2nd modulus Class 2 Equilibrium and Complexation ... · TITRATION CURVES WITH EDTA : Ex: Determination of total hardness in water (Ca2 + and Mg2 + content) Before the Equivalence

ADVANCED ANALYTICAL CHEMISTRY – 1S 2018

2nd modulus

Class 2

Equilibrium and Complexation Titrations

Samples Preparation

Class notes : www.ufjf.br/baccan

Prof. Rafael Arromba de Sousa

Departamento de Química - ICE

[email protected]

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The experimental aspects

of complexometric titrations

are similar to those of other volumetries ...

Last class ...

Titration curves profile

TITRATION CURVES WITH EDTA :

Ex: Determination of total hardness in water (Ca2 + and Mg2 + content)

Before the Equivalence point (EP)

n Ca2+ conc. of non reacted Ca2+ (excess of FREE metallic ion)

EQUIVALENCE POINT

n total Ca2+ ≈ n added EDTA

(low conc. of FREE METALLIC ION: dissociatio)

AFTER the EP (very low conc. of FREE METALLIC ION and

an excess of EDTA)

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PROFILE OF EDTA TITRATION CURVES...

Besides selectivity, the pH influences the profile of the curves

Complexes MY n-4 stability

The pH determines the intensity of the

change in analyte concentration near the EP

For Ca 2+

* pH > 8 for the FP to be appropriate

** Kf´ decreases with decreasing pH

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in the complexometry the pH of the sample solution is important to

guarantee the selectivity and, therefore, the accuracy of the analysis

However,

metals whose complexes with EDTA have very high Kf can be titrated at a

pH lower than the "ideal“ one

On the other hand,

if it is necessary to increase the pH of the sample solution, do not arbitrarily

do so by avoiding precipitation (or co-precipitation) of the analyte in the

form of hydroxides

Certain bases may be used to form complexes with the analyte

(eg. auxiliary ligand)

Prevents analyte hydrolysis

Enables the formation of the analyte complex with EDTA at

the appropriate pH

So ...

Example 1:

Calculate Ca2+ concentrations in the titration of 50.00mL of Ca2+ 0.0100mol L-1

with EDTA 0.0100 mol L-1, in pH 10, after the addition of 20.00, 50.00 and 60.00 mL

of titrant. Data: K f´ CaY 2-= 1.75 10 10

After the addition of 20,00 mL of EDTA

BEFOR THE EP ?

ON THE EP ?

AFTER THE EP ?

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After the addition of 20,00 mL of EDTA BEFORE THE EP

n free Ca2+ = n Ca2+ initial – n added EDTA

n free Ca2+ = (0.0100x0.0500) - (0.0100x0.0200)

n free Ca2+= 0.0003

[Ca2+] = 0.0003 mol / 0.0700 L

= 0.0043 mol L-1

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Example 1:

Calculate the Ca2+ concentrations in the titration of 50.00 mL of Ca2+ 0.0100 mol L-1

with EDTA 0.0100 mol L-1, in pH 10, after the addition of 20.00, 50.00 and 60.00 mL

of titrant. Data: K f´ CaY 2-= 1.75 10 10

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Answer.: [Ca2+] = 5.34 10 -7 mol L-1

Example 1:

Calculate the Ca2+ concentrations in the titration of 50.00 mL of Ca2+ 0.0100 mol L-1

with EDTA 0.0100 mol L-1, in pH 10, after the addition of 20.00, 50.00 and 60.00 mL

of titrant. Data: K f´ CaY 2-= 1.75 10 10

After the addition of 50.00 mL of EDTA

on EP [ M ] n+ = [Ca] :

[CaY2-] (0,01x50)/ 100

[Ca 2+] = [Ca 2+] 2 = [Ca 2+] = 5,34 10-7 mol L-1

1,75 10 10 Ca 1,75 10 10

(≈ 18700x smaller!)

(Ca = Ca 2+)

[CaY2-] = n CaY 2- / V

n CaY 2- = n EDTA = [EDTA] V EDTA

[ MY n-4 ]

K f´ = [M] n+ Ca

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on EP

Calculate the Ca2+ concentrations in the titration of 50.00 mL of Ca2+ 0.0100 mol.L-1

with EDTA 0.0100 mol L-1, in pH 10, after the addition of 20.00, 50.00 and 60.00 mL

of titrant. Data: K f´ CaY 2-= 1.75 10 10

After the addition of 60.00 mL of EDTA after the EP

[CaY2-] ( n Ca2+ initial / V sol) 0.0005 / 0.1100

[Ca2+] = [Ca2+] = =

1.75 1010 Ca 1.75 1010 x 9.09 10 -4 1.59 10 7

= 2.86 10 -10 mol L-1

Ca = C excess EDTA

C excesso EDTA = (n EDTA – n Ca 2+)/ V sol. = 9.09 10 -4

[MY n-4]

K f´ =

[M] n+ Ca

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Calculate the Ca2+ concentrations in the titration of 50.00 mL of Ca2+ 0.0100 mol.L-1

with EDTA 0.0100 mol L-1, in pH 10, after the addition of 20.00, 50.00 and 60.00 mL

of titrant. Data: K f´ CaY 2-= 1.75 10 10

End Point Determination

• Metallochromic indicators (give color to titrated solution)

• Mercury electrode (potentiometric titration) or selective ion (depending on sample and interferents)

Metallochromic Indicators ?

