Sciencemadness Discussion Board

Chlorine solubility in hydrochloric acid

chornedsnorkack - 29-12-2016 at 09:03

Chlorine dissolves in water, partly as molecules but partly with hydrolysis:
Cl2+H2O<->HCl+HClO
How does chlorine dissolve in hydrochloric acid, compared to solubility in water?
Is the solubility diminished, because HCl suppresses solution of chlorine by hydrolysis?
Or is the solubility increased because of reaction
HCl+Cl2<->HCl3?
Also, the equilibrium freezing point of hydrochloric acid is as low as -75 degrees at 23 % acid. Between 15 and 35 % hydrochloric acid freezes under -34 degrees.
Pure dry chlorine boils at -34 degrees and freezes at -101 degrees.
How does liquid chlorine coexist with aqueous hydrochloric acid at conditions where both are liquids? Is chlorine miscible with hydrochloric acid, or does it undergo limited solution and remain immiscible liquid, like bromine in bromine water?

unionised - 29-12-2016 at 13:47

It gets more complicated than that Chlorine forms a hydrate.
https://en.wikipedia.org/wiki/Clathrate_hydrate

softbeard - 29-12-2016 at 14:09

What you're really looking for is a chlorine/HCl/water phase diagram giving partial pressures and phases of each component over a range of temperatures.
As unionised mentioned, hydrates of chlorine and HCl are also involved.
Google is your friend here, and a quick search led me to a patent with a rough phase diagram: http://www.google.ca/patents/US3568409

AJKOER - 30-12-2016 at 20:40

I would not be surprised if in strong light, if there is a change in the solubility of chlorine in aqueous HCl due to a radical pathway as well.

Possible paths:

Cl2 + H2O = H+ + Cl- + HOCl

Cl2 + hv → Cl• + Cl•

Or: HOCl + hv → OH• + Cl•

OH• + Cl- = OHCl•-

OHCl•- + H+ → H2O + Cl•

Cl• + Cl- = Cl2•-

Source: See, for example, http://pubs.acs.org/doi/abs/10.1021/j100497a003

[Edit] replaced 'increase' with 'change'

[Edited on 31-12-2016 by AJKOER]

unionised - 31-12-2016 at 03:54

Quote: Originally posted by AJKOER  
I would not be surprised if in strong light, the apparent solubility of chlorine in aqueous HCl increased due to a radical pathway as well...

[Edited on 31-12-2016 by AJKOER]


I would.
Equilibrium concentrations of radicals in solution due to sunlight are tiny.
The effect of the IR from the sun warming the sample up a bit would be much bigger.

AJKOER - 31-12-2016 at 05:56

Quote: Originally posted by unionised  
Quote: Originally posted by AJKOER  
I would not be surprised if in strong light, the apparent solubility of chlorine in aqueous HCl increased due to a radical pathway as well...

[Edited on 31-12-2016 by AJKOER]


I would.
Equilibrium concentrations of radicals in solution due to sunlight are tiny.
The effect of the IR from the sun warming the sample up a bit would be much bigger.


In the short term, I agree.

In the system cited of HCl/H2O/Cl- with added UV, there is no time dimension. Add to my radical reactions the formation of the ClO dimer, and one can create chlorate in a day:

·OH + HOCl = H2O + ·ClO (See Table 3.5, p. 53 at https://www.google.com/url?sa=t&source=web&rct=j&... )

·ClO + ·ClO = Cl2O2 (See http://pubs.acs.org/doi/abs/10.1021/j100286a035?journalCode=... )

Cl2O2 + HOCl = HClO3 + Cl2

Reference for the above reaction, please see Eq 8 in "Effect of Chloride Ion on the Kinetics and Mechanism of the Reaction between Chlorite Ion and Hypochlorous Acid" by Balazs Kormanyos, et al., 2008, at https://www.researchgate.net/publication/23141635_Effect_of_... ).

and with more UV even perchlorate (see https://www.ncbi.nlm.nih.gov/pubmed/17723378 ) which have even been found on Mars (see http://onlinelibrary.wiley.com/doi/10.1029/2012GL051239/full ). The formation of perchlorate constitutes a sink altering the equilibrium.

As such, an astrochemist for one may dispute the word 'tiny' if implying insignificance of the products of photolysis induced radicals.

[Edited on 31-12-2016 by AJKOER]

unionised - 31-12-2016 at 06:43

Given time I think the major effect would be the net oxidation of water to oxygen.
That's not a solubility thing is it?

An "astrochemist" clearly has nothing to do with the discussion in hand.
Did you introduce that irrelevance to try to cover up the fact that you were wrong about light making a difference?

AJKOER - 31-12-2016 at 08:27

Here is an argument not based on radical chemistry. The equilibrium of chlorine and water produces HCl and HOCl. The hypochlorous acid is unstable, liberating O2, with respect to light, dust, heat, transition metal impurities,....and also undergoes self disproportionation. Any of the latter factors would thereby effect the equilibrium of a system of Cl2/H2O/HCl and chloride.

[Edited on 31-12-2016 by AJKOER]

unionised - 31-12-2016 at 08:32

As I said, the photolysis of HOCl will lead to HCl and O2

Do you understand that dissolving aspirin in water is not the same as hydrolysing it to acetic and salicylic acids?

AJKOER - 31-12-2016 at 09:27

The breakdown of HOCl to HCl and O2 is one pathway observed upon the photolysis of HOCl.

