LECTURE 3

CHAPTER 5: CLASSICAL METHODS OF ANALYTICAL CHEMISTRY: TITRIMETRICMETHODS OF ANALYSIS

CO4: ABILITY TO DIFFERENTIATE VARIOUS USED OF COMPLEXATION, REDOX AND PRECIPITATION BASED TITRATION AND TO CALCULATE CONCENTRATION OF ANALYTES USING THE RESPECTIVE

TITRIMETRIC METHODS.

What is titrimetry?

• Any method in which volume is the signal.Also known as volumetric method.

Terminology:Titrant:Equivalence point:End point:Indicator:

Type of Titrimetric Method

- Classified into four groups based on type of reaction involve;

1. Acid-base titrations (has been covered by Pn. Alina)

2. Complexometric titrations

3. Redox titrations

4. Precipitation titrations

Assignment

• Group 10

• Explain different types of reactions involved in titrimetry.

- Refer to page 139 to 147 in Harvey, D.

- Prepare assignment in powerpoint and printed form

- From now on all groups have to do the same thing.

Complexometric Titrations

• In this titration, the titrant is a complexing agent and forms water-soluble complex with the analyte which is a metal ion. The titrant is often a chelating agent.

Terminology1. Complex ions: A complex ion is a

polyatomic charged aggregate consisting of a positively charged metal ion combined with either a neutral or negatively charged chemical species called ligand.

It is a complex ion that has a metal ion at its centre with a number of other molecules or ions attached to it by co-ordinate bonds.

….Terminology

2. Ligands- They are neutral or negatively charged

species (molecules or ions) containing lone pairs electron that can be donated to metal ion. All ligands are electron donors.

- Classified as uni/monodentate, bidentate or multidentate according to the number of bonding sites (lone pair electrons) that they have.

….Terminology

3. Chelates

A chelate is a complex ion that involves ligands with two or more bonding sites forming a ring structure complex

4. Chelating agent

A substance whose molecules can form several bonds to single metal ion. In other words, a chelating agent is a multidentate ligand

The complex is formed when

1. The central atom accepts an electron

pair from one or more ligands.

2. The ligand possesses at least one electron pair to donate.

3. The bonding (coordinate covalent bonding) occurs.

The EDTA ligand• EDTA = Ethylenediaminetetraacetate

• A Lewis acid

• Has six bonding sites (the four carboxylate groups and the two amino groups providing six lone pairs electrons)

• Metal-ligand complex forms a cage-like structure around the metal ion.

• It is a very stable complex

• All metal-EDTA complexes have a 1:1 stoichiometry

EDTA-Titration

Mn+ + Y4- MY(n-4)+

Ag+ + Y4- AgY3-

Al3+ + Y4- AlY-

Indicator in EDTA titrations

1. Eriochrome Black T (EBT)- a typical metal indicator

- Mln- + HY3- MY3- + Hln- (grape-red) (blue)The indicator form grape-red complex with

the metal ionThe first slight excess of EDTA will turn the

solution to blue due to the color of free (uncomplexed) indicator

- Disadvantage of EBT is that the indicator solution decompose slowly on standing

2. Calmagite

Similar in structure to Eriochrome Black, but does not decompose as easily. Similar color behavior to Eriochrome Black.

EDTA-Application

Water hardness determination

EDTA titrations are routinely used to determine water hardness in lab. In this reaction, the EDTA ligand will react with the minerals present in the water (mainly calcium and magnesium carbonates, sulfates, etc.)

Water hardness

• Hard water is water that has a high mineral content. Hard water usually consists of calcium (Ca2+), magnesium (Mg2+) ions, and possibbly other dissolved compounds such as bicarbonates and sulfates.

• The mineral ions along with other metal ions such as Fe3+ and Pb2+ can be removed from hard water by the addition of EDTA

Other EDTA Application

1. Food industry

EDTA is used as a stabilizing agent in the food industry

- EDTA deactivates enzymes (that contain metal) responsible for food spoilage by removing the metal ions from them and forming stable chelates with them.

3. Medical

i) As an anticoagulant for stored blood in blood banks; it prevents coagulant by sequestering the calcium ions required for clotting.

ii) As an antidote for lead poisoning, calcium disodium EDTA exchanges its chelated calcium for lead, and the resulting lead chelate is rapidly excreted in the urine.

Calculation-example 9.8

Solution:1. Remember:- stoichiometry for the

titration involving EDTA is always 1:1 moles EDTA = moles Ca2+ equation involve/use: M1V1 = M2V21 = EDTA2 = Ca2+ have to find the molarity, M = molarity

M = moles CaCO3 = g CaCO3

V flask FW CaCO3 X V flask

= 0.4071 g

100.09 g/mol X 0.50 L

= 8.135 x 10-3 M Ca2+

MEDTA = MCa VCa

VEDTA

= (8.135 x 10-3 M)(50.0 mL)

42.63 mL

= 9.541 x 10-3 M EDTA

Tutorial 6

Find solution for problems 36 and 37 in page 364.

Group 6 & 11 prepare the solution to be presented in the class.