Q.1 What is the condition must require during titration?
Q.2 What is the different type of reaction usually occurs during titration?
Q.3 What is the requirements of primary standard?
Q.4 What are the substances commonly used as primary standard?
A secondary standard is a substance which may be used for standardizations, and whose content of the active substance has been found by comparison against a primary standard.
It follows that; a secondary standard solution is a solution in which the concentration of dissolved solute has not been determined from the weight of the compound dissolved but by reaction of a volume of the solution against a measured volume of a primary standard solution.
1. Acid-Base reactions:
ü Na2CO3,
ü Na2B4O7(Sodium tetrabromate),
ü KH(C8H404) (Potassium hydrogen phthalate),
ü KH(I03)2 (Potassium hydrogen iodate),
ü HCl,
ü C6H5COOH(Benzoic acid).
2. Complex formation reactions:
ü Ag,
ü AgNO3,
ü NaCl,
ü Various metals (pure Zn, Mg, Cu, Mn) and their salts.
3.Precipitation reactions:
ü Ag,
ü AgNO3,
ü NaCl,
ü KCl,
ü KBr.
4.Oxidation-reduction reactions:
ü K2Cr2O7,
ü KbrO3 (Potassium borate),
ü KIO3,
ü KH(I03)2 (Potassium hydrogen iodate),
ü Sodium Oxalate (Na2C2O4),
ü Arsenic (׀׀׀) oxide(As2O3)
A primary standard should satisfy the following requirements:
1. Purity:
It should be available in highly pure form. It should also preserve in a pure state.
2. Stability:
It should be stable (at 110˚c- 120˚c). There is no chemical change occurred.
3. Unaltered:
The substance should be unaltered in air during weighing, this condition implies that it should be hygroscopic, oxidized by air, or affected by CO2.The standard should maintain an unchanged composition during storage.
4. Test procedure:
The substance should be capable of being tested for impurities by qualitative and other tests of known sensitivity. (The total amount of impurities should not exceed 0.01%-0.02%)
5. Molecular weight:
It should have a high molecular weight so that the weighing errors may be negligible.
6. Solubility:
It must be readily soluble in the solvent.
7. Titration error:
The reaction with the standard solution should be stoichiometric and practically instantaneous. The titration error should be negligible, or easy to determine accurately by experiment.
A primary standard is a compound of sufficient purity from which a standard solution can be prepared by direct weighing of a quantity of it, followed by dilution to give a defined volume of solution.
Normality of a solution is defined as the number of equivalents of solute per litre of solution. It is denoted by ‘N’.
In titrimetry certain chemicals are used frequently in defined concentration as reference solution. Such substances are referred as primary standards or Secondary standards.
Molality of a solution is defined as the number of moles of solute per 1000gm of solvent. It is denoted by ‘m’.
No. Of mole of solute
Molality (m) =
“Molarity” is defined as the number of moles of solute per litre of solution.
Moles of solute
Molarity (M) =
Volume of solution in litre
A standard solution is one, which contains a known weight of the reagent in a definite volume of solution, and for many years concentrations were expressed in terms of molarity (i.e., number of moles per litre) and normality (i.e., number of equivalents per litre).
So, the standard solutions are now commonly expressed in terms of molar concentration or molarity (M).
The reactions employed in titrimetric analysis fall into two main classes…
a. Those in which no change in oxidation state occurs; these are dependent upon the combination of ions.
b. Oxidation-reduction reaction; these involve a change of oxidation state or a transfer of electrons.
For purpose of convenience, these two types of reaction are divided to four main classes. These are ..
ü Neutralization (acid-base) reactions,
ü Complex-forming reactions
ü Precipitation reactions
ü Oxidation-reduction reactions.
1.Neutralization reaction( or Acidimetry and alkalimetry):
These reactions involve the combination of hydrogen and hydroxide ions to form water. These reactions are two types ….
v Acidimetry
v Alkalimetry
Acidimetry:
These include the titration of free bases or those formed from salts of weak acids by hydrolysis, with a standard acid.
Alkalimetry:
The titration of free acids or those formed from by hydrolysis of salts of weak bases, with a standard base.
2.Complex formation reactions:
These depend upon the combination of
ü Ions,
ü Other than hydrogen or hydroxide ions, to form a
o Soluble,
o Slightly dissociated ion or compound.
For example:
In the titration of a solution of a cyanide with silver nitrate
2CN- + Ag+ [Ag (CN) 2] -
or, in the titration of chloride ion with mercury(ІІ) nitrate solution.
2Cl¯ + Hg2+ HgCl2
3.Precipitation reactions:
These depend upon the combination of ions to form a simple precipitate.
For example:
Ag+ + Cl¯ AgCl
4.Oxidation-Reduction reactions:
These include all the reactions involving change of oxidation number or transfer of electrons among the reacting substances.
The standard solutions are either oxidizing or reducing agent.
The principle oxidizing agents are…
ü KMnO4,
ü K2Cr2O7,
ü I2,
ü KI,
ü KBr.
The principle reducing agents are…
ü Fe(ΙΙ),
ü Sn(ΙΙ),
ü As(ΙΙΙ) oxide,
ü Sodium thiosulphate,
ü Mercury(Ι) nitrate,
ü Chromium(ΙΙ) chloride or Sulphate,
Standard Solution:
The solution of accurately known strength is called the standard solution. Standard solutions are expressed by normality (N), molarity (M) etc.
Titrand & Titrant:
In titrimetric analysis the regent of known strength is called ‘Titrant’ and the substance being titrated is called ‘Titrand’. ‘Titrant’ taken in Burette and ‘Titrand’ taken in conical flask.
Example: If a known volume of NaOH solution is titrated with 0.1N HCl, then NaOH solution called ‘Titrand’ and HCl solution called ‘Titrant’.
Titration:
The process of adding standard solution to an unknown solution until the reaction is complete i.e. until the end point is achieved is termed as titration.
Equivalence/End point:
The point at which all the reagent of unknown solution (Titrand) react with known solution (Titrant) is called ‘Equivalence’ or ‘End point’.
At the end point, titration is detected by some physical change (usually color change) produced by indicator.
Indicator:
Indicator is a standard solution, which changes its color at the end point of a titration, or indicator is an organic dye that signals the end point by visual change in color.
Actually indicator formed a complex with excess Titrant, which added after the end point and change their color. For example-
Phenolphthalein in acidic solution
No color change
Addition of base excess than end point
Phenolphthalein in basic solution
Color change due to Phenolphthalein formed complex with base
Titration error:
100% accurate measure is impossible. Practical value and theoretical value is never same.
So, the difference between the theoretical value and practical value of a titration is known as Titration error.
i.e. Titration error = Theoretical value – Practical value.
Example: If the theoretical value of a titration is 5 but the examination value or practical value is 4.91.
Then titration error = 5 – 4.91
= 0.09
Condition of titrimetric analysis:
For use in titrimetric analysis a reaction must fulfill some conditions. They are as follows –
1.Chemical equation:
There must be a simple reaction, which can be expressed by a chemical equation. The substance to be determined should react completely with the reagent in stoichiometric (molar basis) or equivalent proportions.
NaOH + HCl NaCl + H2O
2.Fast:
The reaction should be relatively fast. In some cases the addition of a catalyst may be necessary to increase the speed of a reaction.
3.Alteration of physical condition:
At the end point there must be a change in some physical condition.
4.Indicator:
An indicator should be available which, by a change in physical properties (color or formation of a precipitate), should sharply define the end point of the reaction.
