COMPLEXOMETRICTITRATION

Objectives:To estimate the hardness. Of water using EDTA 'as a titrant .

Define complex formation reaction.

Explain conditions of complexometric titrations.

Determine the concentration of calcium in a was

Define properties of EDTA titrations.

Complexometric titration (sometimes chelatometry) is a form of volumetric analysis in which the

formation of a colored complex is used to indicate the end point of a titration. Complex metric titrations

are particularly useful for the determination of a mixture of different metal ions in solution. An indicator

capable of producing an unambiguous color change is usually used to detect the end-point of the

titration.

In theory, any complexation reaction can be used as a volumetric technique provided that:

1. The reaction reaches equilibrium rapidly after each portion of titrant is added.

2. Interfering situations do not arise. For instance, the stepwise formation of several different

complexes of the metal ion with the titrant, resulting in the presence of more than one complex

in solution during the titration process.

3. Acomplex metric indicator capable of locating equivalence point with fair accuracy is available.

In practice, the use of EDTA as a titrant is well established.

Complex titration with EDTA

EDTA, ethylenediaminetetraacetic acid, has four carboxyl groups and two amine groups that can act as

electron pair donors, or Lewis bases. The ability of EDTA to potentially donate its six lone pairs of

electrons for the formation of coordinate covalent bonds to metal cations makes EDTA a hexadentate

ligand. However, in practice EDTA is usually only partially ionized, and thus forms fewer than six

coordinate covalent bonds with metal cations.

Disodium EDTA is commonly used to standardize aqueous solutions of transition metal cations.

Disodium EDTA (often written as Na2H2Y) only forms four coordinate covalent bonds to metal cations at

pH values ≤ 12. In this pH range, the amine groups remain protonated and thus unable to donate

electrons to the formation of coordinate covalent bonds. Note that the shorthand form Na4-xHxY can be

used to represent any species of EDTA, with x designating the number of acidic protons bonded to the

EDTA molecule.

EDTA forms an octahedral complex with most 2+ metal cations, M2+, in aqueous solution. The main

reason that EDTA is used so extensively in the standardization of metal cation solutions is that the

formation constant for most metal cation-EDTA complexes is very high, meaning that the equilibrium for

the reaction:

M2+ + H4Y → MH2Y + 2H+

lies far to the right. Carrying out the reaction in a basic buffer solution removes H+ as it is formed, which

also favors the formation of the EDTA-metal cation complex reaction product. For most purposes it can

be considered that the formation of the metal cation-EDTA complex goes to completion, and this is

chiefly why EDTA is used in titrations / standardizations of this type.

Indicators

To carry out metal cation titrations using EDTA, it is almost always necessary to use a complex metric

indicator to determine when the end point has been reached. Common indicators are organic dyes such

as Fast Sulphon Black, Eriochrome Black T, Eriochrome Red B, Patton Reeder, or Murexide. Color change

shows that the indicator has been displaced (usually by EDTA) from the metal cations in solution when

the end point has been reached. Thus, the free indicator (rather than the metal complex) serves as the

endpoint indicator.

Water hardness is a measure of the amount of calcium and magnesium salts dissolved in water. There

are no health hazards associated with water hardness, however, hard water causes scale, as well as the

reduced lathering of soaps. Hard water should be not used for washing (it reduces effectiveness of

detergents) nor in water heaters and kitchen appliances like coffee makers (that can be destroyed by

scale). It is also not good for fish tanks. In general, there are many applications where ability to easily

determine water hardness is very important.

Complexometric titration is one of the best ways of measuring total water hardness. At pH around 10

EDTA easily reacts with both calcium and magnesium in the same molar ratio (1:1). Stability constant of

calcium complex is a little bit higher, so calcium reacts first, magnesium later. Thus, for the end point,

we should use the same indicator we use when titrating magnesium - that is Eriochrome Black T. In the

case of water that doesn't contain magnesium at all, to be able to detect end point we should add small

amount of magnesium complex MgEDTA2+. Magnesium will be displaced by identical amount of

calcium, and it will be titrated later, not changing final result. However, this is a very rare situation.

If solutions contain carbonates, they should be removed as they can interfere with end point detection.

To do so we can acidify the solution with hydrochloric acid, boil it, and then neutralize with ammonia.

Small excess of ammonia doesn't hurt, as we finally add ammonia buffer and change of pH by several

tenths is not a problem.

MATERIAL AND CHEMICAL USED

MATERIALS CHEMICALS Bottle 0.01M EDTA

Ring stands Distilled water Burette clamps Ammonia buffer solution

thin-stemmed funnels MgSO4 25-mL burets

EBT

50-mL graduated cylinders Hard water 100-mL beakers

flasks to be used as waste container

Steam bath

PROCEDURE-1

Standardization of the EDTA Solution

1. 25ml burette was attached to a ring stand.