Organic compounds which form colored complexes with the analyte

with a different color than the one of the free indicator

M n+ + Ind M Ind n+

blue red

presenting K f < K f´ MY n-4

the EDTA shifts the equilibrium to the left side 11

Eg. metallochromic Indicators :

Eriocrome Black T

(“Ério T”)

Erochrome Blue R

(“Calcon”)

Other carachteristic: the color of free species is sensitive to the sample pH ...

When such complexes does not dissociate, these "indicators" are said to be "BLOCKED"

** Ério T is blocked by Al 3+, Cr 3+, Co 2+, Cu 2+ and Ni 2+

(can´t be used for these elements, except for masking effects)

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Exemplifying the use of na indicator

Ca and Mg titration, at the same concentration and pH= 10,

by use of the Ério T as indicator:

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FP more evident for Ca

titulation (Kf MEDTA x pH)

AUXILIARY COMPLEXATION AGENTS :

NH3, CN-, F- ...

M n+ + Y 4- MY n-4

Used to:

- avoid the metallic ion precipitation in alkaline pH

Eg: Zn(NH3)42+ (K f´ Zn(NH3)4

2+ < K f´ ZnY2-)

- eliminate interferencies (MASKING)

Eg of MASKING:

1)Titration of Pb in the presence of Ni: CN - (K f´ Ni(CN)42- > K f´ NiY2-)

2)Titration of Mg in the presence de Zn: CN - (K f´ Zn(CN)42- > K f´ ZnY2-)

To manipulate CN- one should use pH> 11 to avoid the formation of HCN

that is toxic... 14

AUXILIARIES COMPLEXATION AGENTS : NH3, CN-, F- ...

EG of “DISMANTLING”:

1) To titrate Zn, after titrating Mg (masked with CN -):

add acetic acid and formaldehyde

Zn(CN)42- + 4H+ + 4HCOH Zn 2+ + 4HOCH2CN

** For each masking agent there is a dismantling agent

Eg: for F- (which masks Al 3+, Fe 3+, Ti 4+ and Be 2+) borates are used :

4F- + BO33- + H+ BF4

- + 3H2O

(reaction used to neutralize HF in digests that will be introduced in ICP

equipment) 15

Titulação Complexométrica (EDTA)

COMPLEXOMETRIC TITRATIONS: DIFFERENT TYPES

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DIRECT TITRATION

BACK-TITRATION

DISPLACEMENT TITRATION

INDIRECT TITRATION

1) DIRECT TITRATION

- The analyte is titrated with EDTA in an appropriate pH

- Depending on the analyte an auxiliary complexation agente is

used

Eg:

Titration of Pb 2+ in pH 10:

- NH3/ NH4Cl buffer

- Sodium or potassium tartarate

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COMPLEXOMETRIC TITRATIONS: TYPES

2) BACK-TITRATION

Analyte precipitates before reaction or reacts slowly with EDTA

Analyte is mixed with EDTA in excess

The excess of EDTA is titrated with a second metal ion

Eg:

Titration of Ni 2+ using Zn 2+ in pH 5.5:

1- Reaction of Ni 2+ with EDTA (in excess)

2- pH ajusted to 5,5 (HAc/ NaAc buffer)

3- Addition of tilenol orange

4- EDTA is titrated with Zn2 + (color change: yellow red) 18

COMPLEXOMETRIC TITRATIONS: TYPES

3) DISPLACEMENT TITRATION

Used when there is no satisfactory indicator for the analyte

- The analyte is treated with a MY 2+ in excess

K f´ analyteY2+ > K f´ MY2+

- M2+ is displaced and titrated with EDTA

Eg:

Determination of Hg2+ with MgY2+

1) Hg2+ + MgY2- HgY2- + Mg2+

2) Mg2+ + Y4- MgY2-

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COMPLEXOMETRIC TITRATIONS: TYPES

4) INDIRECT TITRATION

Used to determine ANIONS (CO32-, SO4

2-, S 2- and CrO42-)

Based on the precipitation of some anions with metallic ions, which

react with EDTA ...

Eg:

Deterrmination of SO42- with Ba 2+:

1- Ba2+ (exc) + SO4

2- BaSO4 (s)

2- BaSO4 (s) + Y4- (exc) BaY2- + Y4-

3- Y4- + Mg2+ MgY2-

** In this eg. the EDTA in excess was titrated. There are procedures

in which the excess of the metal ion (used in the precipitation) is titrated at the end

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COMPLEXOMETRIC TITRATIONS: TYPES

Complexometric titration (EDTA)

EDTA complexes can be formed with most of the metal ions (depending on

pH)

Auxiliary complexing agents, masking and unmasking agents can be used

as needed Titrations with EDTA The reaction stoichiometry is always 1: 1

The FP is generally identified with metallochromic indicators (Erio-T)

There are different types of titration with EDTA

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REFERENCES USED

QUÍMICA ANALÍTICA QUANTITATIVA ELEMENTAR N. Baccan, J. C. Andrade, O. E. S. Godinho, J. S. Barone 3ª Ed, Editora Edgard Blücher Ltda: São Paulo, 2001

ANÁLISE QUÍMICA QUANTITATIVA D. C. Harris 7ª Ed, LTC: Rio de Janeiro, 2008 FUNDAMENTALS OF ANALYTICAL CHEMISTRY D. A. Skoog e col. 8th Ed, Thomsom Broks Cole: Belmont, 2004 INTERNET: www2.iq.usp.br/docente/gutz/curtipot.html 22

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