How to increase pH of lime water

Caccioppoli

Hi to all,
I'm in this problem:
I've got to prepare a solution of Ca(OH)2 with pH=13.2 .
On Perry I read that the solubility value of Ca(OH)2 at T=25°C is 1.65 g/L, so a solution with water and Calcium Hydroxide cannot reach pH=13.2.
So I was suggested to prepare the mixture in the following manner:
distilled water + 4g Ca(Oh)2 + 1g Na2SO4 + 1g NaCl + 1g NaNO3 .

I'd like to know if this solution is correct and why.

Thank you very much,
Caccioppoli

Borek

I don't see how sodium sulfate, chloride and nitrate can help in changing pH (other than increasing ionic strength of the solution, but it won't change the result much).

Perhaps start with saturated Ca(OH)₂ and add NaOH till pH goes up to 13.2? That will mean precipitating part of the calcium, but you can't cheat on K_sp.

Caccioppoli

Thank you very much for the answer.
An european guide line (etag) says that I have to prepare a solution with pH 13.2 having Ca(OH)2 powder and distillated water.
Since theoretically and experimentally I have verified that it is impossible to reach that pH, I asked for information.
They said that I have to add those substances in these quantities to obtain pH 13.2.
I have not yet prepared the mixture as they said, but for me is really strange, and I have thought about adding NaOH too.
By the way I think I should find an explanation for their suggestion.

Borek

If they say that's the standard procedure, you have not much choice but to try. I don't see how it can work, but can be I am missing something.

uby

two issues at play that should not be conflated. first is the solubility of the species in water and second is the dissociation fraction.

with regards to the additions of sodium sulfate and nitrate ... it may have to do with the difference in dissociation fraction between sodium and calcium salts. the sodium salts, dissolved in water, may react with undissolved Ca(OH)2 to produce NaOH (which will dissociate to a large degree and increase pH) and soluble CaSO4/Ca(NO3)2.

Borek

I don't see how. You can't cheat on Ca(OH)₂ K_sp. Introducing Na⁺ doesn't change OH^- concentration, so it can't shift Ca(OH)₂ dissolution equilibrium.

That's all not entirely true, as both Na⁺ and Ca²⁺ get complexed by OH^- to some extent, so it is not only about K_sp - but these complexation reactions don't increase OH^- concentration, so pH is not going up.

chemisttree

It can if calcium sulfate precipitates or if sulfate complexes the calcium. Complexing agents like sucrose can change the solubility of CaOH2. Ammonia can change the solubility of silver chloride, etc...

Borek

Right - but I have problems calling it the lime water after that.