Quantitative Measurement Of An Analyte In Solution Biology Essay

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Recall that titration is the quantitative measurement of an analyte in solution by reacting it completely with a standardized reagent. Complexes form in a fixed stoichiometry so a standard solution of a ligand can be used to titrate a metal ion in solution. Similarly, a standard solution of a metal ion can serve as the titrant for a species that acts as a ligand.

Many metal ions form slightly dissociated complexes with various ligands (complexing agents).

Complexes used to mask undesired reactions.

Its formation can serve as the basis of accurate and convenient titrations for metal ions.

Complexometric titrations are useful for the determination of a large number of metals.

Most complexing agents are weak acids or weak bases whose equilibria influenced by the pH.

Nomenclature

Ligand

Neutral or anionic species with unpaired electrons that can bond to a metal ion. Common ligands are CN-, NH3, OH-, and halides.

Complex

An association of a central metal ion and surrounding ligands in solution - also called coordination complex.

Coordination number

The number of bonds with the central metal ion, usually 2, 4, or 6.

Chelate

A specific type of complex in which at least one ligand contains more than one atom with unpaired electrons so it can make multiple bonds with the central metal ion. Ligands that make two bonds are called bidendate, ligand that make three bonds are tridendate, and so on.

Complexes; formation constant:

Many cations will form complexes in solution with a variety of substances that have a pair of unshared electrons (e.g. N, O, S) capable of satisfying the coordination number of the metal.

The number of molecules of complexing agent (ligand) will depend on the coordination number of the metal and on the number of complexing groups on the ligand molecule.

Cu2+ + NH3 [CuNH3]2+

Cu2+ is a Lewis acid (electron pair acceptor) and NH3+ is a Lewis base (electron pair donor).

When a complex is formed, the solvating molecules (H2O) are replaced one by one by the new ligands (in the above example: NH3). The complex formation takes place step by step where every step corresponds to the substitution of one H2O by NH3 .

Every step is characterized by an equilibrium constant Ki , called the stepwise stability constant (or stepwise formation constant).

Example:

Cu2+ + NH3 [CuNH3]2+

[CuNH3]2+ + NH3 [Cu(NH3)2]2+

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

[Cu(NH3)3]2+ + NH3 [Cu(NH3)4]2+

     K1, K2, K3 and K4 are the stepwise stability constants. These equilibrium constants are described by a general expression

Chelates:

A Chelating agent (the Ligand) is an organic agent that has tow or more groups which are capable of complexing with a metal ion. The complex formed is called a Chelate. The titration with a chelating agent is called a Chelometric titration a type of Complexometric titration.

The most generally useful titrating agent is amino carboxylic acids, in which nitrogen and carboxylate groups serve as ligands. The amino nitrogens are more basic and are protonated (NH3+) more strongly than carboxylate groups. When these groups bind o the metal atoms, they will lose their protons. The metal complexes formed with these multidentate complexing agents are often 1:1 regardless of the charge on the metal ions, because there are sufficient complexing groups on one molecule to satisfy the coordination sites of the metal ions.

The most widely used chelating agent in titration is Ethylene Diamine Tetra Acetic acid -EDTA.

Ethylenediaminetetraacetic acid (EDTA)

A pair of unshared electrons capable of complexing with a metal ion is contained on each of two nitrogens and each of the four COOH groups. Thus, there are six complexing groups in EDTA. EDTA can be represented by the symbol H4Y since it is a tetraprotic acid and the H in H4Y refer to the four ionisable hydrogen.

EDTA equilibria:

We can represent EDTA as having four Ka values corresponding to the stepwise dissociation of the four protons:

Ka1= 1.0x10-2

Ka2= 2.2x10-3

Ka3=6.9x10-7

Ka4=5.5x10-11

Formation constant:

Consider the formation of the EDTA chelate of Ca+2. This can be represented by

Ca+2 + y-4 ↔ CaY2

The effect of pH on EDTA equilibria:

The equilibrium is shifted to the hydrogen ion concentration is increased due to competition for the chelating anion by the hydrogen ions.

Detection of the end point:

Indicators:

Indicators used for Chelometric titrations are themselves chelating agents, they are usually dyes of the O,O- dihydroxy azotype.

Erichrome Black T (EBT):

Is a typical indicator, it contains 3ionizable protons, so it can be represented by H3In. EBT can be used for the titration of Mg+2 with EDTA.

MgIn- + H2Y2 → MgY2 + HIn2- +H+

red colorless colorless blue

Eriochrome black T

pK1=6.3

pK1=11.6

The metal indicator complex should be 10-100 times less stable than the metal-titrant complex so metal titrant will displace the indicator from the metal giving good and sharp end point.

EBT can not be used to indicate the direct titration of Ca2+ alone with EDTA, however, because the indicator forms too weak complex with Ca2+ to give a sharp end point.

The Ca2+ titrates first since its EDTA chelate is more stable. When Mg-EDTA is added to the sample, the Ca2+ in the sample displaces the EDAT from Mg2+ so that the Mg2+ is free to react with the indicator. At the end point, an equivalent amount of EDTA displaces the indicator from Mg+2 (Ca2+ Titer) causing the color change, and no correction is required for the added Mg-EDTA.

Ca2+ + EDTA → Ca-EDTA

(x mole) Ca2+ + (y mole) EDTA-Mg

x mole > y mole

K(EDTA-Mg) = 4.9 x 108

K(EDTA-Ca) = 5.0 x 1010

(y mole) Ca-EDTA + (y mole) Mg2+ + (x-y mole) Ca2+

EDTA

(y mole) Ca-EDTA + (y mole) Mg-EDTA + (x-y mole) Ca-EDTA

mole EDTA= C x V = (y mole) + (x-y mole) = x mole

Since EBT and other indicators are weak acids, their color will depend some what on he ph, because the different ionized species of the indicator have different colors.

For example with EBT,

H2In- is red (pH <6)

HIn-2 is blue (pH 6-12)

In-3 is yellow orange (pH >12)

An indicator is useful for indication of titrations of only those metals that form a more stable complex with titrant than with the indicator at the given pH.

Calmagite indicator gives a some what improves end point over EBT for the titration of Ca+2 and Mg+2 with EDTA; it also has a longer shelf life.

Chelating agents such as EDTA are used in the treatment of heavy metal poisoning, for example, when children ingest chipped paint that contains lead. Heavy metals such as lead form more stable EDTA chelates than Ca+2 does. Chelated lead is excreted via kidneys.

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