# Regarding Analysis for CO2 Absorption in NaOH

Background

Lab experiment, packed glass column (random packing of metal/ceramic material), countercurrent gas absorption, flowrates measured using rotameters, $$rt \approx 27^\circ C$$.

2 N NaOH flows downwards at 10-30 mL/s; Air + $$\ce{CO2}$$ (~10% $$\ce{CO2}$$ (v/v)) flows upwards at total rate of ~350-500 mL/s.

Samples were collected as (I) 10 mL + (II) 10 mL from the bottom of the column. An excess of $$\ce{BaCl2}$$ was added to (II). Both are titrated against 0.5 N HCl with phenolphthalein ($$\ce{HPh}$$) as indicator.

Expected Reactions

$$\ce{2NaOH + CO2 \to Na2CO3}$$ $$\ce{Na2CO3 + BaCl2 \to BaCO3 + 2NaCl}$$

Colour changes:

$$\ce{HPh + HO- \to Ph- + H2O}$$

$$\ce{HPh}$$ is colourless and $$\ce{Ph-}$$ is pink

The Problem

• The first few readings were as expected and no irregularities were observed. The volumes of HCl added were $$<$$ 15 mL.

Later

• On adding indicator and waiting for some time, the colour disappeared (pink $$\to$$ colourless) for both (I) and (II). Shaking sped up the colour change process.

• On adding indicator and starting titration immediately, the colour lightened (did not fade away completely) for a considerable volume of HCl added, then darkened considerably, and after a few more drops, suddenly became colourless (white in case of (II)). The volume of HCl added was $$>$$ 40 mL. Such a great change is not expected.

One Possibility:

• $$\ce{CO2}$$ from the atmosphere reacts with the sample to increase the acidic nature of the solution and causes the disappearance of colour.
• This would explain speeding up of colour change by shaking.

But, is the rate of absorption of CO2 into a still sample of $$\ce{NaOH + Na2CO3}$$ this fast?

Other possibilities

• Formation of buffer: $$\ce{Na+}$$, $$\ce{CO3^{2-}}$$, $$\ce{HO-}$$ ions are present in (I). (II) additionally contains $$\ce{Ba^{2+}}$$, $$\ce{Cl-}$$. The tap water used might be a source of $$\ce{Cl-}$$ as well. $$\ce{HCO3-}$$ formation during titration (?)
• The packing material might have reacted into the system somehow. This seems unlikely, and the packings did not show signs of fouling.
• The chemicals used contain impurities causing these weird effects, in which case, which type of impurites could cause this?