chemistry

QUANTITATIVE ORGANIC ANALYSIS PART-2

3.           Estimation of halogens (Carius method)

•   In this method a known weight of organic compound is heated with fuming nitric acid in the presence of Ag NO₃in a hard glass tube called Carius tube.

•    Carbon and hydrogen of the compound are oxidised to CO₂and H₂O. Halogen forms Silver halide (AgX). It is filtered, washed dried & weighed.

• % of Halogen ( X ) =

At.wtof X

×

Wt.of AgX

×100

Mol.wtof AgX

Wtof Oraganic compound

• % of Cl =

35.5

×

Wt.of AgCl formed

×100

143.5

Wtof Oraganic compound

• % of Br =

80

×

Wt.of AgBr formed

×100

188

Wtof Oraganic compound

• % of I =

127

×

Wt.of Ag I formed

×100

Wt of Oraganiccompounds

235

4.           Estimation of sulphur:-

• A known weight of organic compound containing sulphur is heated with Na₂O₂ or fuming HNO₃

in a carius tube. The sulphur in it is oxidised to H₂SO₄. The acid is precipitated as BaSO₄by adding excess of BaCl₂ solution. The precipitate is filtered, washed, dried and weighed.

•	% of S =	32	×	Wt. of BaSO4	×100
		233		Wt.of Oraganiccompound

5.           Estimation of Phosphorus:

•   To estimate phosphorus in the given organic compound, a known mass of organic compound is heated with fuming nitric acid in a Carius tube. Phosphorus is oxidised to phosphoric acid. The

acid is precipitated as ammonium phosphomolybdate (NH₄)₃PO₄ .12MoO₃by adding ammonia and ammonium molybdate solutions.

•   Sometimes, the acid is precipitated by adding magnesia mixture.

•   Magnesia mixture is obtained by dissolving 100.0g. of MgCl₂. 6 H₂O and 100.0g.of NH₄Cl in

water, then adding 50.0 ml of conc.NH₄OH and diluting the solution to 1000 ml.

• A Precipitate of magnesium ammonium phosphate ( Mg NH₄PO₄ ) is formed which on ignition gives magnesium pyrophosphate (Mg₂P₂O₇).

•

% of P =

2 ×31

×

Wt. of Mg 2 P2 O7

×100 (or)

222

Wt.of Oraganic compound

•

% of P =

31

×

Wt. of (NH 4 )3 PO 4 .12MoO3

×100

1877

Wt.of Oraganic compound

6.   Estimation of Oxygen:

• The percentage of oxygen in an organic compound is found indirectly by the difference between 100 and total percentage of other elements.

• In the direct method, a known mass of organic compound is decomposed by heating in a stream of nitrogen gas. The mixture of gaseous product containing oxygen is passed over red hot coke to convert all oxygen in those oxides to CO. Then the mixture is passed through warm iodine pentoxide ( I₂ O₅) to convert CO to CO₂ and liberating iodine.

• Compound   ^{Heat → O2 +}other gaseous products ;

^N2

2C + O₂    ^1373K →2CO ; I₂ O₅ + 5CO → I ₂ +5CO₂