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Quantitative Analysis of Organic Compounds

Methods to determine the percentage composition (by mass) of elements in organic substances.

By chemca Team • Updated Jan 2026

Quantitative analysis determines how much of each element is present in a compound. This data is used to calculate the Empirical and Molecular formulas.

Standard Notation:
Let the mass of the organic compound taken be $W$ g.

1. Estimation of Carbon and Hydrogen (Liebig's Method)

Principle

A known mass of the compound is burnt in the presence of excess Cupric Oxide ($CuO$) and Oxygen. Carbon oxidizes to $CO_2$ and Hydrogen to $H_2O$.

• $H_2O$ is absorbed by anhydrous $CaCl_2$ or $Mg(ClO_4)_2$ (Increase in mass = $m_{H_2O}$).
• $CO_2$ is absorbed by $KOH$ solution (Increase in mass = $m_{CO_2}$).

$$ \% C = \frac{12}{44} \times \frac{m_{CO_2}}{W} \times 100 $$

$$ \% H = \frac{2}{18} \times \frac{m_{H_2O}}{W} \times 100 $$

2. Estimation of Nitrogen

A. Dumas Method

The compound is heated with $CuO$ in an atmosphere of $CO_2$. Nitrogen yields free $N_2$ gas, which is collected over KOH solution.

Let Volume of $N_2$ collected be $V_1$ mL at temperature $T_1$ and pressure $P_1$. First, convert to STP ($V_{STP}$).

$$ \% N = \frac{28}{22400} \times \frac{V_{STP} (\text{in mL})}{W} \times 100 $$

B. Kjeldahl's Method

Compound is heated with conc. $H_2SO_4$ to convert N to $(NH_4)_2SO_4$. This is treated with alkali to release Ammonia ($NH_3$), which is absorbed in a known excess of standard acid.

Limitation: Not applicable to compounds containing N in the ring (Pyridine), Nitro groups ($-NO_2$), or Azo groups ($-N=N-$).

Let $N$ = Normality of Acid, $V$ = Vol. of Acid consumed by NH3

$$ \% N = \frac{1.4 \times N \times V}{W} $$

3. Estimation of Halogens (Carius Method)

Principle

Heated with fuming $HNO_3$ in the presence of $AgNO_3$. Halogens form precipitates of Silver Halides ($AgX$).

Let mass of $AgX$ formed be $m_{AgX}$.

$$ \% X = \frac{\text{Atomic Mass of X}}{\text{Molar Mass of AgX}} \times \frac{m_{AgX}}{W} \times 100 $$

Chlorine (Cl)
Factor: $35.5 / 143.5$

Bromine (Br)
Factor: $80 / 188$

Iodine (I)
Factor: $127 / 235$

4. Estimation of Sulfur (Carius Method)

Principle

Heated with fuming $HNO_3$. Sulfur oxidizes to $H_2SO_4$, which is precipitated as Barium Sulfate ($BaSO_4$) by adding $BaCl_2$.

Let mass of $BaSO_4$ formed be $m_{BaSO_4}$. (Molar Mass of $BaSO_4 = 233$)

$$ \% S = \frac{32}{233} \times \frac{m_{BaSO_4}}{W} \times 100 $$

5. Estimation of Phosphorus

Phosphorus is oxidized to phosphate and precipitated as either:

1. Ammonium Phosphomolybdate: $(NH_4)_3PO_4 \cdot 12MoO_3$ (Molar Mass = 1877).
   $\% P = \frac{31}{1877} \times \frac{m_{ppt}}{W} \times 100$
2. Magnesium Pyrophosphate: $Mg_2P_2O_7$ (Molar Mass = 222).
   $\% P = \frac{62}{222} \times \frac{m_{ppt}}{W} \times 100$

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