Complexometric Titration

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Complexometric Titration

Reaction result is complex substanceCommon standard solution : EDTA

(Ethylene Diamine Tetra acetic Acid )

What is complex substance?

A substance formed by covalent coordination bond;

A bonding formed by pair electrons sharing, while the source of pair electrons is only one atom/group.

The covalent coordinate in a complex substance

Between the center atom (metal ion) and the ligand (ion/atom/group that give the pair electrons).

Center atom give the orbital.Ligand give the pair electrons.

Complex stabilityIf Mn+ + Lx- [ML]n-x While [ML]n-x is complex substance

then [ML]K = ----------- is Kstab [M] [L]

Kstab= stability constant

The more stable the substance will has bigger Kstab

The complex is difficult to dissociated to be its ions

(Compare with weak acid or base)

EDTA(Ethylene Diamine Tetra acetic Acid)

Ligand that have 6 pairs of electron.Other name : trilon B, complexon III,

sequestrene, versene dan chelaton 3.Release 2 H+ from 2 carboxylic.Most available in disodium salt form.Wrote as H2Y=

EDTA concentration always termed in Molar, because

EDTA has 6 electrons pair which could be used in covalent coordinate bond one-by-one.

EDTA reacts with any metal with mole ratio 1 : 1.

REACTIONS

M2+ + H2Y= MY= + 2H+

M3+ + H2Y= MY- + 2H+

M4+ + H2Y= MY + 2H+

M5+ + H2Y= MY+ + 2H+

Mn+ + H2Y= MY(n-4) + 2H+

pH influence to metal-EDTA complex stability

Every reaction of metal ion with EDTA resulted H+

As Le Chatelier, if [H+] increased, the equilibrium goes to the left (ions)

meansAt low pH or high [H+] , the stability

of metal-EDTA is decreased.

Some of Kstab of metal-EDTA2+ Ions 3+ IonsMg 8.7 Al 16.3Ca 10.7 Fe 25.1Sr 8.6 Y 18.2Ba 7.8 Cr 24.0Mn 13.8 Ce 15.9Fe 14.3 La 15.7Co 16.3 Lu 20.0Ni 18.6 Se 23.1

The Equivalence Point DetectionInstrumental : spectrophotometric,

amperometric, conductimetric or potentiometric

Conventional : Using indicator Indicator available: metal ion

indicator (metalochromic indicator)This indicator is sensitive to very low

concentration of metal ion (pM)

Metal ion Indicator

Ligand in the form of organic ion which its free ion colour differ from its complex.

Could also function as indicator pH.Its Colour change depend on the pH

( owning typical pH job).Complex metal-indicator own the Kstab

expressed by Kin.

Criterion of Choosing Indicator

For Choosing the metal ion indicator, we have to know:

1. pH work indicator. 2. pH titration ( barbed with the

nature of metal ion and metal complex stability – EDTA.

Metal stability – EDTA must > Metal ̶ Indicator

Some of metal ion indicators.EBT (Eriochrome Black T)/Solochrome

Black T/WDFAMurexidXylenol OrangePyrocathecol VioletBromopyrogalol RedVariamin Blue B

EBT (Eriochrome Black T) Solochrome Black T / WDFA

At pH < 5.5 red (H2D-).At pH 7 -11 blue (HD=).At pH > 11.5 yellowish-orange (D3-).At pH 7-11 reacting with the metal

ion. M++ + HD=(blue) MD- (red)+ H+

Preparation of EBT Solution.0.2 gram EBT dissolved in 15 ml three

ethanol amine and 5 ml alcohol absolute.

This solution is stable for a few months.

0.4 gram EBT dissolved in 100 ml methanol, stable at least 1 month.

1 gram of EBT attenuated with 200 - 500 gram of KCL or NaCl or K2So4 or Na2So4 . Most stable during carried through to run dry in bottle call a meeting to order.

Murexide

Until pH 9 reddish violet (H4D-)pH 9 -11 Violet (H3D=)Above pH 11 blue-violet or blue

(H2D3-)At pH baseNi-murexide, Co-murexide yellowCu-murexide orangeCa-murexide red

Preparation of murexide Solution. 0.5 g of the powdered dyestuff in

water, shaking thoroughly, and allowing the undissolved portion to settle. This solution can only be used on making of. Everyday the old supernatant liquid is decanted and the residue treated with water as before, to provide a fresh solution of the indicator.

1 g murexide attenuated with 500 gram pure NaCl, most stable.

Xylenol Orange

The indicator at acid pH (1 – 5) is coloured lemon – yellow.

XO Metal at acid pH is intensely red.Titration is generally conducted at pH

1-3, titration at pH 5 conditioned by hexamine.

0.5 % XO solution in water, most stable.

Types of Complexometric Titrations

1. Direct Titration.The solution containing the metal ion

to be determined is buffered to the desired pH (e.g. to pH = 10 with NH:-aq. NH,) and titrated directly with the standard EDTA solution. It may be necessary to prevent precipitation of the hydroxide of the metal (or a basic salt) by the addition of some auxiliary complexion agent, such as tartrate or citrate or triethanolamine.

2. Back Titration.

Metal ion reacted by excessive EDTA, then at certain pH, the excess of EDTA titrated with the standard metal ion solution such as Mgso4 or Znso4.

At equivalence point obtained mmol of the rest of EDTA = mmol metal std, so that countable.

mmol metal ion = mmol (EDTA –metal std).

Why these Types of titrations are selected.

1. Metal ion precipitated at pH titration.

2. Its Complex forming is too slow. 3. There are not appropriate metal

ion indicators ( complex stability of metal indicators are ineligible).

End Point determined with the sensitive metal ion indicator to metal standard.

3. Substitution Titration. Metal ion titrated by EDTA with the

other metal ion attendance as an aid of determination of the end point ( For example, determination of Ca2+

with aid of Mg2+).

The titration is conducted if:1. Metal ion does not react ( or its

reaction dissatisfactory) with indicator, or

2. Complexion of metal EDTA more stable than other.

Examples:

Stability of Ca EDTA>Mg EDTA

Stability of Ca EBT>Ca EDTAHence titration Ca by EBT

have to entangle the Mg, so that the end point can be reached.

4. Alkalimetric Titration.

When metal ion reacted by EDTA will be formed by a number of ion H+. This ion H+ titrated by NaoH solution uses acid-base indicator.

Its condition, metal ion solution have to really neutral before is reacted by EDTA.