[curiouscat]

I have a saturated solution of NaCl at 30°C (~26% w/w i.e. 36 gm/100gm H2O) & want to precipitate out as much NaCl as I can by bubbling HCl (gas) through it.

Does anyone know what fraction of the initial NaCl can be precipitated out? I'm getting approximately 46% of the initial NaCl can be precipitated out i.e. the resulting solution  will be 14% w/w NaCl or 2.6 M

Here's my calculation:

Solubility Product of NaCl =37 M²

Let's assume HCl has a max solubility in H2O @30°C of ~36% w/w i.e. 11.5 Molar

Ksp = [Na][Cl]

37 = x (x + 11.5)  where x is max solubility of NaCl in a 32% HCl solution.

Solve giving x=2.6 M. i.e. NaCl conc. = 14% w/w

Density of 36% HCl @30 C approx = 1.17 gm/cc

[Borek]

Approach makes sense, but I would not trust the result for two reasons. One, I doubt solubility of HCl doesn't change in the brine, second, because of the very high ionic strength of the solution. That being said, Na⁺, Cl^- and H⁺ are quite common ions, chances are you will be able to find coefficients and use SIT (specific interactions theory) or some other model (Bromley, Pitzer) to calculate a more reasonable answer.

[curiouscat]

One, I doubt solubility of HCl doesn't change in the brine, second,

Thanks @Borek. Neither do I! But I dug around & cannot find any references to NaCl + HCl co-solubility.

If anyone can find anything I'd love to use.

because of the very high ionic strength of the solution. That being said, Na⁺, Cl^- and H⁺ are quite common ions, chances are you will be able to find coefficients and use SIT (specific interactions theory) or some other model (Bromley, Pitzer) to calculate a more reasonable answer.

Right. I'll read up more on that. Thanks.

What's funny is that the experiment of using HCl to ppt out NaCl (common ion demo.) is mentioned in dozens of undergrad labs / texts but no one reports how much NaCl they got out etc.

[curiouscat]

I found one interesting paper but too bad that it isn't not open to read.

http://pubs.acs.org/doi/abs/10.1021/je60084a015

But if I'm reading correctly, this table says that at 35°C even in a fairly weak,  4% w/w HCl solution only 20% w/w NaCl can dissolve.

Normally, at that temperature 26% w/w NaCl ought to have dissolved. If this observation is correct, then there seems a pretty strong interaction.

Too bad that they did not try a 35% HCl solution.

[curiouscat]

@Borek:

On looking deeper into Table-1 in that article, they report the NaCl solubility as 3.68 gm NaCl / 100 gm solution in an approx 20% w/w HCl solution. (at 36 C)

Essentially that'd mean that if I took a saturated NaCl solution (approx 25% w/w) and then passed enough HCl gas through it at one point I should end up with this solution where only 4% NaCl remains in solution and the rest is a 20% HCl solution.

i.e. Almost 84% of the initial dissolved NaCl content can be precipitated out. That would be pretty nice if it is indeed true.

Maybe someone can verify my interpretation?