TERM ASSESSMENT -2

TOPIC : CONVENTIONAL

METHODS

OF

QUANTITATION

SUMMITED BY: HIMANSHU(000FSM141513)M.Sc. FORENSIC SCIENCE SEMESTER-2

Analytical Chemistry

Analytical Chemistry deals with methods for•Identification - of molecules / functional groups •Structural Determination - determination of structure of a molecule •Quantification - Amount present in a sample/mixture •Qualitative analysis – What is present/Identity of species in a impure sample/mixture?•Separation - Separation of mixtures – Chromatographic Techniques.

Analytical chemistry has evolved from an art of court magicians to alchemist’s into a science with applications throughout industry, medicine, and all the sciences.

Qualitative Analysis• Recognition of chemical species by means of colour, reaction

producing a colour, reaction producing a precipitate, reactioninvolving a change of a physical parameter.

• Colored ions are: Cu2+ (blue), Cr3+ (green), CrO4=(yellow), Cr2O7

=

(orange), MnO4-(viole), MnO4

=(green), Ni2+(green), Co2+(pink, or blu),Mn2+(pink), and generally ions of transition metals.

• Precipitates (slight soluble compounds): sulphurs of eavy metals (like:As, Sb, Hg, Cu, Pb, Cd, Sn, Bi, Zn, Ni, Co, Mn), BaSO4, Hg2Cl2, AgCl,PbCl2, Ag2CrO4, many hydroxides of eavy metals.

Quantitative Analysis provides numerical information of analyte(quantitate the exact amount or concentration).

Quantitative Analysis

Conventional - Quantitative analysis

Volumetric analysis

A + B = CA solution of B, at knownconcentration is added to a knownvolume of A, until the number ofequivalent of B is equal to those ofA. The added volume of B ismeasured and the concentration ofA can be calculated.

Gravimetric analysis

A + B = C

An excess of B, at unknownconcentration, is added to A so that A iscompletely transformed in C, that can beweighed.

Volumetric Analysis

It is a general term for a method in quantitative chemicalanalysis in which the amount of a substance is determined bythe measurement of the volume that the substance occupies.

It is commonly used to determine the unknown concentration ofa known reactant.

Volumetric analysis is often referred to as titration, a laboratorytechnique in which one substance of known concentration andvolume is used to react with another substance ofunknown concentration

Basic Terminology

TitrationA process in which a standard reagent is added to a solution of analyte until the reaction between the two is judged complete.

Primary StandardA reagent solution of accurately known concentration is called a standard solution.

StandardizationA process to determine the concentration of a solution of known concentration by titrating with a primary standard.

End point• The point at which the reaction is observed to be completed is the

end point.• The end point in volumetric method of analysis is the signal that

tells the analyst to stop adding reagent and make the final readingon the burette.

• Endpoint is observed with the help of indicator.

Equivalent pointThe point at which an equivalent or stoichiometric amount of titrant

is added to the analyte based on the stoichiometric equation.

Since the solute is constant, then

molesinitial = molesfinal

So the dilution equation becomes

Cinit.Vinit = Cfinal.Vfinal

where Vfinal = Vinit + Vwater

Dilution

Basic Calculation

There depending on the nature of the reaction between the analyte are two different ways of using a volumetric determination, and the titrant.

1. Direct titration – titrant is added to the analyte until the end point is reached.

2.Back titration *– a measured excess amount of a standard reagent is added to the analyte, followed by titration with a second standard reagent to determine the amount of unreacted first reagent.

*Back titrations are useful whenever there is not a suitable indicator or the kinetics or equilibrium constant are not extremely favorable for the direct titration.

Equipment Used

Volumetric analysis involves a few pieces of equipment:

Pipette – for measuring accurate and precise volumes of solutions

Burette – for pouring measured volumes of solutions

Conical flask – for mixing two solutions

Wash bottles – these contain distilled water for cleaningequipment

Funnel – for transfer of liquids without spilling

Volumetric flasks – a flask used to make up accurate volumes for solutionsof known concentration

Experimental Setup

• The burette is attached to a clamp stand above a conical flask

• The burette is filled with one of the solutions (in this case a yellow standard solution)

• A pipette is used to measure an aliquot of the other solution (in this case a purple solution of unknown concentration) into the conical flask

• Prepare a number of flasks for repeat tests• Last, an indicator is added to the conical

flask

What happened during Titration

Complexometrictitration

Redox titrationPrecipitation

titration

Acid-base titration

Acid-base Titration • The objective of an acid-base titration is to determine “Ca”, the nominal

concentration of acid in the solution. The operation is carried out by measuring the volume of the solution of strong base required to complete the reaction in which ‘n’ is

the number of replaceable hydrogen in the acid

Types Of Acid-base Titration

1. Titration of strong acid with strong base.2. Titration of strong acid with weak base.3. Titration of weak acid with strong base.4. Titration of weak acid with weak base.

Acid-base Titration Curve

Acid-base Titration Calculation

Redox Titration • A titration which deals with a reaction involving oxidation and

reduction of certain chemical species.

Redox Titration Calculation

• A titration based on the formation of a coordination complex is known as a complexometric titration.

Ø Complex formation titrations are used to titrate cations via complex formation reagents.

Ø Most, if not all, metals form coordination complexes with anions or molecules. For example,

Fe2+ + 6 CN1- Fe(CN)64-

Ø Molecules/anions that react with metal ions must donate an unshared pair of electrons to form a coordinate covalent bond

• Molecules composed of metals and chelates or metals and coordinating agents are known as coordination complexes.

• Chelating agents form strong 1:1 complexes with metal ions.

• Most common chelating agents belong to a group of compounds called polyaminocarboxylic acids.

Complexometric Titrations

Complex Formation Titrations

The most commonly used chelon (or titrant) in metal ion titrations is EDTA (ethylenediaminetetracetic acid).

EDTA

Ethylene diamine tetraacetic acid. H4EDTA => H4Y

Ethylenediaminetetraacetate anion

EDTA-4 => Y-4

Complexometric Titration Calculation

A Gravimetric analysis is based upon the measurement of the weight of asubstance that has a known composition and is chemically related to theanalyte.

Gravimetric AnalysisGravi – metric

(weighing - measure)

Criteria :

(1)The desired substance: completely precipitated. "common ion" effect can be utilized:

Ag+ + Cl- AgCl(s) excess of Cl- which is added (2) The weighed form: known composition. (3) The product: "pure", easily filtered..

Procedure :7 Steps in Gravimetric Analysis

1) Dry and weigh sample2) Dissolve sample 3) Add precipitating reagent in excess 4) Coagulate precipitate usually by heating 5) Filtration-separate precipitate from mother liquor 6) Wash precipitate 7) Dry and weigh to constant weight (0.2-0.3 mg)

Suction Filtration

• Filter flask • Buchner funnel • Filter paper • Glass frit • Filter adapter • Heavy-walled rubber tubing • Water aspirator• Mother liquor

Gravimetric Titration Calculation