[curiouscat]

Normally Cl₂ has a low solubility in H₂O (0.7 gm in 100 ml) as per this reaction:

Cl₂ + H₂O    HClO + HCl

In order to use hypochlorous acid as a chlorinating species a patent uses a 2% (w/w) NaOH solution to enhance HClO content. Idea being HCl combines with NaOH giving NaCl and driving the reaction forward.

To design a lab experiment I need to estimate how much Cl₂ I can expect to dissolve in various NaOH conc. solutions.

What's a good way to do this? I suppose I need some equilibrium constants or solubility products but I'm  confused about which.

[Corribus]

You can determine the equilibrium constant for this reaction by looking up standard enthalpies of formation and standard entropies for the reactants and products, and calculating the standard Gibbs energy change for the reaction. (Data for aqueous hydrochlorous acid may be hard to find - you may need to back-determine them from the acid dissociation constant for HClO.)

From this you should be able to determine how much HClO is formed as a function of NaOH content.  But keep in mind NaOH will react with HClO as well, and this isn't a strong acid like HCl is, so you'll have to deal with that equilibrium.

That's the way I'd approach the problem.  You may run into a dead-end along the way, though.  If you do, let me know and we'll see if we can't work through it.

[opsomath]

And of course there's the Captain Obvious answer of "assume Cl2 combines 1:1 with OH-", and calculate it from there. This might not be a terrible zeroth-order approximation, actually, since NaOCl solutions can go at least up to 50% w/v:

http://en.wikipedia.org/wiki/Sodium_hypochlorite#Nerve_Agent_Neutralization

[curiouscat]

You can determine the equilibrium constant for this reaction by looking up standard enthalpies of formation and standard entropies for the reactants and products, and calculating the standard Gibbs energy change for the reaction. (Data for aqueous hydrochlorous acid may be hard to find - you may need to back-determine them from the acid dissociation constant for HClO.)

Sounds logical but no luck finding data yet.

I could get ΔG for this reaction

ClO^- + H⁺  HOCl

ΔG^r₀ = 1461 kJ/mol (gas phase)

[Corribus]

I did find most of the values but there are a few I haven't located yet.  In particular I believe you would need thermodynamic values for aqueous chlorine gas.  There's probably a way to get at those from the solubility, but on my first mental pass at is I wasn't sure how.  I will give it some thought tomorrow.  Right now I'm bushed.

I found aqueous standard enthalpy and entropy of HClO to be -26.2 kJ/mol and +36 J/mol K, respectively, from an old JACS paper, if that helps.

[curiouscat]

But keep in mind NaOH will react with HClO as well, and this isn't a strong acid like HCl is, so you'll have to deal with that equilibrium.

Since Ka for HOCL is ~10^-8 wouldn't it be very little dissociated for NaOH to substantially react with? i.e. Won't most of it selectively consume HCl?

Or am I getting something wrong?

PS. If it will indirectly help I also found this:

2 HOCl (aq.)  2 HCl (aq.) + O2 ΔH=-92 kJ/mol

[curiouscat]

I read this in a book which confuses me further. It seems to say adding NaOH will not help increase HOCl concentration. If so, I'm confused why the patent is doing it.

[Borek]

Adding NaOH will help to dissolve more chlorine - but it will be not in the form of HOCl.

[curiouscat]

Adding NaOH will help to dissolve more chlorine - but it will be not in the form of HOCl.

Ah!

As Cl^-?

PS. In case it interests anyone here's an extract from the patent:


The process of this invention requires a first step of contacting a chlorine source with an aqueous pH adjusting source under conditions sufficient to form a chlorinating species In a second step. the chlorinating species is contacted with at least one of propylene or butylene under conditions sufficient to form the chlorohydrin. During this process the aqueous pH adjusting source must be sufficient to maintain a pH of between 7.5 and less than 10 throughout the second step (the "chlorohydrin forming step") This process results in a reduction of byproduct formation and a production of higher yields of the desired chlorohydrins.
.
The chlorine source is preferably Cl2, more preferably, Cl2 gas.
.
The pH adjusting source may be any composition capable of maintaining a pH of 7.5 or greater than 7.5 throughout the chlorohydrin forming step. The pH is maintained at less than 10.0 and preferably less than 9.5. When the pH is greater than 7.5 conditions are favored in the chlorohydrin forming step for the reaction of the chloronium ion intermediate with water or hydroxide ion, thus increasing yield of chlorohydrin.
...
 For example, preferable aqueous pH adjusting sources comprise aqueous mixtures containing compounds such as calcium hydroxide (Ca(OH)2), sodium hydroxide (NaOH)...
...
Irrespective of which chlorine source and pH adjusting source are used, concentration of the pH adjusting source must be sufficient to maintain a pH of greater than 6.0 in the chlorohydrin forming step of this invention. For example. a preferable concentration of pH adjusting source in water ranges from 0.1 normal (N) to 1.5 N and more preferably from 0.2 to 0.5 N.

[Borek]

As both Cl^- and OCl^-. Think in

Cl₂ + 2OH^- Cl^- + OCl^- + H₂O

terms.