Quantitative Analysis

Estimation of Carbon and Hydrogen by Liebig’s Methods

1.    Both carbon and hydrogen are estimated together in one operation.

2.    A known weight of an organic compound is burnt in the presence of excess of oxygen and copper (II) oxide.

3.    Carbon and hydrogen are oxidised to carbon dioxide and water respectively.

Estimation of Nitrogen

(i) Dumas method:

 This method is based on fact that the nitrogenous compounds when heated with copper oxide in an atmosphere of carbon dioxide yield free nitrogen,

(2) Kjeldahl method

Limitations:

Kjeldahl method is not applicable to compounds containing nitrogen in nitro and azo groups and nitrogen present in rings (e.g. pyridine, quinoline, isoquinoline) as nitrogen of these compounds does not convert to ammonium sulfate under the conditions of this method.

(ii) Kjeldahl’s Methods

1.     Kjeldahl’s method is based on the fact that when an organic compound containing nitrogen is heated with con. H₂S0₄ the nitrogen in it is converted to ammonium sulphate.

2.    The resultant liquid is then treated with excess of alkali and the liberated ammonia gas absorbed in excess of standard acid.

3.    The amount of ammonia is determined by finding the amount of acid neutralised by back filtration with some std. alkali.

Estimation of Halogen (Carius Method)

In this method organic compound containing halogen is heated in a carius tube with fuming HNO₃ in presence of AgNO₃, Silver halide (AgX) is precipitated.

By Knowing the wt. of silver halide & organic Compound percentage of halogens can be calculated (x=-Cl, -Br,-I)

Estimation of sulphur

Carius method 

 When an organic compound containing sulphur is heated with fuming nitric acid, sulphur is oxidised to sulphuric acid. This is taken as barium sulphate by adding barium chloride solution. From the amount of barium sulphate, percentage of sulphur can be calculated.

Estimation of Phosphorus

Carius method

1.    The organic compound containing phosphorus is heated with fuming nitric acid. Phosphorus is oxidised to phosphoric acid.

2.    It is precipitated as magnesium ammonium phosphate, MgNH₄PO₄, by the adding up of magnesia mixture (MgSO₄ + NH₄OH + NH₄Cl).

3.    The magnesium ammonium phosphate is washed, dried and ignited when it is converted to magnesium pyrophosphate (Mg₂P₂O₇).

Sublimation:

1.    This method is applicable for organic substance which has high vapour pressure at temperature below their melting point.

2.    Organic substances like benzoic acid, naphthalene and camphor when heated pass directly from solid to vapor without melting into liquid. On cooling the vapours will give back solids.

3.    It is a useful technique to separate volatile and non-volatile solid.

4.    It has limited application because only a few substance will sublime.

Crystallization:

1.    Crystallization is defined as a process by which a chemical is converted from a liquid solution into a solid crystalline state.

2.    An impure solid is completely dissolved in a minimal amount of hot, boiling solvent, and the hot solution is allowed to slowly cool.

3.    The developing crystals ideally form with high purity, while impurities remain in the saturated solution surrounding the solid (called the “mother liquor”).

4.    The crystallized solid is then filtered away from the impurities.

Distillation

1.    This method is to purify liquids from non-volatile impurities, and used for separating the constituents of a liquid mixture which differ in their boiling points.

2.    There are various methods of distillation depending upon the difference in the boiling points of the constituents.

3.    The methods are (i) Simple distillation (ii) Fractional distillation (iii) Steam distillation (iv) Distillation under reduced pressure

4.    Simple distillation

Liquids with large difference in boiling point (about 40K) and do not decompose under ordinary pressure can be purified by simply distillation

Fractional distillation

This is used to purify and separate liquids present in the mixture having their boiling point close to each other.

In the fractional distillation, a fractionating column is fitted with distillation flask and a condenser. Petroleum, coal-tar and crude oil.

Steam distillation:

1.    This method is applicable for solids and liquids. If the compound to be steam distilled the it should not decompose at the steam temperature, should have a fairly high vapour pressure at 373K, it should be insoluble in water and the impurities present should be non-volatile.

2.    This method is used to recover essential oils from plants and flowers, also in the manufacture of aniline and turpentine oil.

Distillation Under Reduced Pressure

Azeotropic Distillation

1.    These are the mixture of liquids that cannot be separated by fractional distillation.

2.    The mixtures that can be purified only by azeotropic distillation are called as azeotropes.

3.    These azeotropes are constant boiling mixtures, which distil as a single component at a fixed temperature. For example ethanol and water in the ratio (95:5)   

4.    In this method the presence of a third component like C6H6, CCl4, ether, glycerol, glycol which act as a dehydrating agent depress the partial pressure of one component of azeotropic mixture and raises the boiling point of that component and thus other component will distil over.

Differential extraction:

1.    The process of removing a substance from its aqueous solution by shaking with a suitable organic solvent is termed extraction.

2.    When an organic substance present as solution in water can be recovered from the solution by means of a separating funnel.

3.    The aqueous solution is taken in a separating funnel with little quantity of ether or chloroform (CHCl3).

4.    The organic solvent immiscible with water will form a separate layer and the contents are shaken gently.

5.    The solute being more soluble in the organic solvent is transfered to it.

6.    The solvent layer is then separated by opening the tap of the separating funnel, and the substance is recovered.

Chromatography:

1.    The principle behind chromatography is selective distribution of the mixture of organic substances between two phases – a stationary phase and a moving phase.

2.    The stationary phase can be a solid or liquid, while the moving phase is a liquid or a gas.

3.    If the stationary phase is solid, the basis is adsorption, and when it is a liquid, the basis is partition.

4.    So the Chromatography is defined as a technique for the separation of a mixture brought about by differential movement of the individual compound through porous medium under the influence of moving solvent.

5.    The various methods of chromatography are

·      Column chromatography (CC)

·      Thin layer chromatography (TLC)

·      Paper chromatography (PC)

Adsorption chromatography:

1.    The principle involved is different compounds are adsorbed on an adsorbent to different degree.

2.    Silica gel and alumina are the commonly used adsorbent.

3.    The components of the mixture move by varying distances over the stationary phase.

4.    Column chromatography and thin layer chromatography are the techniques based on the principle of differential adsorption.

Partition chromatography:

1.    Paper chromatography (PC) is an example of partition chromatography.

2.    The same procedure is followed as in thin layer chromatography except that a strip of paper acts as an adsorbent.

3.    This method involves continues differential portioning of components of a mixture between stationary and mobile phase.

4.    In paper chromatography, a special quality paper known as chromatography paper is used. This paper act as a stationary phase.