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This question came up in figuring out what the fundamental differences are between redox chemistry and acid/base chemistry (if any).

In redox chemistry, strong oxidants and strong reductants can co-exist in solution because they react slowly with each other (or not at all). An example from biology is a low NADH/NAD+ ratio in the same compartment as a high NADPH/NADP+ ratio.

Are there substances that act as acid or base that are far from equilibrium in aqueous solution for a long (minute, hours) time period? Or are all acid/base reactions in aqueous solution fast and reach equilibrium within seconds?

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  • $\begingroup$ Tautomerisation can be that slow. $\endgroup$
    – Mithoron
    May 6, 2020 at 14:27
  • $\begingroup$ The cleavage of esters in aqueous solution can be slow. $\endgroup$
    – MaxW
    May 6, 2020 at 16:38
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    $\begingroup$ @MaxW Yes, but I would argue that the slow steps are not the acid/base ones but the carbon-oxygen bonds forming and breaking. $\endgroup$
    – Karsten ♦
    May 6, 2020 at 17:15
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    $\begingroup$ I guess the Grotthus mechanism is winning every time. At least in those media where you could actually observe proton movement and acid base equilibration. But the key word to the comment is still guess. $\endgroup$
    – Martin - マーチン ♦
    Sep 11 at 23:40
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    $\begingroup$ Would proton transport into the cleft of a protein count? $\endgroup$
    – Buck Thorn ♦
    Sep 12 at 9:45

1 Answer 1

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Since water is a protic solvent, the predominant form of aqueous acid-base reactions involve proton transfer and that mechanism tends to be quite fast.

But not always.

Most readers are familiar with the use of phenolphthalein as an indicator for titrating acid solutions with a strong base such as sodium hydroxide. When the acid is neutralized, the next increment of sodium hydroxide deprotonates the phenolphthalein molecules and they rapidly turn pink/fuchsia. We proceed to record the data, unaware that with too much additional sodium hydroxide the phenolphthalein color may fade.

That is the slow reaction, for instead of a relatively rapid proton transfer this excess stage is a much slower nucleophilic addition of hydroxide ions to the deprotonated phenolphthalein. From Wikipedia:

The lactone form (H2In) is colorless between strongly acidic and slightly basic conditions. The doubly deprotonated (In2-) phenolate form (the anion form of phenol) gives the familiar pink color. In strongly basic solutions, phenolphthalein is converted to its In(OH)3− form, and its pink color undergoes a rather slow fading reaction[1] and becomes completely colorless when pH is greater than 13.

How slow is "rather slow" is indicated in the educational article by González-Arjona et al.[2]. The table below, taken from this reference, indicates that the second-order rate constant for the nucleophilic reaction is about $0.02\text{ mol}^{-1}\text{ s}^{-1}$.

enter image description here

At pH $13$ we multiply by $0.1\text{ M}$ hydroxide ion concentration to get a pseudo-first order constant of $0.002\text{ s}^{-1}$, so the time constant here is about $500$ seconds — certainly long enough for kinetic control and convenient for laboratory kinetic measurements.

So if you want to use phenolphthalein indicator for a chemically inspired "It's a girl!" reveal, sodium hydroxide is not the best base to set off the color reveal. A small amount of milk of magnesia in water has the optimal pH to make the pink color but not set off the slow fading reaction.

Reference

  1. Kunimoto, Ko-Ki (February 2001). "Molecular structure and vibrational spectra of phenolphthalein and its dianion". Spectrochimica Acta Part A. 57 (2): 265–271. Bibcode:2001AcSpA..57..265K. doi:10.1016/S1386-1425(00)00371-1. PMID 11206560.

  2. González-Arjona, D. & Domínguez, Manuel & López-Pérez, German & Mulder, Willem. (2019). "Primary Kinetic Salt Effect on Fading of Phenolphthalein in Strong Alkaline Media: Experimental Design for a Single Lab Session". The Chemical Educator. 24. 126-132.

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    $\begingroup$ (+1) Marvelous answer! I bumped into this one during a lecture demo I was doing! $\endgroup$
    – Ed V
    Sep 12 at 15:53
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    $\begingroup$ I remember we were discussing this earlier. Lime can be too strong/soluble a base too if you want to keep the color, hence my recommendation of magnesia. $\endgroup$
    – Oscar Lanzi
    Sep 12 at 15:54
  • $\begingroup$ At the pH of a phenolphthalein end point ~8-8.5 the decolorization is absorption of CO2 or possibly ionization of a weak acid present not further reaction of the indictor. $\endgroup$
    – jimchmst
    Sep 13 at 7:05
  • $\begingroup$ Still a magnesia-based system would last longer due to the lower solubikity of Mg(OH)2 vs Ca(OH)2. Next time we cover the beaker after the reaction is over. $\endgroup$
    – Oscar Lanzi
    Sep 13 at 10:50

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