General principles of Volumetric Analysis, Chemistry tutorial

Introduction:

Titrimetric analysis (that is, Titration) is one of the core and the most helpful analytical procedures which make up quantitative methods in analytical chemistry. This is fairly rapid with good degree of accuracy. It comprises measuring the volume of the reagent (that is, titrant) required to react by the analyte (that is, test substance or titrand).

Definition of Volumetric Analysis:

Volumetric analysis is the analytical method which deals with reactions between the measured volumes of a regent termed as titrant against the test substance termed as analyte in a stoichiometric way. This is a quantitative study.

Principles and technical terms involved in Volumetric Analysis:

In a titration, the addition of the reagent solution (that is, titrant) of known concentration to analyte continues till their reaction is complete. Titrant is generally added from burette to the titrand or the analyte in a conical flask.

If the concentration of H+ titrant is identified, the reaction between the analyte and titrant is as well known, and then the amount of the analyte can simply be computed.

General (Basic) Requirements for Titration:

1) The titration reaction must encompass large equilibrium constant that is, each addition of titrant should be fully used up by the analyte.

2) The reaction should be rapid. 

3) There must be known reaction pattern between the analyte and titrant.

4) There must be no side or parallel reaction that is, the reaction must be specific without interference.

5) The reaction must be quantitative.

6) There must be different features in some property of the solution if the reaction is complete.

7) The end point must coincide by the equivalence point and be reproducible.

Different Methods of Detecting Completion of a Titration Reaction:

1) Noticing rapid colour change in the indicator.

2) Monitoring the Spectrophotometry absorbance change.

3) Detecting a rapid change in the current or voltage between a pair of electrodes.

4) Observation of marked change of pH in the titration of an acid by a base.

Various technical terms in Volumetric Analysis:

There are quite a few terms used in the volumetric techniques in analytical chemistry.

1) Indicator: It is a compound having a physical property (that is, colour) which changes suddenly close to the equivalence end point.  The change is colour is due the complete consumption of analyze close to the equivalence point whose concentration is acknowledged.

2) Standardization: It is a method via which the precise concentration of a solution is found out.

3) Primary Standard: This is the purest form of reagent that is employed to prepare a standard solution. The purity is above 99.9%

4) Equivalence point: It is the point in which the quantity of titrant added is the precise amount essential for stoichiometric reaction by the analyte or the titrand.

5) End point: It is the actual point if a reaction is observed to be complete.

6) Titration Error: It is the difference between the equivalence point and end point. It is at times termed as indicator error, if indicator is employed as a means of detecting the end point.

7) Blank titration: It is the kind of titration in which the solution doesn't include the analyte of interest. This is for all time carried out to find out the amount of titration error.

8) Direct titration: This is the most general form of titration in which the titrant is added to the analyte till reaction is complete.

9) Back titration: This is the kind of titration essential when direct titration doesn't give clear or sharp end point.  It comprises adding a known surplus of the standard reagent to the analyte. Then a second standard reagent is employed to titrate the surplus of the first reagent so as known the amount of first standard reagent which is consumed via analyte.

Features of standard solution:

The ideal standard solution for volumetric analysis should encompass the given properties.

1) Its concentration must remain constant for months or years after preparation so as to evade the requirement to re-standardize. 

2) Its reaction with analyte must be fast.

3) The reaction by the analyte should be describable via equation.

4) A process should exist for identifying the equivalence point between the reagent and analyte.

Forms (Types) of Volumetric Procedures:

1) Acid-Basic titration:

This is the determination of concentration of an acid or base by precisely neutralizing the acid or base having acid or base of known solution. It permits for quantitative analyses of the concentration of the unknown solution.

The most apparent application of acid base (that is, neutralization) titration comprises determination of innumerable inorganic, organic and biological species which possess inherent basic or acidic properties. Elemental and kjeldahl analysis are quite a few of the other application and they are of research and industrial significance.

2) Oxidation- reduction (redox) titration:

This is a kind of titration characterized via the transfer  of  electron  from  one  substance  to the other  (that is, from  reductant  to  the oxidant) by the end-point determined calorimetrically or potentiometrically. The principle is mainly based on reacting the analyte of interest by a standard solution of oxidizing or reducing agent.