2. The burettevalve was opened and the waste inside it was drained completely into a "waste"

beaker. Squirt down theinsides with deionized water a couple of times.

3. The burettevalve was closed and over-filed the burette with standard EDTA solution.

4. 10ml MgSO4solutionwas Pipetedinto 250ml Erlenmeyer flasks.

5. The initial volumeon the burette was read, and 3mL of ammonium buffer was added, and

3dropsof Eriochrome Black T indicator was added.

6. And heated for 40min constantly on water bath.

7. The solution was immediately titrated with EDTAuntil the red wine solution was turned a

SKYBLUE.

8. The final volume was read at least twice.

DATA -1

species mass Molar mass mole volume color Molarity

EBT - - - 3dropes Red wine Ammonium

buffer

- - - 3ml colorless

MgSO4 - 122g/mole 10ml colorless

EDTA - 16.4ml colorless 0.01M

CALCULATION -1

At the equivalent point,

MEDTA*VEDTA=MMgSO4*VMgSO4,from titration

VEDTA=16.4ml, and VMgSO4=10ml given, thus

MEDTA*VEDTA/VMgSO4=MMgSO4

MMgSO4=0.01M*16.4ml/10ml

MMgSO4=0.0164M…………………………………………………………………………………………………………Ans

PROCEDURE-2

25ml burette was attached to a ring stand.

The burette valve was opened and the waste inside it was drained completely into a

"waste" beaker. Squirt down the insides with deionized water a couple of times.

The burette valve was closed and over-filed the burette with standard EDTA solution.

40ml of water sample was Pipeted into 250ml Erlenmeyer flasks.

The initial volume on the burette was read, and 3mL of ammonium buffer was added,

and 3drops of Eriochrome Black T indicator was added.

The solution was immediately titrated with EDTA until the red wine solution was turned

a SKYBLUE.

The final volume was read at least twice.

DATA-2

species mass Molar mass mole volume color Molarity

EBT - - - 3dropes Red wine Ammonium

buffer

- - - 3ml colorless

Hard water - 18g/mole 40ml colorless EDTA - 5ml colorless 0.01M

CALCULATION-2

MH2O*VH20=MEDTA*VEDTA

MH2O=5ml*0.01M/40ml

MH2O=0.o0125M, and from this data

MOLE of H2O=MH2O*VH2O

=0.00005mole Reactions taking place during titration are:

Ca2+ + EDTA4- → CaEDTA2- and

Mg2+ + EDTA4- → MgEDTA2-

MCa2+=MEDTA*VEDTA/V sample

=0.01M*5ml/45ml

=0.0011M……………………………………………………..Ans

AND, from the reaction

nEDTA=nCa2

nEDTA=5ml*0.01M

=0.05mmole Thus,

I, ( m mole/L H2O)=0.05mmole/0.04L

=1.25mmole/l H2O…………………………………………………..Ans

II, mg/L H2O=1.25mmole/L H2O*100mg/mmole

=125mg/L H2O…………………………………………………………………….Ans

III,(125mg/L H2O)*(1L H2O/1000ml H2O)*(1ml H2O/1g H2O)*(1g/1000mg)

=125ppm……………………………………………………………………………………………….Ans

RESULTS AND DISCUSSION

In our first work, we standardize the EDTA solution by unknown concentration of MgSO4 and,

we determined that the molarity of 10ml of MgSO4 that reacted with 0.01M EDTA and volume of 16.4ml is to be 0.016M. This was happened at the end point, where the red wine color

changed into deep blue color. In our second lab work we , titrate hard water against the standard EDTA, and we observed that here the end point reach fastly ,only by addition of 5ml of EDTA into the solution of hard water.

And also we observed that from 0.05mmole of water and volume of 40ml, there is 1.25mmole Ca2+ /L H2O.

As observed, the standard was allowed to react in a basic medium by the addition of the basic buffer of pH 10. A buffer was added so that the pH while the whole reaction occurs is constant.

A constant pH is needed in the titration process since the EDTA and EBT have polyprotic properties, therefore unstable; and only a single endpoint is needed to be observed (EDTA can be

protonated up to six while EBT is usually up to three. As water hardness is usually reported in terms of mg/L of calcium carbonate (even if water contains both calcium and magnesium), we will use for calculations slightly strange reaction

equation:

CaCO3 + EDTA4- → CaEDTA2- + CO32- That allows direct calculation of calcium carbonate mass for known amount of titrant used.