Q.Why is a solution of potassium hydroxide used to absorb the carbon dioxide evolved during the estimation of carbon present in an organic compound?

Concept: Carbon Dioxide Absorption — KOH is a strong base that reacts specifically and completely with acidic CO₂ gas.

1. During combustion analysis, the organic compound is burnt in excess oxygen. All carbon is converted to CO₂ gas.
2. The gaseous mixture (CO₂, H₂O, excess O₂, etc.) is passed through a pre-weighed U-tube containing a concentrated solution of KOH or solid KOH pellets.
3. KOH reacts with CO₂ to form potassium carbonate: $2\text{KOH} + \text{CO}_2 \rightarrow \text{K}_2\text{CO}_3 + \text{H}_2\text{O}$ This reaction is quantitative — all CO₂ is trapped, while other gases (O₂, N₂) pass through unabsorbed. …

The question is about the estimation of carbon in an organic compound using the combustion method (often called Liebig’s method or a modern variant). You burn the compound completely in a stream of oxygen, converting all carbon into carbon dioxide. The key challenge: how do you measure how much CO₂ was produced? You can’t just collect it as a gas and weigh it easily — it’s dilute, mixed with other gases, and hard to handle. So you absorb it into a chemical that reacts with it completely and irreversibly, then weigh the absorption apparatus before and after. The increase in mass gives you the mass of CO₂ absorbed.

Why potassium hydroxide (KOH) specifically? Let’s walk through the reasoning.

1. CO₂ is an acidic oxide. Carbon dioxide dissolves in water to form carbonic acid ($\mathrm{H_2CO_3}$), a weak acid. To trap it efficiently, you want a strong base that will neutralise it completely. KOH is a strong, water-soluble base.

2. The reaction is clean and quantitative. When CO₂ gas is passed through a concentrated solution of KOH, it forms potassium carbonate:

This reaction goes essentially to completion. Every molecule of CO₂ that contacts the solution is trapped. No CO₂ escapes, so the mass gain of the KOH bulb (or U-tube) equals the mass of CO₂ produced.

3. KOH is more effective than other bases. You might wonder: why not use NaOH (sodium hydroxide) or Ca(OH)₂ (lime water)?

   • NaOH works similarly, but KOH is preferred because its carbonate ($\mathrm{K_2CO_3}$) is more soluble in water than sodium carbonate. This prevents clogging or precipitation inside the absorption tube, which would ruin the measurement.
   • Ca(OH)₂ (lime water) also absorbs CO₂, but it forms insoluble calcium carbonate ($\mathrm{CaCO_3}$), which quickly blocks the apparatus and stops further absorption. So it’s useless for a continuous, quantitative process.

4. The absorption must be complete and weighable. In the experiment, the combustion gases (CO₂, H₂O, N₂, excess O₂) are passed first through a drying agent (like anhydrous CaCl₂ or P₂O₅) to trap water, then through a KOH solution (often on pumice or in a U-tube) to trap CO₂. The KOH bulb is weighed before and after. The increase in mass is the mass of CO₂. From that, you calculate the mass of carbon: …