Qualitative Analysis of Organic Compounds

What Is Qualitative Analysis?

Quantitative analysis is an analysis method used to determine the number of elements or molecules produced during a chemical reaction.  Organic compounds comprise carbon, hydrogen, oxygen, nitrogen, phosphorus, sulphur and halogens. The various methods used for the measurement of the percentage composition of elements in an organic compound are explained here.

Table of Contents

• Detection of C and H
• Test for Phosphorous
• Liebig’s Combustion Method
• Carius Method
• Estimation of Sulphur
• Estimation of Phosphorus
• Estimation of Nitrogen by Dumas Method
• Estimation of Nitrogen by Kjeldahl Method
• Estimation of Oxygen by Aluise’s Method

Qualitative analysis is the analysis of the species present in a given compound. For example, if a compound is taken, the qualitative analysis would be more focused on finding the elements and the ions present in the compound rather than study as to how much they are present.

Detection of C and H

C and H are detected by heating the compound with CuO in a dry test tube. They are oxidised to CO₂ and H₂O, respectively. If the CO₂ turns lime water milky, and H₂O turns anhydrous CuSO₄ blue, then the presence of C and H is confirmed.

Detection of Carbon and Hydrogen

Test for Phosphorous

The organic compound is heated with an oxidising agent to oxidise phosphorous to phosphate. The solution is then boiled with concentrated HNO₃ and treated with ammonium molybdate. Yellow precipitate confirms the presence of phosphorous.

The reaction is given below:

Na₃PO₄ + 3HNO₃ → H₃PO₄ + 3NaNO₃

H₃PO₄ + 12(NH₄)₂MoO₄ + 21HNO₃ → (NH₄)₃PO₄.12MoO₃ + 21NH₄NO₃ + 12H₂O

Quantitative analysis is more towards finding out how much of the elements are present, that is, their amounts.

Also, Read

• Salt Analysis
• Organic Chemistry
• Inductive Effect
• Purification of Organic Compounds
• Lassaigne’s Test
• Determine Molecular Masses

Estimation of C and H

Liebig’s Combustion Method

A known mass of the compound is heated with CuO. The carbon present is oxidised to CO₂ and hydrogen to H₂O. The CO₂ is absorbed in the KOH solution, while H₂O is absorbed by anhydrous CaCl_2, and they are weighed.

Percentage of C = 12/44 x ((Mass of CO₂)/(Mass of compound)) x 100

Percentage of H = 2/18 x ((Mass of H₂O)/(Mass of compound)) x 100

Liebig’s combustion method

Estimation of Halogens

Carius Method

A known mass of the compound is heated with Conc. HNO3 in the presence of AgNO3 in a hard glass tube called the Carius tube. C and H are oxidised to CO2 and H2O. The halogen forms the corresponding AgX. Then, it is filtered, dried and weighed.

Estimation of Halogens by Carius method

Percentage of X =

((Atomic mass of X)/(Molecular mass of AgX))x((Mass of AgX)/(Mass of the compound))x100

Calculations:

Let the mass of the given organic compound be m g.

Suppose the mass of AgX formed = m₁ g.

We know that 1 mol of AgX consists of 1 mol of X.

So, in m₁ g of AgX , mass of halogen

Percentage of halogen

Estimation of Sulphur

A known mass of the compound is heated with conc. HNO₃ in the presence of BaCl₂ solution in the Carius tube. Sulphur is oxidised to H₂SO₄ and precipitated as BaSO₄. It is then dried and weighed.

Percentage of S = ((Atomic mass of S)/(Molecular mass of BaSO₄)) x (( Mass of BaSO₄)/(Mass of the compound)) x 100

Calculations:

Suppose the mass of organic compound = mg

Let the mass of barium sulphate formed = m₁ g

We know that 32 g sulphur is present in 1 mol of BaSO₄

Therefore, 233 g BaSO₄ contains 32 g sulphur:

Percentage of sulphur

Estimation of Phosphorus

A known mass of the compound is heated with HNO₃ in a Carius tube, which oxidises phosphorous to phosphoric acid. It is then precipitated as ammonium phosphomolybdate ((NH₄)₃PO₄.12MoO₃) by adding NH₃ and ammonium molybdate ((NH₄)₂MoO₄). Then, it is filtered, dried and weighed.

