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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?

Note: Do comment if you more information is needed.

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  • $\begingroup$ What was the concentration of NaOH added to the column, 1 N? How long is it in contact with the 0.1 atm carbondioxide? The pKa of your indicator is 9.4? Can you do a control with 0.001 N NaOH (pH ~ 11) plus indicator to see if the carbondioxide in air is sufficient to give a color change upon shaking? $\endgroup$
    – Karsten ♦
    Jan 23, 2019 at 18:55
  • $\begingroup$ @KarstenTheis 2N NaOH was added. It's a trickle bed type of arrangement. Normal shaking of NaOH solution + phenolphthalein does not spontaneously go colourless in my experience. $\endgroup$
    – Eashaan Godbole
    Jan 26, 2019 at 13:25

1 Answer 1

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Phenolphthalein becomes colorless again (from pink) in strongly basic medium with time. It is a known phenomenon called the alkaline fading of phenolphthalein. https://scholar.google.com/scholar?hl=en&as_sdt=0%2C44&q=fading+of+phenolphthalein+in+alkaline+solution&btnG=

It is not a good choice to use phenolphthalein in the presence of sodium carbonate or bicarbonate. Methyl orange is typically used when NaOH and Na2CO3 are present together. Please consult the classic Vogel's "Quantitative Inorganic Analysis" for more details. Alternatively, you may try a potentiometeric titration (pH vs. volume of titrant). Measure the pH after every 0.5 mL addition of titrant and construct a titration curve.

The reaction of CO2 with NaOH is very fast.

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