[tonydoss]

Hi,

can someone tell me what is the solubility of H3BO3 in 7N HNO3 or 6N HCl, or where I could find this information, please?
Thanks.

Regards,

Tony

[Albert]

I've never heard something like that.
However, there's a distinct possibility you are talking about the following reaction:

H₃BO₃ + 2H₂O <-> [B(OH)₄ ]^- + H₃O⁺

Orthoboric acid ionization constant (Ka) is approximately 5.37 10^-10

Hence:  Ka = ([B(OH)₄ ]^- * H⁺)/H₃BO₃

I think you can safely assume [H+] = 7N or 6N

[tonydoss]

I suppose you assume that all the H+ come from HNO3 or HCl.
To calculate [H3BO3] using the Ka equation I will need to know [B(OH)₄^- ] too, won't I? Any idea how I do that? Or I don't need, in which case... how do I do?
Thanks a lot.

Tony

[Albert]

I suppose you assume that all the H+ come from HNO3 or HCl.

Yes, I do.

On this particular occasion, I'd be tempted to use the ionization constant as a Ksp and to solve the problem as when you have to calculate solubility of a salt such as AgCl.

[tonydoss]

   
Correct me if/where I'm wrong:
Ksp = 6.4x10^-10 = [B(OH)₄^- ]x[H₃O⁺ ] with [H₃O⁺ ] = 7 mol/L for instance
then [B(OH)₄^- ] = 6.4x10^-10 / 7

Ka = 5.37x10^-10 = [B(OH)₄^- ]x[H₃O⁺ ]/[H3BO3]
then [H3BO3] = (6.4x10^-10 / 7) x 7 / 5.37x10^-10
which means that [H3BO3] = Ksp/Ka = 6.4x10^-10 / 5.37x10^-10 = 1.19 mol/L = 73.9 mg/mL

Is that right? It seems a lot to me as generally I would be able to dissolve a few mg in 5 mL of 7N HNO3 only.

Thanks

Tony

[Albert]

Ksp = 6.4x10^-10 = [B(OH)₄^- ]x[H₃O⁺

Where does this Ksp come from? Could you display only the information given?

And, last but not least, what exactly do you want to know?

[Borek]

Saturated solution of boric acid in water is about 48 g/L in 20 deg C. That's about 0.77M. Such a solution has pH 4.62 - and boric acid is dissociated in 0.003% (assuming pK_a1 = 9.14).

This means that dissociation of H₃BO₃ is meaningless - and very low pH will not change solubility of boric acid due to its dissociation.

[tonydoss]

I just found the Ksp somewhere on the web. I don't remember where.

I want to know how much H3BO3 crystals I can dissolve in 7N HNO3 or 6N HCl without having any excess crystals.

Following what Borek said, I guess it's 48 g/L whatever the concentration of HNO3 or HCl. Right? It seems a lot, though, as generally I can't dissolve more than a few mg in 5 mL of HNO3 solution, for instance.

[Borek]

I guess it's 48 g/L whatever the concentration of HNO3 or HCl. Right? It seems a lot, though, as generally I can't dissolve more than a few mg in 5 mL of HNO3 solution, for instance.

Perhaps thete is some other chemistry involved, although I have no idea what could it be. 4.8g H₃BO₃ dissolves in 100g of water at 20 deg C, this is taken from solubility tables.