Percentage of P=

((Atomic mass of P)/(Molecular mass of (NH₄)₃PO₄.12MoO₃)) x ((Mass of (NH₄)₃PO₄.12MoO₃)/(Mass of compound)) x 100

Estimation of Nitrogen

Estimation of Nitrogen by Dumas Method

A known mass of the compound is heated with CuO in an atmosphere of CO₂, which yields free nitrogen along with CO₂ and H₂O.

C_xH_yN_z + (2x+ 0.5y) CuO → xCO₂ + 0.5y H₂O + 0.5z (N₂) + (2x+ 0.5y)Cu

The gases are passed over a hot copper gauze to convert trace amounts of nitrogen oxides to N₂. The gaseous mixture is collected over a solution of KOH, which absorbs CO₂, and nitrogen is collected in the upper part of the graduated tube.

Estimation of Nitrogen by Dumas Method

Let the volume of N₂ collected be V₁ mL.

Then, volume of N₂ at STP = (P₁V₁ x 273)/ (760 x T₁) = V mL

Where P₁ and V₁ are the pressure and volume of N₂.

P₁= Atmospheric pressure – aqueous tension

22.4 L of N₂ weighs 28 g,

Therefore, V ml of N₂ weighs

=(28 x V)/22400 grams

The percentage of N would be,

= (28/22400) x (V/ Mass of compound) x 100.

Estimation of Nitrogen by Kjeldahl Method

A known mass of an organic compound (0.5 g) is mixed with K₂SO₄ (10 g), CuSO₄ (1.0 g) and conc.H₂SO₄ (25 mL), and heated in a Kjeldahl’s flask.

CuSO₄ acts as a catalyst, while K₂SO₄ raises the boiling point of sulphuric acid. The nitrogen in the compound is quantitatively converted to (NH₄)₂SO₄. The resulting mixture is reacted with excess of NaOH solution, and the NH_3, so evolved, is passed into a known but excess volume of standard acid.

The acid left unreacted is estimated by titration with some standard alkali. Thus, the percentage of nitrogen can be calculated.

Estimation of Nitrogen by Kjeldahl Method

The reactions are given below:

1. C + H + S → CO₂ + H₂O + SO₂
2. N → (NH₄)₂SO₄
3. (NH₄)₂SO₄ + 2NaOH → Na₂SO₄ + 2NH₃ + 2H₂O
4. 2NH₃ + H₂SO₄ → (NH₄)₂SO₄

Calculation of the percentage of N

Let the mass of the organic compound be mg.

Volume of H₂SO₄ (Molarity M) = V mL

The volume of NaOH of molarity M used for titration excess of H₂SO₄ = V₁ mL

mEq of excess H₂SO₄ = mEq of NaOH

= MV₁ mEq

Total mEq of H₂SO₄ taken = 2MV

mEq of H₂SO₄ used for neutralisation of NH₃ = (2MV – MV₁)

Therefore,

mEq of NH₃ = (2MV-MV₁)

1000 mEq or 1000 mL of NH₃ solution contains = 17 g of NH₃ (or) 14 g of N

Therefore,

(2MV-MV₁) mEq of NH₃ contains

=(14 x (2MV-MV₁)) / 1000 g of N

Percentage of N= ((14x(2MV-MV₁)) x (100/(1000xm)))

Qualitative Analysis of Organic Compounds – Kjeldahl’s Method

Estimation of Oxygen by Aluise’s Method

A known mass of the compound is decomposed by heating it in the presence of N₂ gas. The mixture of gases so produced is passed over red hot coke. This is done so that all the O₂ is converted to CO. This mixture is heated with I₂O_5, in which CO is oxidised to CO_2, liberating I₂.

The reactions are given below:

Organic compound → Other gaseous products + O₂

2C + O₂ → 2CO

I₂O₅ + 5CO → 5CO₂ + I₂

Percentage of O = ((Molecular mass of O₂/Molecular mass of CO₂) x (Mass of CO₂/Mass of the compound) x 100