chemistry

QUANTITATIVE ORGANIC ANALYSIS PART-1

1.           Estimation of Carbon and hydrogen:- (Leibig method)

•    A known weight of the organic substance is taken and completely burnt in excess of air & Copper (II) oxide. Carbon changes to CO₂and hydrogen changes to H₂O as follows.

	y			y
C x Hy + x +		O 2	→ xCO 2	+		H2 O
	4				2

• The CO₂and H₂O obtained are passed through weighed U tubes Containing anhydrous CaCl₂and caustic potash respectively. The increased weight of these two tubes give the weight of H₂O formed and weight of CO₂formed.

% of C =				12			×	Wt.of CO 2 formed		×100
			44					Wt. of Organic compound
% of H=			2		×			Wt.of H2 Oformed	×100
	18						Wt. of Organic compound

•   After calculating the percentage composition all C and H in the compound if their sum is not 100, then the difference is assumed as oxygen.

2.           Estimation of Nitrogen:

•  There are two methods to estimate nitrogen in the given compound. They are

i. Duma’s method          ii. Kjeldahl’s method

i.             Duma’s method:-

•   In this method a known weight of organic compound is heated strongly with dry curpic oxide. Carbon and hydrogen get oxidised to carbondioxide and water vapour. Nitrogen if present is converted to N₂is collected over KOH solution and its volume is determined at S.T.P

y

y

z

y

C x H y Nz +

2x

+

CuO → xCO 2 +

H2 O +

N2 +

2x +

Cu

2

2

2

2

•  % of N =

28

×

Volume of Nitrogen atS.T.P

×100

22400

Weight of organic compound

ii.           Kjeldahl’s method:-

• In this method the given organic compound is treated with Conc.H₂SO₄ in the presence of small

amounts of CuSO₄ to convert nitrogen into ammonium sulphate. Then ammonium sulphate is treated with excess of NaOH to liberate NH₃ gas.

• The ammonia evolved is neutralised with excess of Conc H₂SO₄. which is relatively more in amount than that is required to neutralise NH₃gas. Now, the excess of acid is titrated with standard alkali solution. From this the amount of H₂SO₄used to neutralise NH₃ formed is

calculated and from that percentage of nitrogen is calculated.

Oraganic compounds + H₂ SO ₄ → (NH₄ )₂ SO₄ (NH₄ )₂ SO ₄ + 2NaOH → Na ₂ SO ₄ + 2H₂ O + 2NH₃

2NH₃ + H₂ SO ₄ → (NH₄ )₂ SO₄

^{• =} 1.4 × N × V % of N

Wt.of organiccompound

•    Where N = Normality of acid

V = Volume of acid (ml) neutralised by ammonia