During the estimation of carbon and hydrogen, a known mass of an organic compound is burnt in the presence of excess oxygen and copper(II) oxide. The carbon in the compound is oxidised to carbon dioxide.

This carbon dioxide is absorbed in a U-tube containing a concentrated solution of potassium hydroxide. The increase in the mass of the potassium hydroxide gives the amount of carbon dioxide produced.

This measured mass of carbon dioxide is essential to calculate the percentage of carbon in the organic compound using the relation: $\text{Percentage of carbon} = \frac{12 \times m_2 \times 100}{44 \times m}$, where $m_2$ is the mass of carbon dioxide and $m$ is the mass of the organic compound.