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,