The interaction of Cl2 and water involves the formation of free Cl2 (or its hydrate), H+, Cl-, HCl and HOCl ignoring the slight dissociation into hypochlorite. The process is slow even in cold water. Dissolving in dilute aqueous HCl is even harder, with the equilibrium shifting to more Cl2.

In contrast, with ClO2 in very dilute HClO3, the solubility of ClO2 is, I would anticipate, only slightly impacted as the reaction:

2 ClO2 + H2O = HClO2 + HClO3

appears to move to the right only at higher pH. In other words, ClO2 is highly physically soluble in water and undergoes little dissociation which is opposite to that of chlorine.

[Edited on 1-1-2017 by AJKOER]

unionised - 31-12-2016 at 09:36


Do you understand that dissolving aspirin in water is not the same as hydrolysing it to acetic and salicylic acids?

AJKOER - 31-12-2016 at 09:51

Quote: Originally posted by unionised  

Do you understand that dissolving aspirin in water is not the same as hydrolysing it to acetic and salicylic acids?


To quote a source (link: https://www.reference.com/science/factors-affect-solubility-... ):

"The solubility, or the amount of a substance that can be dissolved in a given solvent, varies based on a variety of factors. Solids, gases and liquids all have different levels of solubility.

Le Chatelier's principle explains why and how substances become more soluble. The principle notes that when a chemical equilibrium is under stress, it will react in a different manner to handle the stress."

So, in the equilibrium reaction of chlorine and water, increasing Cl- adds stress and results in more Cl2/H2O. With an added high chloride concentration, we are observing close to the pure physical solubility of chlorine in water at that temperature and pressure.

[Edited on 31-12-2016 by AJKOER]

unionised - 31-12-2016 at 10:15

And turning chlorine into something else by photolysis is equivalent of hydrolysis of aspirin; it's not the same stuff any more.
Do you accept that while you said "in strong light, the apparent solubility of chlorine in aqueous HCl increased due to a radical pathway as well..." the reality is that it will have practically no effect whatsoever?

AJKOER - 31-12-2016 at 11:27

OK, per my quoted reference, light is but another 'factor'. Also, adding NaCl to water 'changes' the salt to a system of sodium and chloride ions.

Assuming in a particular instance, that by the application of Le Chatelier's principle, the solubility of Cl2 in dilute aqueous HCl is in fact reduced from that in pure water, then from the instability of HOCl due to various factors including light, we thus increase the concentration of HCl via HOCl. This should result in a further reduction of the ability of chlorine to chemically react with water.

The effect of such may be small (when expressed as a relative percent) or not depending on temperature, pressure, length of light exposure, starting concentrations of Cl2 and HCl.
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[Edit] Found a paper, "Solubility of chlorine in aqueous hydrochloric acid solutions", PDF available from Journal of Hazardous Materials 119(1-3):13-8 , April 2005.

Link to full paper with graphs: https://www.researchgate.net/publication/7981068_Solubility_...

Apparently, the solubility of chlorine in aqueous HCl decreases until the following postulated complexation reaction reverses the trend at higher concentrations of HCl:

Cl2 + Cl- = Cl3- (or perhaps more correctly, in my opinion, as Cl2 + HCl = HCl3 )

However, with further research either the world wide web is ignorant of the Cl3- complex or it exists as HCl3, as I can find but one other reference to its formation with a case of in situ generated chlorine and HCl, see http://pubs.acs.org/doi/abs/10.1021/acs.iecr.6b00778?journal... .

Note: My cited complexation reaction upon photolysis could impact the slope of the later part of the solubility curve based on the difference in reaction rates between the above and the radical reaction:

Cl• + Cl- = Cl2•-

and also any photo conversion of HOCl to HCl would could be manifested as an HCl axis shift in primarily the downward sloping part of the solubility curve.
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As a side note, the claimed reaction:

Cl2 + Cl- = Cl3- (or perhaps more correctly, Cl2 + HCl = HCl3 )

may be important in the photolytic production of the chlorine radical per an experiment I once performed. Upon adding an excess of CaCl2 (this divalent chloride is more powerful than NaCl in increasing ionic strength of a solution) to freshly created HOCl placed in a thin wall vessel exposed to strong solar light quickly formed a more deeply colored solution. Some expected reactions of interest:

HOCl + heat → HCl + 1/2 O2

HCl + HOCl = Cl2 + H2O

Cl2 + Cl-(aq) = Cl3-(aq) (or perhaps, Cl2 + HCl(aq) = HCl3(aq) )

HOCl + hv = OH• + Cl•

OH• + Cl- = OHCl•-

OHCl•- + H+ → H2O + Cl• (see http://pubs.acs.org/doi/abs/10.1021/j100497a003 )

Cl2 + hv = Cl• + Cl• (gas phase reaction is more efficient at higher pressure)

Cl3-(aq) + hv =?= 2 Cl• + Cl- (as the existence of Cl3- is itself speculative, likely unstable, and may be more effective than the prior gas phase reaction of gaseous Cl2)

Or: HCl3 (aq) + hv =?= 2 Cl• + HCl (speculation, but as the very existence of HCl3 is questioned, it lack of stability on irradiation seems plausible)

Cl• + Cl- = Cl2•- (this radical has increased longevity over Cl• but lower reactivity)

As: Cl• + HOCl = HCl + ClO• (see, for example, http://pubs.acs.org/doi/abs/10.1021/acs.jpca.5b01273 )

I would similarly expect: Cl2•- + HOCl = HCl + ClO• + Cl-

[Edited on 1-1-2017 by AJKOER]