

(red) (colorless) (colorless) (blue)

Structure and molecular model of Eriochrome Black T(left) and Calmagite (right).

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EDTA titration curves for 50.0 ml 0f 0.00500 M Ca2+ (K’CaY = 1.75 ×1010) and Mg2+ (K’MgY= 1.72 × 108) at pH 10.00.920220


EDTA titration techniques

1) Direct titration

2) Back titration

3) Displacement titration

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1) Direct titration A) There is suitable indicator Analyte Suitable indicator Buffer EDTA----------------------------------------------------------------------------------B) There isn’t suitable indicator Analyte Mg2+ Or Mg-EDTA Indicator Buffer EDTA

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100 Ca2+

1 I99 Ca2+

1 Ca-I100 E

105 E

100 Ca2+

4 Mg2+

1 Mg-I 5 Mg2+

- =99 Ca2+

1 Ca-I

99 Ca2+

1 Ca-I

10 Mg-E

Ca-EMg-I Ca2+

Mg2+

90

10

9

1

Ca-E >Mg-E >Mg-I >Ca-I

Rplacement

Kf Ca-I100 E

100 E

Added Mg2+ or Mg-EDTA



2) Back titration : Slow reaction Analyte Excess EDTA Time Standard Zn2+ or Mg2+

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3) Displacement titration :: Suitable indicator????? Analyte Excess EDTA- Mg Indicator EDTA

• determine Fe3+:Fe3+ + MgY2- ⇌ FeY- + Mg2+

• liberated Mg2+ can be titrated with standard EDTA where:

Fe3+ ≡ Mg2+

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Masking Agents• auxillary ligand that forms stable complex

with potential interferenceat pH=10,

CN- masks

Co2+ , Ni2+ , Cu2+ , Zn2+ , Cd2+ , Hg2+

Kf Co-CN>Kf Co-EDTA

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EDTA Titration Techniques

Direct Titration In a direct titration, analyte is titrated with standard EDTA. Conditional formation constant for the metal-EDTA complex is large The color of the free indicator is distinctly different from that of

the metal-indicator complex.

Back Titration In a back titration, a known excess of EDTA is added to the analyte.

The excess EDTA is then titrated with a standard solution of a second metal ion.

Necessary if the analyte precipitates in the absence of EDTA, if it reacts too slowly with EDTA under titration conditions, or if it blocks the indicator.

The metal ion used in the back titration must not displace the analyte metal ion from its EDTA complex

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Displacement Titration For metal ions that do not have a satisfactory indicator,

a displacement titration maybe feasible.Indirect Titration

Anions that precipitate with certain metal ions can be analyzed with EDTA by indirect titration.

Alternatively, an anion can be precipitated with excess metal ion. The precipitate is filtered and washed, and the excess metal ion in the filtrate is titrated with EDTA. Anions such as CO3

2-, CrO42-,

S2-, and SO42- can be determined by indirect titration with EDTA.

Masking• A masking agent is a reagent that protects some component of the analyte from reaction with EDTA.• Demasking release metal ion from a masking agent.

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Titration Methods Employing EDTA

Direct Titration: Many of the metals in the periodic table can be determined by titration with standard EDTA solution. Some methods are based on indicators that respond to the analyte itself, whereas others are based on an added metal ion.

Methods Based on Indicators for an Added metal Ion: In case where a good, direct indicator for the analyte is unavailable, a small amount of a metal ion for which a good indicator is available can be added. The metal ion must form a complex that is less stable than the analyte complex.920220


…continued…

Back-Titration Methods: Back-titrations are useful for the determination of cations that form stable EDTA complexes and for which a satisfactory indicator is not available; the determination of thallium is an extreme example. The method is also useful for cations such as Cr(III) and Co(III) that react only slowly with EDTA. A measured excess of standard EDTA solution is added to the analyte solution. After the reaction is judged complete, the excess EDTA is back-titrated with a standard magnesium or zinc ion solution to an Eriochrome Black T or Calmagite end point.920220


…continued…

Displacement methods: In displacement titrations, an unmeasured excess of a solution containing the magnesium or zinc complex of EDTA is introduced into the analyte solution. If the analyte forms a more stable complex than that of magnesium or zinc, the following displacement reaction occurs:

MgY2- + M2+ MY2- + Mg2+

where M2+ represents the analyte cation. The liberated Mg2+ or, in some cases Zn2+ is then titrated with a standard EDTA solution. Displacement titrations are used when no indicator for an analyte is available.920220

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Complexometric Titrations 1920220http:\\asadipour.kmu.ac.ir 41 slides.

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