Quantitative Analysis of Organic Compounds
Quick Notes
- Carbon and Hydrogen: These are estimated by burning the compound in oxygen. CO2 is absorbed by KOH and H2O by CaCl2. Their percentages are calculated from the mass of CO2 and H2O formed.
- Nitrogen:
- Dumas Method: Converts N into N2, measured volumetrically.
- Kjeldahl’s Method: Converts N to NH3, absorbed in acid and titrated.
- Halogens: Carius method involves oxidising halogens to halide ions and precipitating them with AgNO3.
- Sulphur: Oxidised to sulphuric acid or BaSO4 in Carius method.
- Phosphorus: Oxidised to phosphoric acid, precipitated as ammonium phosphomolybdate.
- Oxygen: Determined by difference method: %O = 100 − (%C + %H + %N + %halogen + %S + %P)
Full Notes
Carbon and Hydrogen
Quantities of carbon and hydrogen in a compound are estimated simultaneously using a combustion method:
The organic compound is oxidised by heating with copper(II) oxide in a current of dry oxygen.
- CO2 is absorbed in potassium hydroxide (KOH) solution.
- H2O is absorbed by anhydrous calcium chloride (CaCl2).
Formulas for calculation:
- %C = (12 × mass of CO2 / 44 × mass of compound) × 100
- %H = (2 × mass of H2O / 18 × mass of compound) × 100
Example Simultaneous estimation of C and H
If 0.167 g of an organic compound gave 0.44 g CO2 and 0.18 g H2O:
- % C = (12 × 0.44 / 44 × 0.167) × 100 ≈ 71%
- % H = (2 × 0.18 / 18 × 0.167) × 100 ≈ 12%
Nitrogen Estimation
Quantities of nitrogen in a compound can be estimated using two methods - Dumas and Kjeldahl's:
Dumas Method
Organic compound heated with CuO in a CO2-free atmosphere. Nitrogen gas (N2) is released and is collected and measured.
- %N = (mass of N2 × 28) / (22400 × mass of compound) × 100
Kjeldahl’s Method
N is converted to NH3 using H2SO4. NH3 is absorbed in standard acid (HCl or H2SO4) and titrated.
- %N = (1.4 × (V1 − V2) × N) / mass of compound × 100
- Where V1 = vol. of acid used, V2 = vol. unused, N = normality
- Note: Kjeldahl’s method does not detect nitrogen in azo, nitro, or diazo groups.
Halogens – Carius Method
Compound heated with fuming nitric acid and silver nitrate (AgNO3). Halogen forms a precipitate of AgX (X = Cl, Br, I).
- %X = (Atomic mass of X × mass of AgX) / (molar mass of AgX × mass of compound) × 100
- Example: %Cl = (35.5 × mass of AgCl) / (143.5 × mass of compound) × 100
Sulphur Estimation – Carius Method
Compound oxidised with fuming HNO3. Sulphur converted into BaSO4 using barium chloride.
- %S = (32 × mass of BaSO4) / (233 × mass of compound) × 100
Phosphorus Estimation
Compound oxidised to phosphoric acid (H3PO4). Precipitated as ammonium phosphomolybdate using ammonium molybdate.
- %P = (31 × mass of precipitate) / (molar mass of precipitate × mass of compound) × 100
Oxygen Estimation
By difference method:
- %O = 100 − (%C + %H + %N + %X + %S + %P)
This method assumes complete and accurate estimation of all other elements.
Summary
- Carbon and hydrogen are determined by combustion with CO2 absorbed in KOH and H2O in CaCl2.
- Nitrogen is estimated by Dumas volumetric N2 or Kjeldahl ammonia titration.
- Halogens and sulphur are analysed via Carius oxidations and gravimetric precipitates.
- Phosphorus is precipitated as ammonium phosphomolybdate.
- Oxygen content is obtained by difference from 100.