Different applications are known. These comprise determination of iron in ore and calcium in the oxalate.

3) Precipitation titration:

It is a titration in which, as it proceeds in the direction of the end point, the substance of interest is precipitate out solution as the insoluble salt, that is, 

Ag+ (aq, unknown) + Cl- (aq, titrant) → AgCl(s)

This generally makes it difficult to find out the endpoint accurately. As an outcome, precipitation titrations frequently have to be completed as back titrations.

4) Complexometric titration:

Complexometric titration (at times chelatometry) is a form of the volumetric analysis in which the formation of a colored complex is employed to point out the end point of a titration. Complexometric titrations are mainly helpful for the determination of a mixture of various metal ions in solution. The indicator is capable of producing an unambiguous color change is generally employed to detect the end-point of the titration.

Volumetric calculation:

The main step is to relate the moles of titrant to the mole of analyte. In this part, a general framework would just be provided, due to the space and time constraints. Though any condition (computation) can be adapted.

Molarity is a main concept needed for volumetric computation.  Though, chemist as well uses the equivalent weight (or the milli equivalent weight) as the base of volumetric calculation.

Equivalent and equivalent weight are employed rather than moles and formula weight. Normal concentration based on the specific reaction and reaction must be specified.

Mole = gram (g)/Molecular mass (F.w.) Or millimole = mg/Fw (molecular mass)

Molar concentration (M) = moles/Litres or M = millimole/mL

Normality, N = equivalent/Litre = Meq/mL

Equivalent = mole x (number of reacting unit per molecule)

Meq = mole x (number of reacting unit per molecule)

Molarity:

The volumetric computation often supposes that the reaction between analyte and titrant is on 1:1 basis, therefore these are valid.

millimole = mL x M

mg = m mole x FW (molecule mass)

Therefore, based on 1:1 

% A = (mg analyte/mg sample) x 100 %

That is,

% A = [M (mole/mL) x ml x F.Wanalyte (mg/mmole) x 100%]/mg sample

Normality:

eq wt. Acid = [F.WAcid (g/mole)]/[1 equivalence/mole] = F.WAcid/No. of reacting unit

eq wt. HCl = [F.WHCl (g/mole)]/[1 equivalence/ml]

eq wt. H2SO4 = [F.WH2SO4 (mg/mole)]/[2 equivalence/ml]

eq wt. H3PO4 = [F.WH3PO4 (mg/mole)]/[3 equivalence/ml]

Therefore, equivalent weight is the weight of a substance in grams which will furnish one mole for the reacting unit.

Equivalence = g/[eq wt (g/eqn)]   Or meq = mg/[eq wt (mg/eq)]

N = eq/Liter = [(g/eq) wt (g/eq)]/Litre

N = Meq/Ml = [(mg/g) wt (mg/meq)]/Ml

It will be noted that the main benefit of this concept of normality is that one equivalent of substance will for all time react by one equivalent of substance.

MeqA = MgA/[eq wtA (mg/meq)] = NT (meq/ml) x MlT

MgA = MeqT x eq wtA (mg/meq)

MgA = NT (Meq/ml) x MlT x eq wtA (mg/meq)

% A = [{NT (Meq/ml) x MlT x eq wtA (mg/meq)}/mg sample] x 100 %

A + B → Product

MeqA = meqB

Thus one can compute the volume of the two substances which react,

NA (meq/ml) x MLA = NT (meq/ml) x MlT

Reacting units in s

Equiv weight = F.W./No. of reacting unit

As there are different specific reactions, the main task is to evolve the reaction unit in each particular reaction, so as to compute the equivalent weight from the relationship.

a)  Acid-base reaction: The reaction unit for acid and bases is the proton H+

eq wt = F.W./No. of H+

b) Reduction-oxidation: The reacting unit of this kind of reaction is electron. The reducing agent discharges e- thus got oxidized, whereas oxidizing agent takes on electron thus got reduced.

eq wt = F.W./Number of mole of e- gains or lost

c) Precipitation and Complexometric Reaction:  In this case, however there is no reacting unit exchanged however reactant just combines, the change on cations (that is, metal ions) is supposed to be the reacting unit.

eq Wtm+ = Atomic weight/Change (+n)

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